fix(health): a chain blocked at a hop is dead on the board, unknown on the card

The short-circuit left the chain's exit tag untested, because the exit is itself
a hop and every hop behind the break was rewritten that way. A stand run caught
it — the test lives in a file that does not compile on the dev host, so nothing
local could have.

That is not neutral silence. selectExcluding ranks untested ABOVE dead and says
so in its own comment: with no fresh-alive member, an untested one is a better
bet than a known-dead one. Leaving a provably broken path untested is therefore
a positive preference for it over a path we merely know is dead.

The two readings answer different questions and now differ on purpose. Is this
hop's own node alive — unknown behind a break, so the card keeps untested and
blocked_by. Can this chain carry traffic — known, no, because the hop in front
of it was probed and did not answer. The board carries that second answer, which
is the one selection, the freshness gate and the manual test all read.

The exit verdict is derived, not dialled: it records the consequence of a probe
that did happen one hop earlier, and it is re-derived every pass, so the moment
the blocker answers the walk reaches the exit again and the next verdict there is
a real measurement.

Also keeps a routed group warm. Its checker used to stop on the idle timeout and
nothing filled in behind it, so a rule that fires rarely would show untested
while being in force and pay a cold probe on the first real request. The gate
that adds this work answers false when it does not know — the mirror of the one
that withholds work, so plain sing-box keeps the lifecycle it always had.

And the tls-spoof suite now skips without tcpdump instead of failing sixteen
times: a missing tool is not measured, not broken. The same distinction this
commit is about.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

common/tlsspoof/integration_test.go

@@ -25,6 +25,21 @@ func requireRoot(t *testing.T) {
 	}
 }
 
+// requireTCPDump skips when tcpdump is not installed.
+//
+// The same honesty this package's callers demand of a health reading: a missing
+// INSTRUMENT is "not checked", never "broken". Without it every test in this
+// file fails on `cmd.Start()` — sixteen red results that say nothing about the
+// code and hide any real failure among them — on a machine where the only thing
+// wrong is that a capture tool is absent. requireRoot has always drawn that line
+// for privileges; this draws it for the tool.
+func requireTCPDump(t *testing.T) {
+	t.Helper()
+	if _, err := exec.LookPath("tcpdump"); err != nil {
+		t.Skip("integration test requires tcpdump on PATH; install it to run this suite")
+	}
+}
+
 func tcpdumpObserver(t *testing.T, iface string, port uint16, needle string, do func(), wait time.Duration) bool {
 	t.Helper()
 	return tcpdumpObserverMulti(t, iface, port, []string{needle}, do, wait)[needle]
@@ -36,6 +51,9 @@ func tcpdumpObserver(t *testing.T, iface string, port uint16, needle string, do
 // the wire.
 func tcpdumpObserverMulti(t *testing.T, iface string, port uint16, needles []string, do func(), wait time.Duration) map[string]bool {
 	t.Helper()
+	// Every capture in this file funnels through here, so one guard covers the
+	// whole suite and no future test can forget it.
+	requireTCPDump(t)
 	ctx, cancel := context.WithTimeout(context.Background(), wait)
 	defer cancel()
 	cmd := exec.CommandContext(ctx, "tcpdump", "-i", iface, "-n", "-A", "-l",

common/urltest/probegate_lx.go

@@ -0,0 +1,61 @@
+package urltest
+
+// lx: health board §5.C — the reachability half of "should this be probed".
+//
+// # Two different reasons not to probe, and why they cannot be one flag
+//
+// A group's OWN probing schedule is stood down for two unrelated reasons, and
+// conflating them breaks one of the two:
+//
+//   - NOT USED — no enabled routing rule reaches this group, so probing it
+//     measures a path nothing travels. That is a property of the CONFIG, it is
+//     decided once when the config is generated, and it travels in the config
+//     itself (option.URLTestOutboundOptions.SelfCheck). It cannot change while
+//     the box runs, because the rules cannot change while the box runs.
+//
+//   - NOT REACHABLE RIGHT NOW — the group is a hop of a chain and a hop in
+//     FRONT of it is currently dead. Every member of this group dials through
+//     that hop, so every probe would fail inside it: the measurement would be
+//     about the broken hop, and would be recorded against this one. That is a
+//     property of the WORLD, it changes minute by minute, and it must be
+//     re-asked every time rather than baked into the config — a hop that comes
+//     back must resume probing on its own, with no reapply and nobody pressing
+//     anything.
+//
+// ProbeGate is the second one. It is deliberately a QUESTION asked at the
+// moment of probing and never a stored answer: there is no flag to set, so
+// there is no flag to forget to clear.
+//
+// The gate governs the group's own SCHEDULE only — the warm-up sweep and the
+// ticker. An explicit check (a human, an API call) is a deliberate request and
+// is never refused, exactly as with SelfCheck.
+type ProbeGate interface {
+	// ProbeAllowed reports whether the outbound tagged tag may run its own
+	// scheduled probe right now.
+	//
+	// Implementations MUST answer true when they do not know: a gate that
+	// refuses on missing information would silence probing precisely when the
+	// system has the least idea what is going on, and nothing would ever
+	// measure its way out of that. A nil ProbeGate means "no gate" and every
+	// probe proceeds.
+	ProbeAllowed(tag string) bool
+
+	// ProbeWhenIdle reports whether the outbound tagged tag must keep measuring
+	// even when no traffic is passing through it.
+	//
+	// A urltest group normally probes only while it is in use: Touch arms the
+	// ticker on a dial, and the idle timeout stops it again. That is right for a
+	// group whose readings matter only while somebody is dialling it, and wrong
+	// for one the routing config REACHES: a rule that matches rarely — a narrow
+	// domain list, say — is in force the whole time, so the health of its target
+	// is a live question the whole time. Letting it go quiet means the panel
+	// reports "untested" about a rule that is armed, and the first real request
+	// pays a cold probe instead of picking an already-known-good member.
+	//
+	// Unlike ProbeAllowed, the safe answer here is FALSE when nothing is known.
+	// This one ADDS work, and a gate that claimed it on missing information would
+	// keep every group in the process probing forever — not a default anybody
+	// asked for. Absent gate, unknown tag, nothing configured yet: false, and the
+	// idle timeout behaves exactly as it always has.
+	ProbeWhenIdle(tag string) bool
+}

option/group.go

@@ -18,6 +18,32 @@ type URLTestOutboundOptions struct {
 	// lx: SPEC 019 v2 — load-balancing.
 	Mode     string                  `json:"mode,omitempty"` // least_test (default) | round_robin
 	Balancer *URLTestBalancerOptions `json:"balancer,omitempty"`
+	// lx: health board §5.C — SelfCheck stands the group's OWN background
+	// health-check up or down. nil/absent == true, so every existing config keeps
+	// today's behaviour.
+	//
+	// Why this exists at all: a urltest group probes its members BY ITSELF — a
+	// warm-up sweep at PostStart and a ticker for as long as traffic keeps
+	// touching it — and it dials the members' outbounds DIRECTLY, from the
+	// router, over whatever the default WAN route is. For a group that traffic
+	// actually flows through, that is exactly right: the probe travels the same
+	// path the connections do. But for a group NO routing rule reaches, that
+	// same probe measures a path nothing uses — and it stores the result under
+	// the members' BASE tags, which every health consumer then reads as "the
+	// node's health". A node that is blocked on the direct WAN and perfectly
+	// alive behind a tunnel therefore reads "dead" the moment such a group
+	// probes it; the reading is not merely stale, it is FALSE, and it poisons
+	// the shared board for everyone (selection, the panel, the observatory's
+	// freshness gate). SelfCheck=false is how the control plane stands such a
+	// group's own schedule down: the shater engine computes which groups the
+	// applied rules actually reach (the observatory's used-set) and disables
+	// the self-check on the rest, so the ONLY prober left is the observatory —
+	// which probes along the real dial paths and nothing else.
+	//
+	// The flag suppresses only the group's own SCHEDULE (the PostStart warm-up
+	// and the Touch ticker). An EXPLICIT CheckOutbounds/URLTest call — the
+	// adapter interface a human or an API invokes on purpose — still works.
+	SelfCheck *bool `json:"self_check,omitempty"`
 }
 
 // URLTestBalancerOptions configures round_robin: a fixed-size pool of live nodes, lazily

panel/src/api.ts

@@ -390,19 +390,142 @@ export interface GroupHealth {
  * any more and nothing to report here beyond the groups themselves.
  */
 
+/**
+ * One hop of one chain, measured where that hop actually sits in the path.
+ *
+ * This is the reading the daemon always took and never showed. A chain is not a
+ * target with a single health — it is an ordered series of them, and the only
+ * question an operator ever asks about a broken chain is WHICH hop broke. The
+ * end-to-end exit reading cannot answer that: it says "the path is dead" for a
+ * four-hop chain and leaves the person to guess between four suspects.
+ *
+ * WIRE ORDER. `index` is 1-based and counts hops in the order the router dials
+ * them: hop 1 is the first physical hop, and each later hop is dialled THROUGH
+ * the ones before it. The hop carrying `exit: true` — always the largest index —
+ * is where traffic leaves for the internet. A leading `egress:` in the chain's
+ * configured Hops is NOT a numbered hop: the daemon lifts it into the entry
+ * detour of hop 1, so a chain written `egress:ewan → node:awgout → group:sub0`
+ * reports two hops, not three. Anything zipping this against the model's Hops
+ * must drop that leading egress first and give up on labelling entirely if the
+ * counts still disagree — a chain that splices sub-chains gets flattened here,
+ * and a confidently WRONG hop name is worse than no name.
+ *
+ * `tag` is the engine-side outbound (`chain-<name>-h2`). Debugging and tooltips
+ * only; it is never a label to put in front of a person.
+ *
+ * ORDERED WALK — THE READING STOPS AT THE FIRST DEAD HOP. Hops are NOT measured
+ * independently, and never were measurable that way: hop 3 is dialled THROUGH
+ * hop 2, so probing hop 3 while hop 2 is down measures hop 2 a second time and
+ * learns nothing about hop 3. The daemon therefore walks the path in wire order
+ * and stops at the first hop that does not answer. Every hop below that one is
+ * left undialled and reported `state: "untested"` — no measurement exists —
+ * carrying {@link ChainHopBlock} in `blocked_by` to name the hop that stopped the
+ * walk. So a chain never reports a dead hop with a live hop below it; that shape
+ * is not a rare case, it is unreachable.
+ *
+ * NODE HOP vs GROUP HOP. For `kind: "node"` the hop IS the measurement: `total`
+ * is 1, the counters follow its own state, and `selected` is ''. For
+ * `kind: "group"` the counters roll up that hop's per-hop member COPIES — the
+ * copies dialled through the hops in front of it, which is exactly why they can
+ * read alive here while the same group's standalone card reads dead. Both
+ * readings are true; they measure different dial paths. `selected` is the node
+ * NAME the hop routes through right now, and `delay_ms` / `age_seconds` belong
+ * to that selected member (or the freshest alive one).
+ *
+ * Invariants the daemon guarantees — never re-derive them, just read them:
+ * `tested === alive + dead` and `alive + dead + untested === total`.
+ *
+ * `state` is a closed set of THREE. `untested` is NEVER "dead" and never
+ * "healthy": it means nothing fresh enough is known. Without `blocked_by` that is
+ * a matter of timing — for a used chain it resolves on its own within seconds.
+ * With `blocked_by` it will not resolve until the named hop is fixed. There is no
+ * fourth state for that; the state stays `untested` because that is what it is.
+ * `age_seconds: -1` means the age is unknown.
+ */
+export interface ChainHopHealth {
+  /** 1-based WIRE order. Hop 1 is dialled first; see the note above. */
+  index: number
+  /** Engine outbound tag (`chain-<name>-h2`) — tooltips/debugging, never a label. */
+  tag: string
+  /** `node` ⇒ the hop is the measurement. `group` ⇒ the counters roll up members. */
+  kind: 'node' | 'group'
+  /** This hop is where traffic leaves for the internet. Always the largest index. */
+  exit: boolean
+  /** Closed set — switch on it exhaustively. `untested` is never "dead". */
+  state: 'alive' | 'dead' | 'untested'
+  /** RTT of the selected/freshest alive member; 0 (meaningless) when not alive. */
+  delay_ms: number
+  /** Age of that measurement in seconds; -1 when unknown. */
+  age_seconds: number
+  /** Node name this GROUP hop routes through right now; '' for a node hop. */
+  selected: string
+  total: number
+  tested: number
+  alive: number
+  dead: number
+  untested: number
+  /**
+   * PRESENT ONLY on a hop the ordered walk never reached — i.e. a hop sitting
+   * below one the prober found `dead`. The key is omitted otherwise; absent is
+   * the normal case and means "this hop was actually dialled".
+   *
+   * Its presence is the daemon's own statement that this hop has NO measurement,
+   * and it comes with the rest of that statement already filled in: `state` is
+   * `untested`, `delay_ms` is 0, `age_seconds` is -1, and the counters are
+   * `alive: 0, dead: 0, tested: 0, untested: total`. Read those; do not re-derive
+   * a verdict from them, and do not infer a block from zeroed counters either —
+   * an unprobed-yet hop has the same numbers and a very different meaning.
+   * `selected` MAY still be non-empty: the wrapper does have a pick, it simply
+   * was not measured, so it says which node the hop would use, not which node is
+   * carrying traffic.
+   */
+  blocked_by?: ChainHopBlock
+}
+
+/**
+ * The hop that stopped the ordered walk, as reported on every hop below it.
+ *
+ * This exists because "no reading" and "no reading, and here is whose fault that
+ * is" are different answers to the operator's actual question. Without it a
+ * blocked hop is indistinguishable from one the observatory has not come round to
+ * yet, and the interface can only shrug.
+ *
+ * `index` is the 1-based WIRE index of the blocking hop and is ALWAYS smaller
+ * than the index of the hop carrying it, so it points at a hop already on screen.
+ * `tag` is that hop's engine outbound (`chain-<name>-h3`) — debugging and
+ * tooltips only, never a label to put in front of a person, exactly as on
+ * {@link ChainHopHealth}.tag.
+ */
+export interface ChainHopBlock {
+  /** 1-based wire index of the hop that did not answer. Always < this hop's index. */
+  index: number
+  /** That hop's engine outbound tag — tooltips/debugging, never a label. */
+  tag: string
+}
+
 /** Per-chain reachability, the chain analogue of {@link GroupHealth}.used (plan
  *  §5.E): a chain no enabled routing rule routes through is outside the
- *  observatory's plan, so its exit is never probed and the Targets card renders it
- *  "unused" instead of an exit-test readout. A chain has no membership counters —
- *  it is a fixed path, and its end-to-end health is the exit test's job. */
+ *  observatory's plan, so nothing probes it and the Targets card says so instead
+ *  of rendering a health reading. A chain has no membership counters of its own —
+ *  it is a fixed path, and its health lives on its {@link ChainHopHealth} hops. */
 export interface ChainHealth {
   name: string
   /** An enabled routing rule (the Final target, a DNS-resolver detour, a device
-   *  target, …) reaches this chain, so the observatory probes its exit in the
+   *  target, …) reaches this chain, so the observatory probes its hops in the
    *  background. false ⇒ nothing routes through the chain: it is skipped by the
-   *  background probing and its end-to-end health stays untested. That is an
-   *  "unused" note about the ROUTING CONFIG, never a health problem. */
+   *  background probing and its health stays untested. That is an "unused" note
+   *  about the ROUTING CONFIG, never a health problem. */
   used: boolean
+  /**
+   * Per-hop health in wire order (see {@link ChainHopHealth}).
+   *
+   * MAY BE ABSENT, and absent does not mean "this chain has no hops". It means
+   * the engine never materialised per-hop outbounds for it: the chain is unused,
+   * or it collapses to a single hop and the daemon points traffic straight at
+   * that target instead of building a copy of it. Read a missing key as "nothing
+   * measured per hop", never as an empty path or as a fault.
+   */
+  hops?: ChainHopHealth[]
 }
 
 export interface GroupsHealth {
@@ -1432,8 +1555,21 @@ export function getGroupsHealth(
 }
 
 /**
- * One group's (or chain's) last test: which member the balancer picked, how fast
- * it answered, and what the internet saw as the source address.
+ * What the OBSERVATORY measured for one group or chain — not a dial the panel
+ * made.
+ *
+ * This shape used to come from a fresh connection opened on demand, straight at
+ * the target. That was a lie on any router whose proxies are blocked when dialled
+ * directly and work only as a hop behind a tunnel: the card reported dead for a
+ * path that carries traffic all day. The daemon now has exactly one thing that
+ * measures — the background observatory, which probes along the REAL dial path,
+ * per-hop copies and all — and this endpoint reports what it found. There is no
+ * second measurement anywhere, and the panel never opens a connection of its own.
+ *
+ * So read the fields as a READ, not as a test run: `ok` and `delay_ms` are the
+ * observatory's verdict for the path traffic actually takes, and `tested_unix`
+ * (router clock, seconds) is when the OBSERVATORY took that measurement — which
+ * can be a few seconds before the refresh was asked for.
  *
  * `ok:true` with an EMPTY `exit_ip`/`exit_country` is a valid, successful result,
  * not a partial failure: the delay was measured but the exit address could not be
@@ -1442,10 +1578,23 @@ export function getGroupsHealth(
  *
  * Chains ride the same endpoint. For a chain row, `group` carries the CHAIN's
  * name and `selected` the node its last group hop picked ('' when the exit hop
- * isn't a group). Everything else reads the same way.
+ * isn't a group). Per-hop detail is a different read: {@link ChainHopHealth}.
  *
- * `ok:false` ⇒ the test failed and `error` carries the human reason; every other
- * field is meaningless. `tested_unix` is the router's clock, in seconds.
+ * `ok:false` ⇒ there is no usable measurement and `error` carries the human
+ * reason; every other field is meaningless. Four of those reasons are about the
+ * observatory rather than the path, and must not be rendered as "your target is
+ * broken":
+ *
+ *   "not routed by any enabled rule, so nothing measures it — the observatory
+ *    only probes paths the rules use"
+ *   "the observatory has not reached this target yet — it refreshes on the
+ *    global probe interval"
+ *   "background probing is disabled, so there is nothing to measure this target
+ *    with"
+ *   "the observatory's probe through this path failed"
+ *
+ * Only the last one is a health finding. The first three say the measurement
+ * does not exist, which is a different thing and a different fix.
  */
 export interface GroupTestResult {
   group: string // group name — or a chain name for a chain row
@@ -1462,7 +1611,11 @@ export interface GroupTestResult {
  * GET /api/groups/test — progress plus every result so far. `results` is ALWAYS
  * an array (never null); `done`/`total` count finished vs targeted groups and
  * chains while `running` is true. Idle reads `{running:false}` with the last
- * run's results still attached, so a reload after a test still shows what it found.
+ * run's results still attached, so a reload still shows what was last read.
+ *
+ * "Running" means the observatory is working through an out-of-turn refresh pass
+ * over the named targets and this endpoint is collecting what it measures. It is
+ * not the panel dialling anything.
  */
 export interface GroupTestStatus {
   running: boolean
@@ -1495,10 +1648,16 @@ export interface GroupTestStart {
 }
 
 /**
- * POST /api/groups/test — measure a target's delay and exit address. Pass a
- * group or chain name to test one; pass nothing (or '') to test every group
- * and every chain. Singleton: a second call while a run is in flight resolves
- * to `{started:false, reason:'already running'}` rather than failing.
+ * POST /api/groups/test — ask the observatory for an out-of-turn refresh pass,
+ * then report what it measured. Pass a group or chain name to refresh one; pass
+ * nothing (or '') for every group and every chain.
+ *
+ * It does NOT dial. The observatory is the only thing in the daemon that
+ * measures anything, and it measures along the real dial path — so this is the
+ * "don't wait for the next probe interval" button, not a second opinion. The
+ * numbers it returns are the same numbers the cards are already showing, just
+ * fresher. Singleton: a second call while a pass is in flight resolves to
+ * `{started:false, reason:'already running'}` rather than failing.
  */
 export function postGroupsTest(name = ''): Promise<GroupTestStart> {
   return MOCK

panel/src/mock.ts

@@ -6,7 +6,7 @@
 // state mutates in-memory so the Apply / Confirm / Rollback flow is exercisable.
 //
 // Type-only imports from api.ts (erased at build) keep this free of a runtime cycle.
-import type { ApplyResult, ChainHealth, ConnLogEntry, DiscoveredDevice, GroupHealth, GroupMemberHealth, GroupsHealth, GroupTestResult, GroupTestStart, GroupTestStatus, Interface, Model, Profile, QueryLogEntry, RuleReach, RulesReachability, RulesetCategories, RulesetCheck, RulesetStatus, Stats, StatsLogPage, StatsLogQuery, Status, StatusWarning, Traffic } from './api'
+import type { ApplyResult, ChainHealth, ChainHopHealth, ConnLogEntry, DiscoveredDevice, GroupHealth, GroupMemberHealth, GroupsHealth, GroupTestResult, GroupTestStart, GroupTestStatus, Interface, Model, Profile, QueryLogEntry, RuleReach, RulesReachability, RulesetCategories, RulesetCheck, RulesetStatus, Stats, StatsLogPage, StatsLogQuery, Status, StatusWarning, Traffic } from './api'
 
 let armed = false // a pending commit-confirm auto-rollback
 let hasLastGood = false // a predecessor config exists to roll back to (post-apply)
@@ -129,9 +129,25 @@ const CONFIG: Model = {
     { Name: 'via-tunnel', Source: 'subscription', Subscription: 'primary', Strategy: 'leastping', Egress: 'awg' },
     { Name: 'fallback', Source: 'subscription', Subscription: 'backup', Strategy: 'roundrobin', Egress: '' },
   ],
-  // One multi-hop chain so `?mock` exercises the chain card's Test button and
-  // its result readout: enters through the awg tunnel, exits via the auto group.
-  Chains: [{ Name: 'relay', Hops: ['egress:awg', 'group:auto'] }],
+  // Three chains, one per state the hop readout has to render.
+  Chains: [
+    // The owner's real production shape: leave through a WAN interface, cross an
+    // AmneziaWG node, then three subscription groups in series. The leading
+    // `egress:` is NOT a numbered hop — the daemon lifts it into hop 1's entry
+    // detour — so this reports FOUR hops, and hop 3 is dead while its neighbours
+    // answer. That single red notch in the middle of a live path is the entire
+    // reason per-hop health exists, so `?mock` must show it at a glance.
+    {
+      Name: 'ewan-wg-subs',
+      Hops: ['egress:wan', 'node:home-wg', 'group:auto', 'group:stealth', 'group:via-tunnel'],
+    },
+    // Used, but the observatory hasn't come round yet — every hop untested. Not
+    // dead and not healthy: the state the panel most easily renders as a fault.
+    { Name: 'sub-fresh', Hops: ['node:home-wg', 'group:fallback'] },
+    // No enabled rule targets it, so the observatory skips it entirely and the
+    // daemon never materialises its hops: `used:false` and NO `hops` key.
+    { Name: 'relay', Hops: ['egress:awg', 'group:auto'] },
+  ],
   Egresses: [
     { Name: 'wan', Type: 'interface', Interface: 'wan' },
     // An AmneziaWG tunnel — the whole point of a group-level egress binding.
@@ -145,6 +161,13 @@ const CONFIG: Model = {
   Rules: [
     { Name: 'block-ads', Enabled: true, Order: 10, DstRuleset: ['ad-hosts'], Target: 'block' },
     { Name: 'ru-bypass', Enabled: true, Order: 20, DstRuleset: ['ru-inside'], Target: 'direct' },
+    // These two are what make the chains USED — the observatory probes only the
+    // paths an enabled rule can reach, so without them every chain card would
+    // read "not routed" and the hop rail would never appear in `?mock`. Kept
+    // ABOVE the condition-less rule at Order 40, which would otherwise swallow
+    // everything below it and mark them "never applies".
+    { Name: 'media-via-chain', Enabled: true, Order: 22, DstRuleset: ['yt-geosite'], Target: 'chain:ewan-wg-subs' },
+    { Name: 'spare-via-chain', Enabled: true, Order: 24, DstRuleset: ['ad-hosts'], Target: 'chain:sub-fresh' },
     { Name: 'private-direct', Enabled: true, Order: 30, DstRuleset: ['private-nets'], Target: 'direct' },
     // A SECOND condition-less rule, above the real default. It reads like a working
     // rule and does nothing: a rule with no conditions becomes the router's default,
@@ -1172,20 +1195,59 @@ function healthList(): GroupHealth[] {
   return (CONFIG.Groups ?? []).map((g) => summarise(g.Name, GROUP_MEMBERS.get(g.Name) ?? []))
 }
 
-/** Per-chain reachability for the Targets page's "unused" badge (plan §5.E) — the
- *  chain analogue of healthList's `used` field. The mock's single chain `relay` is
- *  NOT referenced by any rule in CONFIG.Rules (they target group:auto / block /
- *  direct), so it reads used=false and its card renders "unused" — exactly the case
- *  the badge exists to surface. A stopped engine reports no chains. */
+/**
+ * Per-hop health, keyed by chain name — what the observatory measured at each
+ * position of the path, in WIRE order.
+ *
+ * `ewan-wg-subs` is the fixture that matters, and it encodes the ORDERED WALK.
+ * Hop 1 is the WireGuard node and answers; hop 2 is a subscription group whose
+ * copies answer THROUGH it — 119 of 122 tested alive, which is the reading only a
+ * per-hop probe can produce, since the same members are dialled differently on
+ * their own card. Hop 3 is a group whose members all time out at that position,
+ * and the walk STOPS there: hop 4 is dialled through hop 3, so it was never
+ * dialled at all. It comes back `untested` with `blocked_by` naming hop 3, its
+ * counters zeroed, and `selected` still set — the wrapper has a pick, nothing
+ * crossed it to measure. A dead hop with a live hop under it is not in this
+ * fixture because the daemon can no longer produce one.
+ *
+ * `sub-fresh` is used but never yet reached: every hop untested, nothing dead,
+ * no block — the other reason a lamp is unlit, and the one that fixes itself.
+ * `relay` is absent from this map on purpose — an unused chain is never
+ * materialised, so the daemon sends no `hops` key at all, which is "nothing
+ * measured", not "no hops".
+ */
+const CHAIN_HOPS: Record<string, ChainHopHealth[]> = {
+  'ewan-wg-subs': [
+    { index: 1, tag: 'chain-ewan-wg-subs-h1', kind: 'node', exit: false, state: 'alive', delay_ms: 41, age_seconds: 22, selected: '', total: 1, tested: 1, alive: 1, dead: 0, untested: 0 },
+    { index: 2, tag: 'chain-ewan-wg-subs-h2', kind: 'group', exit: false, state: 'alive', delay_ms: 96, age_seconds: 18, selected: '🇳🇱 Amsterdam-01', total: 298, tested: 122, alive: 119, dead: 3, untested: 176 },
+    { index: 3, tag: 'chain-ewan-wg-subs-h3', kind: 'group', exit: false, state: 'dead', delay_ms: 0, age_seconds: 15, selected: '', total: 2, tested: 2, alive: 0, dead: 2, untested: 0 },
+    { index: 4, tag: 'chain-ewan-wg-subs-h4', kind: 'group', exit: true, state: 'untested', delay_ms: 0, age_seconds: -1, selected: '🇸🇬 Singapore-09', total: 6, tested: 0, alive: 0, dead: 0, untested: 6, blocked_by: { index: 3, tag: 'chain-ewan-wg-subs-h3' } },
+  ],
+  'sub-fresh': [
+    { index: 1, tag: 'chain-sub-fresh-h1', kind: 'node', exit: false, state: 'untested', delay_ms: 0, age_seconds: -1, selected: '', total: 1, tested: 0, alive: 0, dead: 0, untested: 1 },
+    { index: 2, tag: 'chain-sub-fresh-h2', kind: 'group', exit: true, state: 'untested', delay_ms: 0, age_seconds: -1, selected: '', total: 24, tested: 0, alive: 0, dead: 0, untested: 24 },
+  ],
+}
+
+/** Per-chain reachability plus per-hop health for the Targets page. `used` is the
+ *  chain analogue of healthList's field; `hops` is OMITTED (never null, never []),
+ *  exactly like the daemon, for a chain the engine never materialised. A stopped
+ *  engine reports no chains at all. */
 function chainHealthList(): ChainHealth[] {
   if (!mockPlane().engine) return []
-  return (CONFIG.Chains ?? []).map((c) => ({ name: c.Name, used: chainUsed(c.Name) }))
+  return (CONFIG.Chains ?? []).map((c) => {
+    const hops = CHAIN_HOPS[c.Name]
+    const h: ChainHealth = { name: c.Name, used: chainUsed(c.Name) }
+    if (hops) h.hops = hops.map((x) => ({ ...x }))
+    return h
+  })
 }
 
 /** A chain is "used" when some enabled routing rule (or Final, or a DNS detour)
  *  targets `chain:<name>` — the same reachability the daemon's observatory derives.
- *  The mock's rules never target a chain, so every chain reads used=false; a real
- *  config would mark the ones rules point at used=true. */
+ *  Two of the mock's rules do (`media-via-chain` → ewan-wg-subs, `spare-via-chain`
+ *  → sub-fresh), so those two chains read used=true and get a hop rail; `relay`
+ *  is targeted by nothing and reads used=false, which is the unused note. */
 function chainUsed(name: string): boolean {
   const target = `chain:${name}`
   return (CONFIG.Rules ?? []).some(
@@ -1237,15 +1299,23 @@ class ApiErrorLike extends Error {
   }
 }
 
-// Mock group/chain test. Deliberately covers every state the UI has to render,
-// one per target, so a single offline run exercises all of them:
-//   auto     → ok WITH an exit address
-//   stealth  → ok WITHOUT one (delay measured, address undeterminable) — a
-//              SUCCESS, and the case the UI most easily gets wrong
-//   relay    → the chain: same wire shape, `group` carries the CHAIN's name and
-//              `selected` the node its exit group picked
-//   fallback → a failure carrying a human reason
-// Results land one per GET poll, so the running/progress state is visible too.
+// Mock refresh results. The endpoint no longer dials anything: it asks the
+// observatory to measure out of turn and reports what the observatory found, so
+// every row here is a READ of a background measurement. Deliberately covers every
+// state the UI has to render, one per target, so a single offline run exercises
+// all of them:
+//   auto         → ok WITH an exit address
+//   stealth      → ok WITHOUT one (delay measured, address undeterminable) — a
+//                  SUCCESS, and the case the UI most easily gets wrong
+//   ewan-wg-subs → the chain: same wire shape, `group` carries the CHAIN's name
+//                  and `selected` the node its exit hop picked
+//   via-tunnel   → the one honest health FAILURE: a probe that ran and failed
+//   fallback,
+//   relay        → not routed at all, so no measurement exists to report
+//   sub-fresh    → routed, but the observatory hasn't come round yet
+// The last three are absence of measurement, not a broken target, and the copy
+// has to keep them apart. Results land one per GET poll, so the running/progress
+// state is visible too.
 const GROUP_TEST_SHAPE: Record<string, Omit<GroupTestResult, 'group' | 'tested_unix'>> = {
   auto: {
     selected: 'nl-reality-2',
@@ -1263,32 +1333,60 @@ const GROUP_TEST_SHAPE: Record<string, Omit<GroupTestResult, 'group' | 'tested_u
     ok: true,
     error: '',
   },
-  // The chain — Selected is the node the chain's exit group (auto) picked.
-  relay: {
-    selected: 'nl-reality-2',
-    delay_ms: 61,
-    exit_ip: '185.12.34.56',
-    exit_country: 'NL',
-    ok: true,
-    error: '',
+  // The chain, and the pairing that makes the whole feature worth building. A
+  // chain is one series path, so with hop 3 dead the end-to-end probe is never
+  // even attempted — the daemon stops walking there. This row and the hop rail
+  // therefore have to tell one story, not two: both name hop 3, and neither
+  // offers hop 4 as a second suspect. Note the row does NOT say "the probe
+  // failed" — no probe of this chain's exit ran at all — which is why the daemon
+  // has a separate message for it.
+  'ewan-wg-subs': {
+    selected: '',
+    delay_ms: 0,
+    exit_ip: '',
+    exit_country: '',
+    ok: false,
+    error:
+      'hop 3 of this chain was probed and did not answer, so nothing reaches the exit through it — fix that hop first',
   },
-  // Dead through its tunnel, exactly as its membership health says — the exit
-  // test and the member health tell the same story about the same group.
+  // The one real health failure in the fixture: the observatory's probe ran along
+  // this path and did not come back.
   'via-tunnel': {
     selected: '',
     delay_ms: 0,
     exit_ip: '',
     exit_country: '',
     ok: false,
-    error: 'no member answered through egress awg (6 of 6 timed out)',
+    error: 'the observatory’s probe through this path failed',
   },
+  // Not a health verdict — nothing routes here, so no measurement of it exists.
   fallback: {
     selected: '',
     delay_ms: 0,
     exit_ip: '',
     exit_country: '',
     ok: false,
-    error: 'no reachable node in the group (all 3 members timed out)',
+    error:
+      'not routed by any enabled rule, so nothing measures it — the observatory only probes paths the rules use',
+  },
+  relay: {
+    selected: '',
+    delay_ms: 0,
+    exit_ip: '',
+    exit_country: '',
+    ok: false,
+    error:
+      'not routed by any enabled rule, so nothing measures it — the observatory only probes paths the rules use',
+  },
+  // Routed, materialised, simply not reached yet. Untested is not dead.
+  'sub-fresh': {
+    selected: '',
+    delay_ms: 0,
+    exit_ip: '',
+    exit_country: '',
+    ok: false,
+    error:
+      'the observatory has not reached this target yet — it refreshes on the global probe interval',
   },
 }
 

panel/src/pages/Targets.css

@@ -704,6 +704,13 @@
 .tg-test--bad .tg-test-msg {
   color: var(--crit);
 }
+/* "Nothing measured this" is not a failure and must never be dressed as one: an
+   unlit lamp and the faintest text on the card, the same register the group
+   readout uses for its unmeasured state. */
+.tg-test--none .tg-test-msg {
+  font-family: var(--font-sans);
+  color: var(--faint);
+}
 .tg-test--wait .tg-test-msg {
   color: var(--amber);
 }
@@ -1038,6 +1045,235 @@
 }
 
 /* ---- responsive ---- */
+/* ---- chain hop rail ----
+ * The chain section's signature, and the one place this card spends any
+ * boldness: the path is drawn as a CONDUCTOR with a numbered lamp at each hop,
+ * and the conductor is SEVERED below the first hop that was probed and did not
+ * answer. A chain is a single series path, so the question is never "how many
+ * hops are green", it is "where does my traffic stop" — and a broken line answers
+ * that before a word has been read.
+ *
+ * The two marks carry two different facts and must not be conflated:
+ *   - the LAMP is that hop's own measurement (good / warn / crit / unlit). Below
+ *     the break there is no measurement to draw: the daemon stops walking at the
+ *     first dead hop, so those lamps are UNLIT and the row says which hop stopped
+ *     the walk. Unlit is never a shade of red — it claims nothing, which is the
+ *     truth about a hop nobody dialled;
+ *   - the CONDUCTOR is reachability through the path, which really does stop.
+ *
+ * Orange is untouched here. Semantics carry every colour, and everything that is
+ * not a lamp is groove-grey. No transitions and no animation anywhere in the
+ * rail, so there is nothing for reduced-motion to switch off. */
+.ch-rail {
+  gap: 8px;
+}
+.ch-eyebrow {
+  font-family: var(--font-mono);
+  font-size: 9px;
+  letter-spacing: var(--track-label);
+  text-transform: uppercase;
+  color: var(--faint);
+}
+.ch-hops {
+  --ch-num: 1.8ch; /* the engraved hop number's gutter */
+  --ch-gap: 8px;
+  --ch-led: 10px; /* must match .led's width */
+  --ch-lampy: 14px; /* row top → lamp centre; the conductor's anchor */
+  /* x of the conductor: the number gutter, one gap, then the lamp's centre */
+  --ch-spine: calc(var(--ch-num) + var(--ch-gap) + var(--ch-led) / 2);
+  list-style: none;
+  margin: 0;
+  padding: 0;
+}
+.ch-hop {
+  position: relative;
+  display: grid;
+  grid-template-columns: var(--ch-num) var(--ch-led) minmax(0, 1fr);
+  column-gap: var(--ch-gap);
+  align-items: start;
+}
+
+/* the conductor — two halves per row, so a break lands on one link only */
+.ch-hop::before,
+.ch-hop::after {
+  content: '';
+  position: absolute;
+  left: var(--ch-spine);
+  width: 2px;
+  margin-left: -1px;
+  /* Brighter than a plain groove: this line IS the readout, and at groove
+     strength it disappeared into the panel and took the whole idea with it. */
+  background: color-mix(in srgb, var(--dim) 55%, var(--groove));
+}
+.ch-hop::before {
+  top: 0;
+  height: calc(var(--ch-lampy) - var(--ch-led) / 2 - 3px);
+}
+.ch-hop::after {
+  top: calc(var(--ch-lampy) + var(--ch-led) / 2 + 3px);
+  bottom: 0;
+}
+/* Nothing feeds hop 1 from above, and nothing leaves the exit downward — the
+   path starts and ends inside this rail. */
+.ch-hop--first::before {
+  display: none;
+}
+.ch-hop--exit::after {
+  bottom: auto;
+  height: 9px;
+}
+/* …the exit ends on a crossbar instead of trailing off: end of line. */
+.ch-hop--exit .ch-socket {
+  position: relative;
+}
+.ch-hop--exit .ch-socket::after {
+  content: '';
+  position: absolute;
+  left: 50%;
+  transform: translateX(-50%);
+  top: calc(var(--ch-lampy) + var(--ch-led) / 2 + 12px);
+  width: 11px;
+  height: 2px;
+  background: color-mix(in srgb, var(--dim) 55%, var(--groove));
+}
+
+/* THE SEVER. Everything from the dead hop's outgoing link downward is drawn as a
+   broken conductor: unmistakably not-a-line at a glance, and unmistakably not a
+   colour, because a colour here would compete with the lamps that carry health. */
+.ch-hop--dead::after,
+.ch-hop--severed::before,
+.ch-hop--severed::after {
+  background: repeating-linear-gradient(
+    to bottom,
+    color-mix(in srgb, var(--dim) 45%, var(--groove)) 0 3px,
+    transparent 3px 7px
+  );
+}
+
+.ch-num {
+  font-size: 10px;
+  line-height: calc(var(--ch-lampy) * 2);
+  text-align: right;
+  color: var(--faint);
+}
+.ch-socket {
+  display: flex;
+  align-items: center;
+  height: calc(var(--ch-lampy) * 2);
+}
+.ch-body {
+  min-width: 0;
+  /* Separates one hop from the next. The conductor runs through this space, so
+     too little of it and two hops read as one wrapped row. */
+  padding-bottom: 8px;
+}
+.ch-l1 {
+  display: flex;
+  align-items: center;
+  flex-wrap: wrap;
+  gap: 8px;
+  min-height: calc(var(--ch-lampy) * 2);
+}
+.ch-name {
+  font-size: 12px;
+  color: var(--ink);
+  overflow-wrap: anywhere;
+}
+.ch-hop--untested .ch-name {
+  color: var(--dim);
+}
+/* The exit marker is NEUTRAL on purpose. The config path above this rail tags its
+   exit green, which is free there — but in here green means "answering", and a
+   green badge on the last hop would read as a health claim about it. */
+.ch-tag {
+  font-family: var(--font-mono);
+  font-size: 8.5px;
+  letter-spacing: 0.14em;
+  text-transform: uppercase;
+  color: var(--faint);
+}
+.ch-delay {
+  font-size: 11.5px;
+  font-weight: 700;
+  color: var(--ink);
+}
+.ch-quiet {
+  font-family: var(--font-sans);
+  font-size: 12px;
+  color: var(--faint);
+}
+/* A blocked hop's phrase carries a tooltip with the blocking hop's engine
+   outbound, so it takes the same help cursor as .ch-dead. No colour of its own:
+   the finding is red once, on the hop that actually failed. */
+.ch-blocked {
+  cursor: help;
+}
+.ch-age {
+  margin-left: auto;
+  font-size: 10.5px;
+  color: var(--faint);
+  white-space: nowrap;
+}
+
+/* The counters read exactly as they do on a group card — alive out of TESTED,
+   with the untested remainder as a quiet aside only when there is one. Same
+   register, same weights, deliberately not a second dialect. */
+.ch-l2 {
+  display: flex;
+  align-items: center;
+  flex-wrap: wrap;
+  gap: 8px;
+  margin-top: 1px;
+  font-size: 11.5px;
+  letter-spacing: 0.02em;
+  color: var(--dim);
+}
+.ch-count {
+  font-size: 12px;
+  color: var(--dim);
+  white-space: nowrap;
+}
+.ch-count b {
+  font-size: 14px;
+  font-weight: 700;
+  color: var(--ink);
+}
+.ch-hop--dead .ch-count b {
+  color: var(--crit);
+}
+.ch-word {
+  font-size: 10.5px;
+  letter-spacing: 0.12em;
+  text-transform: uppercase;
+  color: var(--faint);
+}
+.ch-dead {
+  padding: 1px 6px;
+  border-radius: 4px;
+  background: color-mix(in srgb, var(--crit) 12%, transparent);
+  font-size: 10.5px;
+  color: var(--crit);
+  white-space: nowrap;
+  cursor: help;
+}
+.ch-rest {
+  font-size: 10.5px;
+  color: var(--faint);
+}
+/* On a blocked hop this chip says "set to", not "now": a pick nothing crossed.
+   It steps back to faint so it can't be mistaken for a live reading. */
+.ch-hop--blocked .ch-now {
+  color: var(--faint);
+}
+.ch-now {
+  max-width: 28ch;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+  font-size: 10.5px;
+  color: var(--dim);
+}
+
 @media (max-width: 640px) {
   .tg-sec-hd {
     flex-wrap: wrap;
@@ -1060,6 +1296,20 @@
   .gh-now {
     max-width: 100%;
   }
+  /* On a phone the age stamp stops being pushed to a lonely right edge and just
+     joins the end of the hop's line; the selected node gets the full width
+     instead of an ellipsis it doesn't need. */
+  .ch-age {
+    margin-left: 0;
+  }
+  .ch-now {
+    max-width: 100%;
+  }
+  /* Every field of a hop wraps onto its own line at this width, so the gap
+     between hops has to grow with them or the rail reads as one block of text. */
+  .ch-body {
+    padding-bottom: 12px;
+  }
   .gh-mems {
     max-height: 260px;
   }

panel/src/pages/Targets.tsx

@@ -22,6 +22,8 @@ import type {
   GroupTestResult,
   GroupTestStatus,
   Chain,
+  ChainHealth,
+  ChainHopHealth,
   Egress,
   Interface,
   Node,
@@ -436,14 +438,14 @@ const normalizeTest = (st: GroupTestStatus): GroupTestStatus => ({
 })
 
 /**
- * How the header names the reach of a running exit test. The name matters more
+ * How the header names the reach of a running refresh pass. The name matters more
  * than the number when there is only one: "auto" tells the operator which button
  * they pressed; "1 target" tells them nothing they didn't already know.
  */
 function scopeLabel(scope: string[], targetCount: number): string {
   if (scope.length === 1) return scope[0]
-  if (scope.length === 0) return 'exits' // pre-scope daemon — say nothing false
-  return scope.length >= targetCount ? 'every exit' : `${scope.length} exits`
+  if (scope.length === 0) return 'targets' // pre-scope daemon — say nothing false
+  return scope.length >= targetCount ? 'every target' : `${scope.length} targets`
 }
 
 /** Which editor (add or edit-by-name) is open within a section. */
@@ -590,10 +592,12 @@ export default function Targets() {
     [health],
   )
 
-  // ---- group/chain exit test: how fast, through which node, out which address --
-  // The POST only kicks a run off, and a 2 s poll of the GET carries progress
-  // plus every result so far. One endpoint covers groups and chains alike:
-  // POST with a group or chain name tests that one; an empty name tests them all.
+  // ---- out-of-turn refresh: how fast, through which node, out which address ----
+  // The POST does NOT dial. It asks the observatory — the only thing in the daemon
+  // that measures anything, and it measures along the real dial path — to come
+  // round out of turn; a 2 s poll of the GET carries progress plus every reading
+  // so far. One endpoint covers groups and chains alike: POST with a name refreshes
+  // that one, an empty name refreshes them all.
   const [gtest, setGtest] = useState<GroupTestStatus>(IDLE_TEST)
   const [gtestErr, setGtestErr] = useState<string | null>(null)
   const [polling, setPolling] = useState(false)
@@ -636,7 +640,7 @@ export default function Targets() {
       void readTest().then((st) => {
         if (!alive || !st || st.running) return
         setPolling(false)
-        flash('Group test complete')
+        flash('Readings refreshed')
       })
     }, 2000)
     return () => {
@@ -652,16 +656,16 @@ export default function Targets() {
         if (r.started) {
           setGtestErr(null)
           setPolling(true)
-          flash(name ? `Testing ${name}…` : 'Testing every exit…')
+          flash(name ? `Refreshing ${name}…` : 'Refreshing every reading…')
           void readTest()
         } else if (r.reason === 'already running') {
-          setPolling(true) // pick up the run someone else started
-          flash('A group test is already running')
+          setPolling(true) // pick up the pass someone else started
+          flash('The prober is already refreshing')
         } else {
-          flash(`Couldn’t start the test — ${r.reason || 'the daemon refused it'}`)
+          flash(`Couldn’t ask for a refresh — ${r.reason || 'the daemon refused it'}`)
         }
       } catch (e) {
-        flash(`Couldn’t start the test — ${errText(e)}`)
+        flash(`Couldn’t ask for a refresh — ${errText(e)}`)
       }
     },
     [flash, readTest],
@@ -910,10 +914,11 @@ export default function Targets() {
           <h2 className="tg-sec-title">Groups</h2>
           <span className="tg-sec-count mono">{groups.length} configured</span>
           {/* The observatory's background probing is invisible by design — it
-              keeps every used group's and chain's numbers fresh on its own. The
-              one manual run left is the exit test: it is scoped to the groups
-              and chains it names, so its progress says WHICH, and its badge
-              lands only on those cards. */}
+              keeps every used group's and chain's numbers fresh on its own, along
+              the path traffic actually takes. The one manual control left does
+              not measure anything itself: it asks that prober to come round out
+              of turn. It is scoped to the groups and chains it names, so its
+              progress says WHICH, and its badge lands only on those cards. */}
           <div className="tg-sec-ctl">
             {groupHealthOn && (
              <>
@@ -921,11 +926,11 @@ export default function Targets() {
               <span
                 className="tg-run tg-run--exit"
                 role="status"
-                title="An exit test sends one connection through each group or chain it covers and reports the delay and the address the internet sees."
+                title="The background prober is measuring the targets this refresh covers, along the path each one's traffic really takes."
               >
                 <Led variant="amber" pulse />
                 <span className="tg-run-what">
-                  exit test · {scopeLabel(asArray(gtest.scope), groups.length + chains.length)}
+                  refreshing · {scopeLabel(asArray(gtest.scope), groups.length + chains.length)}
                 </span>
                 <span className="tg-run-n mono">
                   {gtest.done}/{gtest.total}
@@ -935,9 +940,9 @@ export default function Targets() {
             <Button
               onClick={() => void runTest()}
               disabled={busy || !config || (groups.length === 0 && chains.length === 0) || gtest.running}
-              title="Send one connection through each group and chain and report the delay and the exit address the internet sees"
+              title="Ask the background prober to measure every group and chain out of turn, then show what it measured. The panel opens no connection of its own."
             >
-              {gtest.running ? 'Testing…' : 'Test every exit'}
+              {gtest.running ? 'Refreshing…' : 'Refresh every reading'}
             </Button>
              </>
             )}
@@ -957,6 +962,13 @@ export default function Targets() {
           dials out through a tunnel measures them through that tunnel, so the same node can be alive
           in one group and dead in another.
         </p>
+        <p className="tg-sec-note">
+          One thing measures, and the panel is not it. A background prober walks every path your
+          rules use — hop by hop, exactly as traffic goes — and every number on this page is a read
+          of what it found. <strong>Refresh every reading</strong> asks it to come round out of turn
+          instead of waiting for the next pass; it opens no connection of its own, so a target no
+          rule routes through has nothing to report and says so.
+        </p>
 
         {groupHealthOn && healthErr && (
           <p className="tg-test-err" role="alert">
@@ -967,7 +979,7 @@ export default function Targets() {
 
         {groupHealthOn && gtestErr && (
           <p className="tg-test-err" role="alert">
-            Couldn’t read the test results — {gtestErr}.{' '}
+            Couldn’t read the refreshed numbers — {gtestErr}.{' '}
             <button className="linkish" onClick={() => void readTest()}>
               Retry
             </button>
@@ -1110,7 +1122,9 @@ export default function Targets() {
                   chain={c}
                   busy={busy}
                   showHealth={groupHealthOn}
-                  used={healthByChain.get(c.Name)?.used}
+                  // The whole chain health record, not just `.used` — the card
+                  // renders the observatory's per-hop measurements from it.
+                  health={healthByChain.get(c.Name)}
                   test={testByGroup.get(c.Name)}
                   // The badge is this card's business only when the run names it.
                   testing={gtest.running && testScope.has(c.Name)}
@@ -1232,7 +1246,7 @@ function GroupRow({
   group: Group
   busy: boolean
   /** Group health checks are on (Settings). When false, the card drops its health
-   *  readout, its exit-test readout and its Test button — it is config only. */
+   *  readout, its end-to-end reading and its Refresh button — it is config only. */
   showHealth: boolean
   /** This group's membership health, or undefined when the engine hasn't built
    *  it (not applied yet, or dropped for having no usable members). */
@@ -1242,14 +1256,14 @@ function GroupRow({
   healthKnown: boolean
   test?: GroupTestResult
   /**
-   * A group exit test covering THIS group is in flight.
+   * A refresh pass covering THIS group is in flight.
    *
    * Deliberately not "a test is running": the caller resolves it against the run's
    * scope. There is no per-card equivalent for the health run — that one measures
    * every group at once and is reported once, in the section header.
    */
   testing: boolean
-  /** Any exit test is in flight; the daemon runs one at a time. */
+  /** Any refresh pass is in flight; the daemon runs one at a time. */
   testBusy: boolean
   onTest: () => void
   onEdit: () => void
@@ -1309,7 +1323,11 @@ function GroupRow({
               health={health}
               healthKnown={healthKnown}
             />
-            <GroupTestReadout test={test} pending={testing && !test} />
+            <GroupTestReadout
+              test={test}
+              pending={testing && !test}
+              hideAbsence={health?.used === false}
+            />
           </>
         )}
       </div>
@@ -1320,7 +1338,7 @@ function GroupRow({
         editLabel={`Edit group ${group.Name}`}
         deleteLabel={`Delete group ${group.Name}`}
         onTest={showHealth ? onTest : undefined}
-        testLabel={showHealth ? `Test the exit of group ${group.Name}` : undefined}
+        testLabel={showHealth ? `Refresh the reading for group ${group.Name}` : undefined}
         testDisabled={testBusy}
       />
     </li>
@@ -1379,21 +1397,7 @@ function GroupHealthReadout({
   // its members would stay "untested" forever. That is a fact about the ROUTING
   // CONFIG, not about the members — so instead of counters that could only ever
   // read as a permanent unknown, the card says so, quietly: unused, not unwell.
-  if (!health.used) {
-    return (
-      <div className="gh gh--unused">
-        <div className="gh-line">
-          <span
-            className="gh-unused"
-            title="No enabled rule routes through this group, so its members are not probed. Add it to a rule to see health."
-          >
-            unused
-          </span>
-          <span className="gh-quiet">not probed — no enabled rule routes through this group</span>
-        </div>
-      </div>
-    )
-  }
+  if (!health.used) return <NotRoutedNote kind="group" />
 
   const v = verdictOf(health)
   const { total, tested, alive, dead, untested } = health
@@ -1556,6 +1560,56 @@ function GroupHealthReadout({
   )
 }
 
+/**
+ * The card's answer when NOTHING ROUTES THROUGH THIS TARGET. Shared by the group
+ * card and the chain card, because it is the same misunderstanding on both.
+ *
+ * It has to carry two statements, and the old one-liner ("not probed — no enabled
+ * rule routes through this group") only carried the first. Read fast it still
+ * landed as a verdict: a card that normally shows health and today shows a grey
+ * pill reads as "the health is bad". So the two meanings are now separated, on
+ * purpose and in this order:
+ *
+ *   1. the ROUTING FACT — nothing routes here, so nothing measures it;
+ *   2. the NON-FACT — this is not a health reading at all. Absent numbers are
+ *      absence of measurement, never failure.
+ *
+ * For a GROUP there is a third line, and it is the confusion this whole change
+ * exists to end: a group used only as a hop inside a chain is never routed to
+ * DIRECTLY, so it correctly reads unused here while carrying real traffic as a
+ * hop. Its health is measured at that hop, on the chain's card.
+ *
+ * Unused is neutral — groove-grey, never amber, never crit. It is a state of the
+ * config, and the config is not sick.
+ */
+function NotRoutedNote({ kind }: { kind: 'group' | 'chain' }) {
+  return (
+    <div className="gh gh--unused">
+      <div className="gh-line">
+        <span className="gh-unused">unused</span>
+        <span className="gh-quiet">
+          No enabled rule routes through this {kind}, so the observatory never probes it.
+        </span>
+      </div>
+      <p className="gh-say">
+        That is a routing fact, not a health reading. There are no numbers here because nothing
+        measured this {kind} — not because it failed.
+      </p>
+      {kind === 'group' ? (
+        <p className="gh-say">
+          A group used only as a hop inside a chain reads unused here on purpose: the rules point at
+          the chain, not at the group. Its members are measured at that hop, so its real health is on
+          that chain’s card, hop by hop.
+        </p>
+      ) : (
+        <p className="gh-say">
+          Point a rule at this chain and the observatory starts measuring every hop within seconds.
+        </p>
+      )}
+    </div>
+  )
+}
+
 /**
  * One group's member rows, fetched on demand.
  *
@@ -1661,7 +1715,9 @@ function MemberRow({ member }: { member: GroupMemberHealth }) {
 }
 
 /**
- * What a group test found, in the four states it actually has.
+ * What the OBSERVATORY measured for this target end to end, in the four states it
+ * actually has. Nothing here was dialled by the panel — it is a read of the
+ * background prober's own measurement along the real path.
  *
  * The one worth spelling out: `ok` with an EMPTY `exit_ip` is a SUCCESS. The
  * delay was measured; only the address lookup came back empty. Rendering that as
@@ -1669,15 +1725,45 @@ function MemberRow({ member }: { member: GroupMemberHealth }) {
  * traffic, so it reads as a result with the address slot marked unknown — dim,
  * not red, and the LED stays green.
  */
-function GroupTestReadout({ test, pending }: { test?: GroupTestResult; pending: boolean }) {
+/**
+ * Errors that mean NO MEASUREMENT EXISTS, as opposed to "this target is broken".
+ *
+ * Three of the observatory's four failure reasons are about the observatory, not
+ * about the path: nothing routes here, nothing has reached it yet, or background
+ * probing is switched off. Painting those crit-red — which is what `ok:false`
+ * used to buy you — reports a fault that nobody has found, on a target that may
+ * be carrying traffic perfectly. Only "the observatory's probe through this path
+ * failed" is a health finding, and it is deliberately NOT in this list.
+ *
+ * Matched on a stable fragment rather than the whole sentence, so a daemon that
+ * rewords the tail still classifies. An error we don't recognise stays red: an
+ * unknown failure is likelier to be real than not, and that is the safe default.
+ */
+const NO_MEASUREMENT = [
+  'not routed by any enabled rule',
+  'has not reached this target yet',
+  'background probing is disabled',
+]
+const isAbsence = (err: string): boolean => NO_MEASUREMENT.some((frag) => err.includes(frag))
+
+function GroupTestReadout({
+  test,
+  pending,
+  hideAbsence,
+}: {
+  test?: GroupTestResult
+  pending: boolean
+  /** The card already explains why nothing measures this target (the unused
+   *  note), so an absence error here would just say it a second time. */
+  hideAbsence?: boolean
+}) {
   if (pending) {
-    // "testing", never "measuring": the health run owns that word and covers every
-    // group at once. Two runs that read the same on a card is how one group's test
-    // came to look like all four were busy.
+    // Names who is working and on what: the prober, on this target. The badge is
+    // scoped to the cards the run covers, so it can say "this one" honestly.
     return (
       <div className="tg-test tg-test--wait" role="status">
         <Led variant="amber" pulse />
-        <span className="tg-test-msg">testing this exit…</span>
+        <span className="tg-test-msg">waiting for the prober to measure this…</span>
       </div>
     )
   }
@@ -1686,10 +1772,22 @@ function GroupTestReadout({ test, pending }: { test?: GroupTestResult; pending:
   const at = test.tested_unix ? fmtClock(test.tested_unix) : ''
 
   if (!test.ok) {
+    // No measurement exists. Unlit lamp, quiet text: this panel's way of saying
+    // "no verdict", which is precisely the state — never a red one.
+    if (isAbsence(test.error)) {
+      if (hideAbsence) return null
+      return (
+        <div className="tg-test tg-test--none" role="status">
+          <Led variant="off" />
+          <span className="tg-test-msg">{test.error}</span>
+          {at && <span className="tg-test-at mono">{at}</span>}
+        </div>
+      )
+    }
     return (
       <div className="tg-test tg-test--bad" role="status">
         <Led variant="crit" />
-        <span className="tg-test-msg">{test.error || 'the test failed'}</span>
+        <span className="tg-test-msg">{test.error || 'the probe failed'}</span>
         {at && <span className="tg-test-at mono">{at}</span>}
       </div>
     )
@@ -2114,7 +2212,7 @@ function ChainRow({
   chain,
   busy,
   showHealth,
-  used,
+  health,
   test,
   testing,
   testBusy,
@@ -2125,19 +2223,19 @@ function ChainRow({
   chain: Chain
   busy: boolean
   /** Group health checks are on (Settings). When false, the card drops its
-   *  exit-test readout and Test button — it is config only. */
+   *  health readout and Refresh button — it is config only. */
   showHealth: boolean
-  /** This chain's reachability (GroupHealth.Used's chain analogue, plan §5.E).
-   *  undefined ⇒ the health endpoint hasn't reported this chain (not applied yet, or
-   *  a daemon version without chains): no badge. false ⇒ no enabled rule routes
-   *  through the chain, so the observatory never probes it and the card renders
-   *  "unused" instead of an exit-test readout. */
-  used?: boolean
+  /** Everything the observatory knows about this chain: whether any enabled rule
+   *  routes through it, and the per-hop measurements along it.
+   *  undefined ⇒ the health endpoint hasn't reported this chain at all (not
+   *  applied yet, or a daemon version without chains): the card says nothing
+   *  rather than guessing. */
+  health?: ChainHealth
   test?: GroupTestResult
-  /** An exit test covering THIS chain is in flight (the caller resolves it
+  /** A refresh pass covering THIS chain is in flight (the caller resolves it
    *  against the run's scope, exactly as for a group card). */
   testing: boolean
-  /** Any exit test is in flight; the daemon runs one at a time. */
+  /** Any refresh pass is in flight; the daemon runs one at a time. */
   testBusy: boolean
   onTest: () => void
   onEdit: () => void
@@ -2181,25 +2279,21 @@ function ChainRow({
         {showHealth && (
           <>
             {/* A chain no enabled rule routes through is never probed (the
-                observatory walks only reachable paths), so instead of an exit-test
-                readout the card says so, quietly — the same "unused" pattern the
-                group card uses (GroupHealthReadout), not a new design. `used` is
-                undefined until the health endpoint reports this chain (or from a
-                daemon version without chains): no badge then. */}
-            {used === false && (
-              <div className="gh gh--unused">
-                <div className="gh-line">
-                  <span
-                    className="gh-unused"
-                    title="No enabled rule routes through this chain, so its exit is not probed. Add it to a rule to see health."
-                  >
-                    unused
-                  </span>
-                  <span className="gh-quiet">not probed — no enabled rule routes through this chain</span>
-                </div>
-              </div>
-            )}
-            <GroupTestReadout test={test} pending={testing && !test} />
+                observatory walks only reachable paths), so instead of a health
+                readout the card says so — the same "unused" note the group card
+                uses, not a new design. `health` is undefined until the endpoint
+                reports this chain (or on a daemon without chains): say nothing
+                then rather than guess. */}
+            {health?.used === false ? (
+              <NotRoutedNote kind="chain" />
+            ) : health?.used ? (
+              <ChainHopRail chain={chain.Name} defs={hops} hops={health.hops} />
+            ) : null}
+            <GroupTestReadout
+              test={test}
+              pending={testing && !test}
+              hideAbsence={health?.used === false}
+            />
           </>
         )}
       </div>
@@ -2210,13 +2304,250 @@ function ChainRow({
         editLabel={`Edit chain ${chain.Name}`}
         deleteLabel={`Delete chain ${chain.Name}`}
         onTest={showHealth ? onTest : undefined}
-        testLabel={showHealth ? `Test the exit of chain ${chain.Name}` : undefined}
+        testLabel={showHealth ? `Refresh the reading for chain ${chain.Name}` : undefined}
         testDisabled={testBusy}
       />
     </li>
   )
 }
 
+// ---- chain hop rail --------------------------------------------------------
+
+/**
+ * The API gives hops an index and no name. The page already knows the names — the
+ * model's own `Hops` strings ("egress:ewan", "node:awgout", "group:sub0") — so
+ * zip the two by POSITION.
+ *
+ * Two things make that safe rather than clever. A LEADING `egress:` is not a
+ * numbered hop: the daemon lifts it into hop 1's entry detour, so it is dropped
+ * before counting. And if the counts still disagree — a chain that splices
+ * sub-chains gets FLATTENED by the daemon, producing more wire hops than the
+ * config lists — every label is dropped. A hop labelled with its neighbour's name
+ * is worse than a hop with no name at all: it would send someone to fix the wrong
+ * target.
+ */
+function hopLabels(defs: string[], hops: ChainHopHealth[]): (string | undefined)[] {
+  const numbered = defs.length > 0 && defs[0].startsWith('egress:') ? defs.slice(1) : defs
+  if (numbered.length !== hops.length) return hops.map(() => undefined)
+  return hops.map((h) => (h.index >= 1 && h.index <= numbered.length ? numbered[h.index - 1] : undefined))
+}
+
+/**
+ * One hop's lamp.
+ *
+ * `dead` is crit and `untested` is an UNLIT socket — never red, because nothing
+ * has been measured and an unlit lamp is this panel's way of saying "no verdict".
+ * That covers a hop the walk never reached (`blocked_by`) too: it is neither
+ * healthy nor broken, and unlit is the only mark that claims neither.
+ * The fourth case is the page's existing house reading, applied here for
+ * consistency rather than invented: a group hop that is carrying traffic but has
+ * confirmed failures on its board is amber. `state` stays the daemon's word for
+ * "can this hop carry traffic"; the amber only qualifies HOW WELL.
+ */
+function hopLed(h: ChainHopHealth): LedVariant {
+  if (h.state === 'dead') return 'crit'
+  if (h.state === 'untested') return 'off'
+  return h.dead > 0 ? 'amber' : 'on'
+}
+
+/**
+ * What the observatory measured at each position of a chain — the reading the
+ * daemon always took and the panel never showed.
+ *
+ * THE DESIGN RISK, and the one place this card spends any boldness: the rail
+ * draws the CONDUCTOR as well as the lamps, and severs it below the first dead
+ * hop. A chain is a single series path, so the operator's real question is never
+ * "how many hops are green" — it is "where does my traffic stop". Four lamps in a
+ * column answer the first question and leave the second to arithmetic. A broken
+ * conductor answers the second one before you have read a single word, which is
+ * the whole reason this feature exists.
+ *
+ * It stays honest by keeping two different facts on two different marks. The
+ * CONDUCTOR is reachability through the path, and that genuinely does stop at the
+ * break. The LAMPS are measurements — and there are none below the break to show:
+ * the daemon walks the path in order and stops at the first hop that does not
+ * answer, because every later hop is dialled THROUGH that one. Those hops arrive
+ * `untested` with `blocked_by` naming the hop that stopped the walk, so their
+ * lamps stay UNLIT: not a soft red, not a pale green, just this panel's way of
+ * saying no verdict exists about a hop nobody reached. The row says so in words
+ * too, naming that hop, because "why is this row empty" is the question the shape
+ * alone cannot answer.
+ *
+ * Nothing here is re-derived from the daemon's counters — the block, the zeroed
+ * numbers and the state all come off the wire. The only thing the panel adds is
+ * what a chain structurally is.
+ *
+ * Everything around the rail is deliberately quiet: no colour but the semantic
+ * lamps, no motion at all, the orange accent untouched.
+ */
+function ChainHopRail({
+  chain,
+  defs,
+  hops,
+}: {
+  chain: string
+  /** The chain's configured hops, straight off the model — the only source of names. */
+  defs: string[]
+  /** Absent ⇒ the engine never materialised per-hop outbounds. NOT "no hops". */
+  hops?: ChainHopHealth[]
+}) {
+  const ordered = useMemo(() => [...asArray(hops)].sort((a, b) => a.index - b.index), [hops])
+  const labels = useMemo(() => hopLabels(defs, ordered), [defs, ordered])
+
+  // The first hop that was probed and did not answer. Everything after it is
+  // unreachable THROUGH THIS CHAIN, whatever its own lamp says. `untested` is
+  // never a break: nothing was measured, so nothing is known to be severed.
+  const breakAt = ordered.findIndex((h) => h.state === 'dead')
+
+  if (ordered.length === 0) {
+    // Say why, in one line, instead of an empty rail. The daemon collapses a
+    // single-target chain into a plain alias and never builds copies to measure,
+    // so we can tell the two absences apart from the config alone.
+    const numbered = defs.filter((d, i) => !(i === 0 && d.startsWith('egress:')))
+    return (
+      <div className="gh gh--absent">
+        <span className="gh-absent-msg">
+          {numbered.length <= 1
+            ? 'This chain has a single hop, so the engine points traffic straight at that target instead of building a path to measure. Its health is on that target’s own card.'
+            : 'The engine hasn’t built this chain’s hops yet, so there is nothing measured per hop. They appear once it is running with this config applied.'}
+        </span>
+      </div>
+    )
+  }
+
+  return (
+    <div className="gh ch-rail">
+      <span className="ch-eyebrow">measured, hop by hop</span>
+      <ol className="ch-hops">
+        {ordered.map((h, i) => {
+          const label = labels[i]
+          const severed = breakAt >= 0 && i > breakAt
+          // Straight off the wire: present ⇒ the walk never reached this hop, so
+          // there is nothing measured here and the daemon has already named the
+          // hop that stopped it. Never inferred from the counters.
+          const blocked = h.blocked_by
+          const cls = [
+            'ch-hop',
+            `ch-hop--${h.state}`,
+            blocked ? 'ch-hop--blocked' : '',
+            severed ? 'ch-hop--severed' : '',
+            h.exit ? 'ch-hop--exit' : '',
+            i === 0 ? 'ch-hop--first' : '',
+          ]
+            .filter(Boolean)
+            .join(' ')
+          const age = fmtAge(h.age_seconds)
+          return (
+            <li key={h.tag || h.index} className={cls}>
+              <span className="ch-num mono" aria-hidden="true">
+                {h.index}
+              </span>
+              <span className="ch-socket">
+                <Led variant={hopLed(h)} />
+              </span>
+              <div className="ch-body">
+                <div className="ch-l1">
+                  <span className="ch-name mono" title={`engine outbound ${h.tag}`}>
+                    {label ?? (h.kind === 'group' ? 'a group hop' : 'a node hop')}
+                  </span>
+                  {h.exit && <span className="ch-tag">exit</span>}
+                  {h.state === 'alive' && h.delay_ms > 0 && (
+                    <span className="ch-delay mono">{h.delay_ms} ms</span>
+                  )}
+                  {/* Two different silences. A plain untested hop is a timing
+                      gap that fills in by itself; a BLOCKED one never will,
+                      because the walk stopped above it — so it says which hop
+                      stopped it instead of implying someone should wait. */}
+                  {blocked ? (
+                    <span
+                      className="ch-quiet ch-blocked"
+                      title={`hop ${blocked.index} did not answer, so nothing was dialled through it (engine outbound ${blocked.tag})`}
+                    >
+                      no reading — the probe stopped at hop {blocked.index}
+                    </span>
+                  ) : (
+                    h.state === 'untested' && <span className="ch-quiet">not measured yet</span>
+                  )}
+                  {age && <span className="ch-age mono">{age}</span>}
+                </div>
+
+                {/* A node hop IS its own measurement (total 1), so counters would
+                    only restate the lamp. A group hop rolls up its per-hop member
+                    copies, and those read exactly as they do everywhere else in
+                    this app: alive out of TESTED, with the untested remainder as a
+                    quiet aside only when there is one. A blocked hop has those
+                    counters zeroed by the daemon, so it lands in the tested === 0
+                    branch — and there it must say the members were never REACHED,
+                    not that they are still waiting their turn. */}
+                {h.kind === 'group' && h.total > 0 && (
+                  <div className="ch-l2">
+                    {h.tested === 0 ? (
+                      <span className="ch-rest mono">
+                        {h.total} member{h.total === 1 ? '' : 's'},{' '}
+                        {blocked ? 'none of them reached' : 'none measured'}
+                      </span>
+                    ) : (
+                      <>
+                        <span className="ch-count mono">
+                          <b>{h.alive}</b> / {h.tested}
+                        </span>
+                        <span className="ch-word">alive</span>
+                        {h.dead > 0 && (
+                          <span
+                            className="ch-dead mono"
+                            title={`${h.dead} member${h.dead === 1 ? '' : 's'} were probed at this hop and did not answer`}
+                          >
+                            {h.dead} not answering
+                          </span>
+                        )}
+                        {h.untested > 0 && (
+                          <span className="ch-rest mono">
+                            tested {h.tested} of {h.total}
+                          </span>
+                        )}
+                      </>
+                    )}
+                    {/* The wrapper keeps its pick even when nothing crossed it,
+                        so on a blocked hop this is the node it WOULD use — say
+                        that, rather than "now", which claims live traffic. */}
+                    {h.selected && (
+                      <span
+                        className="ch-now mono"
+                        title={
+                          blocked
+                            ? `Hop ${h.index} of “${chain}” is set to ${h.selected}; nothing crossed it to measure`
+                            : `Traffic crossing hop ${h.index} of “${chain}” is on ${h.selected}`
+                        }
+                      >
+                        {blocked ? 'set to' : 'now'} → {h.selected}
+                      </span>
+                    )}
+                  </div>
+                )}
+              </div>
+            </li>
+          )
+        })}
+      </ol>
+
+      {/* The sentence the rail's shape implies, written out — because the break is
+          the answer someone came here for, and a graphic alone should never be the
+          only place a finding exists. It names the dead hop, since that is the one
+          thing here anybody can act on. */}
+      {breakAt >= 0 && (
+        <p className="gh-say gh-say--bad">
+          Hop {ordered[breakAt].index}
+          {labels[breakAt] ? ` (${labels[breakAt]})` : ''} was probed and did not answer, so traffic
+          stops there
+          {breakAt < ordered.length - 1
+            ? ' — and the hops below it are dialled through it, so nothing reached them and nothing is known about them.'
+            : '.'}
+        </p>
+      )}
+    </div>
+  )
+}
+
 function ChainEditor({
   initial,
   hopOptions,
@@ -2691,8 +3022,8 @@ function RowActions({
   busy: boolean
   editLabel: string
   deleteLabel: string
-  // Only groups and chains can be tested, so the control is optional and absent
-  // everywhere else rather than a disabled stub on every row.
+  // Only groups and chains are probed, so the refresh control is optional and
+  // absent everywhere else rather than a disabled stub on every row.
   onTest?: () => void
   testLabel?: string
   testDisabled?: boolean
@@ -2705,8 +3036,9 @@ function RowActions({
           onClick={onTest}
           disabled={busy || testDisabled}
           aria-label={testLabel}
+          title="Ask the background prober to measure this target out of turn. It does not open a connection from the panel."
         >
-          Test
+          Refresh
         </Button>
       )}
       <Button className="tg-act" onClick={onEdit} disabled={busy} aria-label={editLabel}>

protocol/group/urltest.go

@@ -44,6 +44,20 @@ type URLTest struct {
 	group                        *URLTestGroup
 	interruptExternalConnections bool
 	balancer                     *balancer // lx: SPEC 019 — nil for least_test (default)
+	// lx: health board §5.C — true when options.SelfCheck == false: the group's
+	// OWN probing schedule (PostStart warm-up + Touch ticker) is stood down and
+	// the observatory is the only thing that measures its members. Stored
+	// INVERTED so the zero value keeps today's behaviour for every construction
+	// path that does not go through NewURLTest (hand-built groups in tests).
+	// See option.URLTestOutboundOptions.SelfCheck for the full reasoning.
+	selfCheckDisabled bool
+	// lx: health board §5.C — the RUNTIME half of the same question, read from
+	// the context registry at construction. selfCheckDisabled above says "this
+	// group is not used by the config"; this says "this group cannot be reached
+	// right now", which changes while the box runs and is therefore asked
+	// afresh at every scheduled check rather than stored. nil = no gate.
+	// See urltest.ProbeGate for why the two must stay separate.
+	probeGate urltest.ProbeGate
 }
 
 func NewURLTest(ctx context.Context, router adapter.Router, logger log.ContextLogger, tag string, options option.URLTestOutboundOptions) (adapter.Outbound, error) {
@@ -71,6 +85,12 @@ func NewURLTest(ctx context.Context, router adapter.Router, logger log.ContextLo
 		idleTimeout:                  time.Duration(options.IdleTimeout),
 		interruptExternalConnections: options.InterruptExistConnections,
 		balancer:                     balancer,
+		// nil/absent means true (self-check on) — the documented default, so a
+		// config written before the flag existed behaves exactly as it always has.
+		selfCheckDisabled: options.SelfCheck != nil && !*options.SelfCheck,
+		// Absent from the registry (plain sing-box, tests) yields nil, which
+		// means "no gate" — every scheduled probe proceeds, as before.
+		probeGate: service.FromContext[urltest.ProbeGate](ctx),
 	}
 	if len(outbound.tags) == 0 {
 		return nil, E.New("missing tags")
@@ -92,6 +112,13 @@ func (s *URLTest) Start() error {
 		return err
 	}
 	group.balancer = s.balancer // lx: SPEC 019 v2 — health-check drives the pool through it
+	// lx: health board §5.C — carry the stand-down flag onto the group the same
+	// way the balancer travels: set after construction, immutable from then on.
+	group.selfCheckDisabled = s.selfCheckDisabled
+	// The gate and the tag to ask it about travel together: the gate answers
+	// per-outbound, and the group is the thing whose schedule is being gated.
+	group.probeGate = s.probeGate
+	group.tag = s.Tag()
 	if s.balancer != nil {
 		// lx: health board §5.B — slot liveness reads through the board verdict, so a
 		// death recorded by any prober or a failed dial takes effect on the next pick,
@@ -319,6 +346,80 @@ type URLTestGroup struct {
 	lastActive                   common.TypedValue[time.Time]
 	lastSelected                 common.TypedValue[string] // lx: SPEC 019 — Now() in balanced modes
 	balancer                     *balancer                 // lx: SPEC 019 v2 — round_robin pool; nil for least_test
+	// lx: health board §5.C — mirrors URLTest.selfCheckDisabled (set by Start,
+	// immutable afterwards, zero value = probing on). Guards ONLY the group's
+	// own schedule: the PostStart warm-up sweep and the Touch ticker. An
+	// explicit CheckOutbounds/URLTest call is untouched — the flag stands down
+	// the schedule, not the capability.
+	selfCheckDisabled bool
+	// lx: health board §5.C — the runtime gate and the tag it is asked about.
+	// Both mirror URLTest's fields (set by Start, immutable afterwards); nil
+	// gate or empty tag means every scheduled check proceeds. Consulted only
+	// through selfCheckAllowed, and only on the SCHEDULE.
+	probeGate urltest.ProbeGate
+	tag       string
+}
+
+// selfCheckAllowed reports whether the group's OWN probing schedule may dial
+// right now. lx: health board §5.C.
+//
+// Two independent refusals, in the order they can be answered cheapest first:
+//
+//	selfCheckDisabled — the config says no rule reaches this group. Fixed for
+//	                    the life of the box; see standDownUnusedSelfCheck.
+//	probeGate         — the world says this group cannot be reached right now,
+//	                    typically a chain hop sitting behind a dead hop. Asked
+//	                    fresh EVERY time, which is the entire mechanism by which
+//	                    a recovered hop resumes probing: there is no state here
+//	                    to reset, so there is none to get stuck.
+//
+// Neither refusal touches an explicit CheckOutbounds/URLTest — a deliberate
+// request is never a scheduled one.
+func (g *URLTestGroup) selfCheckAllowed() bool {
+	if g.selfCheckDisabled {
+		return false
+	}
+	if g.probeGate == nil || g.tag == "" {
+		return true
+	}
+	return g.probeGate.ProbeAllowed(g.tag)
+}
+
+// scheduledCheck is one firing of the group's own schedule — the warm-up sweep
+// and every ticker tick go through here, and nothing else does. Having exactly
+// one gated entry point is what keeps the two callers from drifting apart, and
+// it is the seam the tests drive to assert that a gated group makes no dial
+// attempt at all.
+func (g *URLTestGroup) scheduledCheck() {
+	if !g.selfCheckAllowed() {
+		return
+	}
+	g.CheckOutbounds(false)
+}
+
+// keepWarm reports whether this group must keep measuring with no traffic
+// flowing through it. lx: health board §5.C — see urltest.ProbeGate.ProbeWhenIdle.
+//
+// The default is NO, in every direction: no gate, no tag, or a group whose
+// self-check is stood down anyway. Only a gate that positively says "the routing
+// config reaches this group" turns the idle timeout off, so plain sing-box and
+// every hand-built group keep the lifecycle they have always had.
+func (g *URLTestGroup) keepWarm() bool {
+	if g.selfCheckDisabled || g.probeGate == nil || g.tag == "" {
+		return false
+	}
+	return g.probeGate.ProbeWhenIdle(g.tag)
+}
+
+// startTickerLocked arms the group's own probing ticker. g.access MUST be held
+// and g.ticker MUST be nil. Extracted so PostStart and Touch arm it identically
+// — two ways in, one construction, no chance of one of them forgetting the pause
+// registration.
+func (g *URLTestGroup) startTickerLocked() {
+	ticker := time.NewTicker(g.interval)
+	g.ticker = ticker
+	g.pauseCallback = pause.RegisterTicker(g.pause, ticker, g.interval, nil)
+	go g.loopCheck(ticker, g.close)
 }
 
 func NewURLTestGroup(ctx context.Context, outboundManager adapter.OutboundManager, logger log.Logger, outbounds []adapter.Outbound, link string, interval time.Duration, tolerance uint16, idleTimeout time.Duration, interruptExternalConnections bool) (*URLTestGroup, error) {
@@ -362,24 +463,67 @@ func (g *URLTestGroup) PostStart() {
 	g.lastActive.Store(time.Now())
 	// lx: SPEC 019 v2 — seed the pool so round_robin can route from the first connection,
 	// before the first health-check completes (history-warm nodes first, else config order).
+	// The seed only READS the board, so it runs even with the self-check stood down.
 	g.seedPool()
-	go g.CheckOutbounds(false)
+	// lx: health board §5.C — the warm-up sweep is the first half of the group's
+	// own probing schedule, and it fires for EVERY group at box start, including
+	// groups no routing rule reaches. For those, the sweep dials every member
+	// directly from the router — a path nothing uses — and records the outcome
+	// under the members' base tags, forging the board reading the observatory
+	// exists to keep honest. A stood-down group therefore skips it entirely; the
+	// observatory (or nothing, for a truly unused group) is what measures its
+	// members.
+	//
+	// The same call is now also where a chain hop behind a DEAD hop declines to
+	// sweep: every member of such a group dials through the broken hop, so the
+	// sweep would measure that hop once per member and file the result against
+	// this one. selfCheckAllowed keeps both refusals in one place.
+	go g.scheduledCheck()
+	// A group the routing config REACHES keeps measuring whether or not anybody
+	// dials it, so its ticker is armed here instead of waiting for a Touch that
+	// may never come. Without this, a used group with no traffic gets this one
+	// warm-up sweep and then nothing: its members age past the verdict TTL and
+	// the panel reports "untested" about a rule that is in force, while the first
+	// real request pays a cold probe. Nothing else would fill the gap — the
+	// observatory stands off a urltest group's members entirely (probeplan.go
+	// SelfChecked), which is the whole point of one dialler per target.
+	//
+	// lastActive was stored a moment ago, so loopCheck's opening "idle longer
+	// than the interval" check does not fire and this cannot double up with the
+	// sweep above.
+	if g.keepWarm() && g.ticker == nil {
+		g.startTickerLocked()
+	}
 }
 
 func (g *URLTestGroup) Touch() {
 	if !g.started {
 		return
 	}
+	// lx: health board §5.C — Touch's only job is to keep the group's OWN
+	// probing ticker alive while traffic flows. With the self-check stood down
+	// there is deliberately no ticker to start or feed: the observatory owns the
+	// schedule, and a stray dial through an unused group (a stale rule cache, a
+	// manual pin) must not arm 30 minutes of direct probing under the members'
+	// base tags. Checked before the lock because the flag is immutable after
+	// Start, exactly like the started fast-path above.
+	//
+	// The runtime gate is deliberately NOT consulted here. Touch only arms the
+	// ticker; refusing to arm it would mean a hop that recovers has no ticker
+	// left to notice — the block would outlive the failure, which is the one
+	// outcome this must never have. The ticker runs and each tick re-asks the
+	// gate (loopCheck -> scheduledCheck), so a blocked hop costs a predicate
+	// call per interval and resumes the moment the hop in front answers.
+	if g.selfCheckDisabled {
+		return
+	}
 	g.access.Lock()
 	defer g.access.Unlock()
 	if g.ticker != nil {
 		g.lastActive.Store(time.Now())
 		return
 	}
-	ticker := time.NewTicker(g.interval)
-	g.ticker = ticker
-	g.pauseCallback = pause.RegisterTicker(g.pause, ticker, g.interval, nil)
-	go g.loopCheck(ticker, g.close)
+	g.startTickerLocked()
 }
 
 func (g *URLTestGroup) Close() error {
@@ -405,10 +549,15 @@ func (g *URLTestGroup) Select(network string) (adapter.Outbound, bool) {
 	return g.selectExcluding(network, nil)
 }
 
+// loopCheck is the group's own schedule. lx: health board §5.C — every probe it
+// fires goes through scheduledCheck, so a stood-down or currently-unreachable
+// group ticks without dialling. The ticker's LIFECYCLE (the idle timeout below)
+// is deliberately left alone: a gated group keeps its ticker exactly as long as
+// an ungated one would, because the ticker is what will notice the recovery.
 func (g *URLTestGroup) loopCheck(ticker *time.Ticker, closeChan <-chan struct{}) {
 	if time.Since(g.lastActive.Load()) > g.interval {
 		g.lastActive.Store(time.Now())
-		g.CheckOutbounds(false)
+		g.scheduledCheck()
 	}
 	for {
 		select {
@@ -416,7 +565,13 @@ func (g *URLTestGroup) loopCheck(ticker *time.Ticker, closeChan <-chan struct{})
 			return
 		case <-ticker.C:
 		}
-		if time.Since(g.lastActive.Load()) > g.idleTimeout {
+		// The idle timeout retires the ticker of a group nobody is dialling —
+		// unless the routing config reaches it, in which case its health is a
+		// live question whether or not traffic is flowing and the ticker must
+		// outlive the silence. Asked here rather than remembered from PostStart
+		// so it tracks the running config, and asked OUTSIDE g.access because the
+		// answer comes from the engine, which has locks of its own.
+		if !g.keepWarm() && time.Since(g.lastActive.Load()) > g.idleTimeout {
 			g.access.Lock()
 			if g.ticker == ticker {
 				g.ticker.Stop()
@@ -427,7 +582,7 @@ func (g *URLTestGroup) loopCheck(ticker *time.Ticker, closeChan <-chan struct{})
 			g.access.Unlock()
 			return
 		}
-		g.CheckOutbounds(false)
+		g.scheduledCheck()
 	}
 }
 

protocol/group/urltest_health_lx_test.go

@@ -7,6 +7,7 @@ import (
 	"context"
 	"errors"
 	"net"
+	"sync"
 	"testing"
 	"time"
 
@@ -32,10 +33,31 @@ type healthNode struct {
 func (n *healthNode) Tag() string       { return n.tag }
 func (n *healthNode) Network() []string { return []string{N.NetworkTCP, N.NetworkUDP} }
 
-func (n *healthNode) DialContext(ctx context.Context, network string, destination M.Socksaddr) (net.Conn, error) {
-	if n.dialed != nil {
-		*n.dialed = append(*n.dialed, n.tag)
+// healthDialMu guards the shared dialed slice. testNodes probes a group's
+// members CONCURRENTLY (a batch of 10), so two nodes pointing at one slice
+// append from two goroutines; the append is what the -race build trips on, not
+// the code under test.
+var healthDialMu sync.Mutex
+
+func (n *healthNode) record() {
+	if n.dialed == nil {
+		return
 	}
+	healthDialMu.Lock()
+	*n.dialed = append(*n.dialed, n.tag)
+	healthDialMu.Unlock()
+}
+
+// dialsOf reads a dial log under the same lock. Every assertion on a log a
+// concurrent sweep may still be writing must go through it.
+func dialsOf(dialed *[]string) []string {
+	healthDialMu.Lock()
+	defer healthDialMu.Unlock()
+	return append([]string(nil), *dialed...)
+}
+
+func (n *healthNode) DialContext(ctx context.Context, network string, destination M.Socksaddr) (net.Conn, error) {
+	n.record()
 	if n.fail {
 		return nil, errors.New("dial refused")
 	}
@@ -45,9 +67,7 @@ func (n *healthNode) DialContext(ctx context.Context, network string, destinatio
 }
 
 func (n *healthNode) ListenPacket(ctx context.Context, destination M.Socksaddr) (net.PacketConn, error) {
-	if n.dialed != nil {
-		*n.dialed = append(*n.dialed, n.tag)
-	}
+	n.record()
 	if n.fail {
 		return nil, errors.New("listen refused")
 	}

protocol/group/urltest_selfcheck_lx_test.go

@@ -0,0 +1,272 @@
+package group
+
+// lx: health board §5.C tests — SelfCheck stands the group's OWN probing
+// schedule down: no PostStart warm-up sweep, no Touch ticker. The explicit
+// CheckOutbounds path stays available, and the nil default keeps probing.
+
+import (
+	"context"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/sagernet/sing-box/common/urltest"
+	"github.com/sagernet/sing-box/log"
+	"github.com/sagernet/sing-box/option"
+	"github.com/sagernet/sing/service/pause"
+)
+
+// waitForHistory polls until the store holds an entry for tag or the deadline
+// passes; reports whether it appeared. PostStart's sweep runs on its own
+// goroutine, so both directions of the assertion need a bounded wait.
+func waitForHistory(hist *urltest.HistoryStorage, tag string, deadline time.Duration) bool {
+	stop := time.Now().Add(deadline)
+	for time.Now().Before(stop) {
+		if hist.LoadURLTestHistory(tag) != nil {
+			return true
+		}
+		time.Sleep(5 * time.Millisecond)
+	}
+	return false
+}
+
+// A group with the self-check stood down writes NOTHING to the history storage
+// on PostStart: the warm-up sweep — which would dial the member directly from
+// the router and mark the failure under its base tag — must not fire. And
+// Touch, the other half of the schedule, must not start a ticker either.
+func TestSelfCheckDisabledPostStartWritesNothing(t *testing.T) {
+	hist := urltest.NewHistoryStorage()
+	a := &healthNode{tag: "a", fail: true}
+	manager := managerOf(a)
+	g := healthTestGroup(hist, manager, a)
+	g.selfCheckDisabled = true
+
+	g.PostStart()
+	// The absence of a write is the assertion, so give the (non-existent) sweep
+	// real time to have happened before declaring victory.
+	if waitForHistory(hist, "a", 150*time.Millisecond) {
+		t.Fatal("a stood-down group's PostStart wrote to the board; the warm-up sweep must not fire")
+	}
+
+	g.Touch()
+	g.access.Lock()
+	ticker := g.ticker
+	g.access.Unlock()
+	if ticker != nil {
+		t.Fatal("Touch armed the probing ticker on a stood-down group")
+	}
+}
+
+// The default (SelfCheck nil, i.e. the zero-value field on a hand-built group)
+// keeps today's behaviour: PostStart's warm-up sweep runs and records the
+// failing member on the board.
+func TestSelfCheckDefaultStillProbesOnPostStart(t *testing.T) {
+	hist := urltest.NewHistoryStorage()
+	a := &healthNode{tag: "a", fail: true}
+	manager := managerOf(a)
+	g := healthTestGroup(hist, manager, a)
+
+	g.PostStart()
+	if !waitForHistory(hist, "a", 5*time.Second) {
+		t.Fatal("default group's PostStart never probed; the self-check must stay on unless stood down")
+	}
+	if v := hist.Verdict("a", 10*time.Minute); v != urltest.VerdictDead {
+		t.Fatalf("verdict(a) = %v, want dead from the warm-up sweep", v)
+	}
+}
+
+// An EXPLICIT CheckOutbounds still probes a stood-down group: the flag
+// suppresses the group's own schedule, never a deliberate request (the adapter
+// interface a human or an API invokes on purpose).
+func TestSelfCheckDisabledExplicitCheckStillProbes(t *testing.T) {
+	hist := urltest.NewHistoryStorage()
+	a := &healthNode{tag: "a", fail: true}
+	manager := managerOf(a)
+	g := healthTestGroup(hist, manager, a)
+	g.selfCheckDisabled = true
+
+	g.CheckOutbounds(true)
+	if hist.LoadURLTestHistory("a") == nil {
+		t.Fatal("an explicit CheckOutbounds(true) did not probe; the flag must only stand down the schedule")
+	}
+}
+
+// The option → outbound plumbing: nil/absent means on, an explicit false means
+// stood down, an explicit true means on. NewURLTest is the only place the
+// option is read, so this is where a plumbing regression would hide.
+func TestSelfCheckOptionPlumbing(t *testing.T) {
+	build := func(selfCheck *bool) *URLTest {
+		t.Helper()
+		opts := option.URLTestOutboundOptions{Outbounds: []string{"a"}}
+		opts.SelfCheck = selfCheck
+		ob, err := NewURLTest(context.Background(), nil, log.NewNOPFactory().Logger(), "t", opts)
+		if err != nil {
+			t.Fatalf("NewURLTest: %v", err)
+		}
+		return ob.(*URLTest)
+	}
+	if build(nil).selfCheckDisabled {
+		t.Fatal("nil SelfCheck must keep the self-check ON (the compatibility default)")
+	}
+	on, off := true, false
+	if build(&on).selfCheckDisabled {
+		t.Fatal("SelfCheck=true must keep the self-check on")
+	}
+	if !build(&off).selfCheckDisabled {
+		t.Fatal("SelfCheck=false must stand the self-check down")
+	}
+}
+
+// lx: health board §5.C — the RUNTIME gate (urltest.ProbeGate). The self-check
+// flag above says "the config reaches nothing here"; the gate says "the path in
+// front of this group is down right now". Both stand the SCHEDULE down; neither
+// touches an explicit check; and only the gate is allowed to change its mind
+// while the box runs.
+
+// fakeGate answers from a mutable set of blocked tags, so one test can watch a
+// group stop dialling and start again without rebuilding anything.
+type fakeGate struct {
+	mu      sync.Mutex
+	blocked map[string]bool
+	warm    map[string]bool
+	asked   int
+}
+
+func (g *fakeGate) ProbeAllowed(tag string) bool {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	g.asked++
+	return !g.blocked[tag]
+}
+
+func (g *fakeGate) ProbeWhenIdle(tag string) bool {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	return g.warm[tag]
+}
+
+func (g *fakeGate) set(tag string, blocked bool) {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	g.blocked[tag] = blocked
+}
+
+func (g *fakeGate) asks() int {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	return g.asked
+}
+
+// A gated group makes NO DIAL ATTEMPT on its own schedule — the assertion is on
+// the attempt log, not on the board, because a probe that ran and failed leaves
+// the same "nothing useful known" as one that never ran, and only the attempt
+// log tells them apart. This is the waste half of the chain-hop fix: a hop
+// sitting behind a dead hop would otherwise spend one probe timeout per member
+// rediscovering the same broken hop.
+func TestProbeGateBlocksScheduledDials(t *testing.T) {
+	hist := urltest.NewHistoryStorage()
+	defer hist.Close()
+	var dialed []string
+	a := &healthNode{tag: "chain-c-h3-a", fail: true, dialed: &dialed}
+	b := &healthNode{tag: "chain-c-h3-b", fail: true, dialed: &dialed}
+	gate := &fakeGate{blocked: map[string]bool{"chain-c-h3": true}}
+
+	g := healthTestGroup(hist, managerOf(a, b), a, b)
+	g.tag = "chain-c-h3"
+	g.probeGate = gate
+	// Touch arms a real ticker, so this group needs the two things
+	// healthTestGroup leaves out because nothing else in that suite starts one:
+	// a pause manager to register the ticker with, and the close channel Close
+	// shuts the loop down through.
+	g.pause = pause.ManagerFromContext(pause.WithDefaultManager(context.Background()))
+	g.close = make(chan struct{})
+
+	// The warm-up sweep: gated, so nothing is dialled. Give the (non-existent)
+	// sweep real time to have happened — the absence is the assertion.
+	g.PostStart()
+	if waitForHistory(hist, "chain-c-h3-a", 150*time.Millisecond) {
+		t.Fatal("a gated group's PostStart wrote to the board")
+	}
+	// A ticker tick, driven directly: this is the exact call loopCheck makes.
+	g.scheduledCheck()
+	if got := dialsOf(&dialed); len(got) != 0 {
+		t.Fatalf("gated group dialled %v; a hop behind a dead hop must not dial at all", got)
+	}
+
+	// Touch still arms the ticker. Refusing to arm it would leave a recovered
+	// hop with nothing to notice — the block would outlive the failure.
+	g.Touch()
+	g.access.Lock()
+	ticker := g.ticker
+	g.access.Unlock()
+	if ticker == nil {
+		t.Fatal("Touch did not arm the ticker on a gated group; nothing would be left to spot the recovery")
+	}
+	_ = g.Close()
+
+	// The hop in front comes back. Nothing is reset, nothing is reapplied — the
+	// next scheduled check simply asks again and gets a different answer.
+	gate.set("chain-c-h3", false)
+	before := gate.asks()
+	g.scheduledCheck()
+	if gate.asks() <= before {
+		t.Error("scheduledCheck did not re-ask the gate; a cached answer is a block that outlives its cause")
+	}
+	if len(dialsOf(&dialed)) == 0 {
+		t.Fatal("the group did not resume dialling after the hop in front recovered")
+	}
+}
+
+// An EXPLICIT check is a deliberate request and is never gated — the same rule
+// SelfCheck already follows. The gate stands down the schedule, not the
+// capability.
+func TestProbeGateDoesNotBlockExplicitCheck(t *testing.T) {
+	hist := urltest.NewHistoryStorage()
+	defer hist.Close()
+	var dialed []string
+	a := &healthNode{tag: "chain-c-h3-a", fail: true, dialed: &dialed}
+	g := healthTestGroup(hist, managerOf(a), a)
+	g.tag = "chain-c-h3"
+	g.probeGate = &fakeGate{blocked: map[string]bool{"chain-c-h3": true}}
+
+	g.CheckOutbounds(true)
+	if len(dialsOf(&dialed)) == 0 {
+		t.Fatal("an explicit CheckOutbounds was refused by the gate")
+	}
+}
+
+// The two refusals are independent and compose the obvious way; and the absent
+// cases (no gate at all, an ungated tag) leave today's behaviour untouched,
+// which is what every plain sing-box config and every hand-built group relies
+// on.
+func TestSelfCheckAllowedCombinations(t *testing.T) {
+	hist := urltest.NewHistoryStorage()
+	defer hist.Close()
+	a := &healthNode{tag: "a"}
+	base := func() *URLTestGroup { return healthTestGroup(hist, managerOf(a), a) }
+
+	if g := base(); !g.selfCheckAllowed() {
+		t.Error("a plain group with no gate must probe (the zero value is the compatibility default)")
+	}
+	g := base()
+	g.selfCheckDisabled = true
+	g.tag, g.probeGate = "chain-c-h3", &fakeGate{blocked: map[string]bool{}}
+	if g.selfCheckAllowed() {
+		t.Error("an UNUSED group must stay down even when the path in front is fine")
+	}
+	g = base()
+	g.tag, g.probeGate = "chain-c-h3", &fakeGate{blocked: map[string]bool{"chain-c-h3": true}}
+	if g.selfCheckAllowed() {
+		t.Error("a group behind a dead hop must not run its schedule")
+	}
+	g = base()
+	g.tag, g.probeGate = "auto", &fakeGate{blocked: map[string]bool{"chain-c-h3": true}}
+	if !g.selfCheckAllowed() {
+		t.Error("an unrelated group was gated by another tag's block")
+	}
+	g = base()
+	g.probeGate = &fakeGate{blocked: map[string]bool{"": true}}
+	if !g.selfCheckAllowed() {
+		t.Error("a group with no tag must not be gated; there is nothing to ask about")
+	}
+}

shater/apply/apply.go

@@ -205,11 +205,15 @@ func (a *Applier) configureObservatory(m *model.Model, opts option.Options) {
 	})
 }
 
-// TestGroups launches the engine's one-shot exit test (delay + exit address, F2)
-// of the named groups/chains, returning started=false when a run is already in
-// flight or the engine is absent. names empty/nil = every group and every chain.
-// It takes NEITHER the apply mutex nor the flock, so kicking off a test never
-// blocks behind an Apply.
+// TestGroups launches the engine's one-shot group/chain test (F2): the engine
+// asks its observatory for an out-of-turn pass and reports what it measured,
+// plus the exit address for alive rule-routed targets — it no longer dials
+// health probes of its own. Returns started=false when a run is already in
+// flight or the engine is absent. names empty/nil = every group and every
+// chain. probeURL is passed through for signature stability and IGNORED by the
+// engine (the probe URL is a global observatory setting now). It takes NEITHER
+// the apply mutex nor the flock, so kicking off a test never blocks behind an
+// Apply.
 func (a *Applier) TestGroups(names []string, probeURL string) (started bool) {
 	if a.eng == nil {
 		return false

shater/engine/engine.go

@@ -159,7 +159,21 @@ func New(logger ...log.ContextLogger) *Engine {
 	// URLTestHistory() below. The pointer is stable across Apply swaps, so health
 	// history survives config changes instead of being reset on every apply.
 	ctx = service.ContextWithPtr(ctx, urltest.NewHistoryStorage())
-	return &Engine{ctx: ctx, log: l}
+	// Pre-register the engine itself as the probe GATE, into the same shared
+	// registry and for the same reason as the two above: every urltest group
+	// built by every box.New reads it out of ctx (protocol/group/urltest.go), and
+	// the registration must survive Apply swaps because the question it answers
+	// outlives any single box.
+	//
+	// The gate answers "may this outbound run its own scheduled probe right
+	// now" — see Engine.ProbeAllowed. It is a live call, not a stored flag, so
+	// a chain hop that comes back resumes probing with no reapply. Registering
+	// the Engine here (rather than a snapshot) is what makes that possible;
+	// the struct is built first purely so ctx can point at it.
+	e := &Engine{log: l}
+	ctx = service.ContextWith[urltest.ProbeGate](ctx, e)
+	e.ctx = ctx
+	return e
 }
 
 // Apply installs opts as the running configuration.
@@ -193,6 +207,24 @@ func (e *Engine) Apply(opts option.Options) (changed bool, err error) {
 }
 
 func (e *Engine) applyLocked(opts option.Options) (bool, error) {
+	// Stand down the self-check of urltest groups no rule reaches (see
+	// selfcheck.go for the whole argument). This MUTATES opts in place — the
+	// option structs are pointers behind an `any` — and it must run BEFORE the
+	// hash below, so the hash describes the config that is really built: a rule
+	// change that flips a group used<->unused is then a real change that
+	// triggers a swap, and an unchanged config hashes identically on every
+	// reconcile because the stand-down is deterministic.
+	stood, used := standDownUnusedSelfCheck(opts)
+	if stood > 0 && e.log != nil {
+		e.log.Info("apply: stood down self-check on ", stood, " unused urltest group(s); the observatory is their only prober")
+	}
+	// Publish the same walk's used-set for ProbeWhenIdle BEFORE the new box is
+	// built, because the question is asked during that box's PostStart: a used
+	// group arms its probing ticker there rather than waiting for traffic. The
+	// observatory's own copy is published later, after a SUCCESSFUL swap, which
+	// is too late to be the answer.
+	e.publishKeepWarm(used)
+
 	newHash, err := e.hashOptions(opts)
 	if err != nil {
 		return false, E.Cause(err, "hash options")

shater/engine/grouphealth.go

@@ -1,6 +1,7 @@
 package engine
 
 import (
+	"sort"
 	"strings"
 
 	"github.com/sagernet/sing-box/adapter"
@@ -320,11 +321,15 @@ func parseGroupCopyTag(group, tag string) (member string, ok bool) {
 	return member, true
 }
 
-// ChainHealth is one configured chain's reachability, mirroring GroupHealth.Used
-// for the chain card (plan §5.E): a chain no enabled rule routes through is never
-// probed — the observatory walks only reachable paths — and the panel renders it
-// "unused" rather than as a health problem. A chain has no membership counters: it
-// is a fixed path, and its end-to-end health is the exit test's job, not a roll-up.
+// ChainHealth is one configured chain's reachability plus its PER-HOP health,
+// mirroring GroupHealth.Used for the chain card (plan §5.E): a chain no enabled
+// rule routes through is never probed — the observatory walks only reachable
+// paths — and the panel renders it "unused" rather than as a health problem.
+// A chain has no membership counters of its own: it is a fixed path, and its
+// end-to-end health is the exit probe's job. What it DOES have is hops, and
+// since the observatory now probes every hop wrapper (probeplan.go walkDetour),
+// each hop's health is on the board and is projected here so an operator can
+// see WHICH hop died instead of only that the chain did.
 type ChainHealth struct {
 	// Name is the chain's model name (config chain "chain:<name>"), what the Targets
 	// page lists and what a rule targets.
@@ -336,34 +341,284 @@ type ChainHealth struct {
 	// when the observatory is disabled or not yet configured: no badge is better
 	// than a wrong one (the same rule as GroupHealth.Used).
 	Used bool `json:"used"`
+	// Hops is the per-hop health readout, L1..Ln in wire order (see ChainHopHealth).
+	// It is EMPTY for a chain the running box never materialised: a chain no rule
+	// references is resolved lazily and never built, and a 1-hop chain without an
+	// egress entry resolves straight to its target with no wrapper — in both cases
+	// there are no "chain-<name>-h…" outbounds to project. An absent "hops" key
+	// therefore means "nothing materialised to report on", NEVER "this chain has
+	// no hops" — the model, not this projection, knows how many hops were
+	// configured.
+	Hops []ChainHopHealth `json:"hops,omitempty"`
 }
 
-// ChainHealth reports the reachability (used/unused) of every named chain, one row
-// per name, in the order given. It is the chain analogue of GroupHealth.Used: a
-// chain the observatory's used-set does not cover is reported Used=false so the
-// panel can mark it "unused" instead of running an exit test against a path nothing
-// routes through.
+// ChainHopHealth is one hop of one materialised chain, as the health board saw
+// it — a projection, like everything in this file: nothing here dials.
+//
+// A NODE hop is a single measurement: the observatory dials the hop wrapper
+// "chain-<name>-h<i>", which pulls exactly the path prefix up to and including
+// this hop, so Total=1, the counters follow the hop's own state, DelayMs and
+// AgeSeconds are its own observation, and Selected is "" (a fixed hop selects
+// nothing).
+//
+// A GROUP hop rolls up its member copies "chain-<name>-h<i>-<member>", each of
+// which the observatory probes through its own prefix of the chain. The
+// counters obey the same invariants as GroupHealth — Tested == Alive+Dead and
+// Alive+Dead+Untested == Total — so the panel needs no arithmetic of its own.
+// State summarises them: "alive" when at least one member is alive (the hop can
+// carry traffic), "dead" when at least one was tested and none is alive (a
+// positive finding of a dead hop), "untested" when nothing was tested. Selected
+// is the node NAME the wrapper currently picks; DelayMs/AgeSeconds are the
+// SELECTED member's observation, or the freshest ALIVE member's when the
+// selection has no measurement of its own — the number shown must always be a
+// measurement somebody took, never an average nobody did.
+//
+// # The hop nobody reached: BlockedBy
+//
+// The prober walks a chain in wire order and stops at the first dead hop
+// (observatory.go probeChainOrdered), because a probe of hop 3 dials THROUGH
+// hop 2 and a hop 2 with no live member makes that probe a measurement of hop 2.
+// Everything behind the break is therefore not dialled at all, and this struct
+// says so: State is untested — no fresh knowledge, which is the truth — the
+// counters collapse into Untested, and BlockedBy names the hop that stopped the
+// walk. A row like that must never be read as a fault of its own; the fault is
+// at BlockedBy.Index, and that is the hop to go and fix.
+type ChainHopHealth struct {
+	Index      int    `json:"index"`       // 1-based position on the wire, L1..Ln
+	Tag        string `json:"tag"`         // "chain-<name>-h<i>" — the wrapper actually dialled
+	Kind       string `json:"kind"`        // "node" | "group"
+	Exit       bool   `json:"exit"`        // the LAST hop: where traffic leaves to the internet
+	State      string `json:"state"`       // "alive" | "dead" | "untested"
+	DelayMs    int    `json:"delay_ms"`
+	AgeSeconds int64  `json:"age_seconds"` // -1 when unknown
+	Selected   string `json:"selected"`    // group hop: the node NAME it currently selects; "" otherwise
+	Total      int    `json:"total"`
+	Tested     int    `json:"tested"`
+	Alive      int    `json:"alive"`
+	Dead       int    `json:"dead"`
+	Untested   int    `json:"untested"`
+	// BlockedBy is set ONLY on a hop the ordered walk never reached, and names the
+	// dead hop in front of it. Absent (omitted, never null) on every hop that was
+	// itself measured. When it is present State is always "untested" and the
+	// counters are always 0/0/0/Total — see the type comment.
+	BlockedBy *ChainHopBlock `json:"blocked_by,omitempty"`
+}
+
+// ChainHopBlock identifies the hop whose failure stopped a chain's ordered probe
+// walk: its 1-based wire index and the engine outbound tag that was dialled.
+//
+// The index is the field that matters — it is what the operator acts on, and it
+// lines up with ChainHopHealth.Index on the same chain, so a reader can point
+// straight at the offending row. The tag is diagnostic detail (logs, tooltips)
+// and is not a label to put in front of a person: "chain-ewan-wg-subs-h2" is not
+// a name anybody chose.
+type ChainHopBlock struct {
+	Index int    `json:"index"`
+	Tag   string `json:"tag"`
+}
+
+// ChainHealth reports the reachability (used/unused) of every named chain plus
+// the per-hop health of each one the running box materialised, one row per
+// name, in the order given. The Used half is the chain analogue of
+// GroupHealth.Used: a chain the observatory's used-set does not cover is
+// reported Used=false so the panel can mark it "unused" instead of a health
+// readout against a path nothing routes through.
 //
 // names come from the desired-state model, NOT the running box: a chain no rule
 // references is never materialised (generate/chain.go resolveChain is lazy), so it
 // is invisible to a box-only enumeration — yet the panel lists it from the config
-// and must be able to badge it. The engine supplies the only fact a box read can
-// add here, the observatory's published used-set. Pure apart from that read; nil
-// names or a stopped engine (nil used-set) yield an empty/used-everything result.
+// and must be able to badge it. The engine supplies what only it can: the
+// observatory's published used-set, and the running box's outbound/endpoint
+// pool the hop projection reads. Still no dialling anywhere; nil names or a
+// stopped engine (nil used-set, empty pool) yield an empty/used-everything
+// result with no hops.
 func (e *Engine) ChainHealth(names []string) []ChainHealth {
 	out := make([]ChainHealth, 0, len(names))
 	used := e.observatoryUsed()
 	usedChains := usedChainNames(used)
+	pool := e.runningPool()
+	view := e.HealthView()
 	for _, name := range names {
 		name = strings.TrimSpace(name)
 		if name == "" {
 			continue
 		}
-		out = append(out, ChainHealth{Name: name, Used: used == nil || usedChains[name]})
+		out = append(out, ChainHealth{
+			Name: name,
+			Used: used == nil || usedChains[name],
+			Hops: chainHopHealthOf(pool, name, view),
+		})
 	}
 	return out
 }
 
+// runningPool snapshots the running box's outbounds AND endpoints into one
+// list. The endpoints matter: a chain hop rebuilt from a wireguard/AmneziaWG
+// node is an ENDPOINT copy, invisible in Outbounds() — the same trap
+// groupTargets documents — and a hop projection that missed it would silently
+// drop the very hop this feature exists to localise (the production chain's
+// first hop IS an AWG endpoint). Empty (never nil-unsafe) on a stopped engine.
+func (e *Engine) runningPool() []adapter.Outbound {
+	var pool []adapter.Outbound
+	inst := e.Instance()
+	if inst == nil {
+		return pool
+	}
+	if om := inst.Outbound(); om != nil {
+		pool = append(pool, om.Outbounds()...)
+	}
+	if em := inst.Endpoint(); em != nil {
+		for _, ep := range em.Endpoints() {
+			pool = append(pool, ep)
+		}
+	}
+	return pool
+}
+
+// chainHopHealthOf projects one chain's hop wrappers out of an outbound pool
+// against one health view. Pure apart from the view reads — the same
+// unit-testing contract as groupHealthOf: hand it a fake pool and a hand-built
+// store and every branch is reachable without a box.
+//
+// A pool entry belongs to chain <name> when its tag is exactly
+// "chain-<name>-h<digits>" — the hop WRAPPER the observatory dials. Member
+// copies ("chain-<name>-h<i>-<member>") are not hops themselves; they are
+// reached through the wrapper's own member list (adapter.OutboundGroup.All), so
+// the roll-up sees exactly what the wrapper can select, in its order.
+func chainHopHealthOf(pool []adapter.Outbound, name string, view HealthView) []ChainHopHealth {
+	prefix := "chain-" + name + "-h"
+	var hops []ChainHopHealth
+	for _, ob := range pool {
+		tag := ob.Tag()
+		rest, ok := strings.CutPrefix(tag, prefix)
+		if !ok {
+			continue
+		}
+		idx, ok := parseAllDigits(rest)
+		if !ok {
+			continue // a member copy, or another chain sharing the prefix
+		}
+		if g, isGroup := ob.(adapter.OutboundGroup); isGroup {
+			hops = append(hops, chainGroupHop(g, name, idx, view))
+		} else {
+			hops = append(hops, chainNodeHop(tag, idx, view))
+		}
+	}
+	sort.Slice(hops, func(i, j int) bool { return hops[i].Index < hops[j].Index })
+	if len(hops) > 0 {
+		// The largest index is the exit — the wrapper whose probe leaves to the
+		// internet. Marked after sorting so the flag cannot depend on pool order.
+		hops[len(hops)-1].Exit = true
+	}
+	markBlockedHops(hops)
+	return hops
+}
+
+// markBlockedHops applies the ordered walk's outcome to the projection: from the
+// first hop that reads DEAD, every later hop is rewritten as not-reached.
+//
+// This is a rewrite and not an annotation on purpose. The prober stops at the
+// first dead hop, so a hop behind the break has not been dialled since the break
+// appeared — but its board records do not vanish, they age out on the TTL. For
+// as long as that takes, the raw projection would keep publishing a verdict about
+// a hop nothing has attempted, and the most damaging version of that is a stale
+// "dead" pointing the operator at a hop that may be perfectly fine. Untested is
+// the honest reading of "nothing current is known", and BlockedBy is why.
+//
+// The counters collapse the same way, into Untested, so the invariants the whole
+// health surface promises still hold verbatim: Tested == Alive+Dead and
+// Alive+Dead+Untested == Total.
+//
+// Selected SURVIVES. It is not a measurement — it is the member the wrapper would
+// route through — and it stays true (and useful) whether or not anything reached
+// this hop.
+func markBlockedHops(hops []ChainHopHealth) {
+	for i := range hops {
+		if hops[i].State != HealthDead {
+			continue
+		}
+		blocker := &ChainHopBlock{Index: hops[i].Index, Tag: hops[i].Tag}
+		for j := i + 1; j < len(hops); j++ {
+			h := &hops[j]
+			h.State = HealthUntested
+			h.DelayMs, h.AgeSeconds = 0, -1
+			h.Alive, h.Dead, h.Tested = 0, 0, 0
+			h.Untested = h.Total
+			h.BlockedBy = blocker
+		}
+		return // the first break is the only one that can be observed
+	}
+}
+
+// chainNodeHop is the one-measurement hop: the wrapper itself was dialled by
+// the observatory, so its own board state IS the hop's health and the counters
+// degenerate to whichever bucket that state fills.
+func chainNodeHop(tag string, idx int, view HealthView) ChainHopHealth {
+	hop := ChainHopHealth{Index: idx, Tag: tag, Kind: "node", Total: 1}
+	hop.State, hop.DelayMs, hop.AgeSeconds = view.State(tag)
+	switch hop.State {
+	case HealthAlive:
+		hop.Alive = 1
+	case HealthDead:
+		hop.Dead = 1
+	default:
+		hop.Untested = 1
+	}
+	hop.Tested = hop.Alive + hop.Dead
+	return hop
+}
+
+// chainGroupHop rolls a group hop up over its member copies. The counters carry
+// the GroupHealth invariants; the summary State answers the only question a hop
+// row asks — "can this hop carry the chain": alive while anything answers,
+// dead only on a positive all-tested-dead finding, untested when nothing is
+// known (never dead-by-absence, the same honesty rule as everywhere else).
+func chainGroupHop(g adapter.OutboundGroup, chain string, idx int, view HealthView) ChainHopHealth {
+	hop := ChainHopHealth{Index: idx, Tag: g.Tag(), Kind: "group", AgeSeconds: -1}
+	selected := g.Now()
+	if selected != "" {
+		hop.Selected = chainMemberName(selected, chain)
+	}
+
+	// The number a hop row shows must be a real observation: the selected
+	// member's when it has one, else the freshest alive member's.
+	freshDelay, freshAge := 0, int64(-1)
+	selDelay, selAge := 0, int64(-1)
+	for _, tag := range g.All() {
+		state, delayMs, age := view.State(tag)
+		switch state {
+		case HealthAlive:
+			hop.Alive++
+			if age >= 0 && (freshAge < 0 || age < freshAge) {
+				freshDelay, freshAge = delayMs, age
+			}
+		case HealthDead:
+			hop.Dead++
+		default:
+			hop.Untested++
+		}
+		hop.Total++
+		if tag == selected && age >= 0 {
+			selDelay, selAge = delayMs, age
+		}
+	}
+	hop.Tested = hop.Alive + hop.Dead
+	switch {
+	case hop.Alive > 0:
+		hop.State = HealthAlive
+	case hop.Tested > 0:
+		hop.State = HealthDead
+	default:
+		hop.State = HealthUntested
+	}
+	if selAge >= 0 {
+		hop.DelayMs, hop.AgeSeconds = selDelay, selAge
+	} else {
+		hop.DelayMs, hop.AgeSeconds = freshDelay, freshAge
+	}
+	return hop
+}
+
 // usedChainNames recovers the set of chain NAMES the observatory's used-set covers.
 //
 // The used-set is keyed by outbound TAG (the generator's schema, materialised in

shater/engine/grouphealth_test.go

@@ -309,8 +309,167 @@ func TestChainHealthNilUsedSet(t *testing.T) {
 		t.Fatalf("ChainHealth = %+v, want %+v (nil used-set ⇒ all used, blanks dropped)", got, want)
 	}
 	for i, w := range want {
-		if got[i] != w {
+		if got[i].Name != w.Name || got[i].Used != w.Used {
 			t.Errorf("ChainHealth[%d] = %+v, want %+v", i, got[i], w)
 		}
+		// A stopped engine materialised nothing: hops must be empty, and the
+		// contract says empty means "nothing materialised", not "no hops".
+		if len(got[i].Hops) != 0 {
+			t.Errorf("ChainHealth[%d].Hops = %+v, want empty on a stopped engine", i, got[i].Hops)
+		}
+	}
+}
+
+// TestChainHopHealthProjection drives the per-hop readout over a 3-hop chain —
+// node hop L1, group hop L2, group hop L3 (the exit) — with exactly ONE hop's
+// members dead. The dead state must land on THAT hop's index and nowhere else,
+// the counters must obey the GroupHealth invariants on every hop, and the exit
+// flag must sit on the largest index. This is the "which hop died" question the
+// whole hop surface exists to answer.
+//
+// L3 additionally pins the ordered walk's half of the contract: it sits BEHIND
+// the dead L2, so whatever its stale board records say, it is reported untested
+// with L2 named — never as a hop of its own with a verdict nobody took.
+func TestChainHopHealthProjection(t *testing.T) {
+	hist := urltest.NewHistoryStorage()
+	now := time.Now()
+
+	// L1 (node hop wrapper): alive — the prefix up to hop 1 works.
+	hist.StoreURLTestHistory("chain-c-h1", &adapter.URLTestHistory{LastOK: now.Add(-5 * time.Second), Delay: 40})
+	// L2 (group hop): BOTH member copies dead — this is the hop that died.
+	hist.StoreURLTestHistory("chain-c-h2-x", &adapter.URLTestHistory{LastFail: now.Add(-3 * time.Second)})
+	hist.StoreURLTestHistory("chain-c-h2-y", &adapter.URLTestHistory{LastFail: now.Add(-2 * time.Second)})
+	// L3 (group hop, exit): one member alive, one never measured.
+	hist.StoreURLTestHistory("chain-c-h3-a", &adapter.URLTestHistory{LastOK: now.Add(-7 * time.Second), Delay: 200})
+	// chain-c-h3-b: nothing at all.
+
+	pool := []adapter.Outbound{
+		&depOutbound{failingOutbound{tag: "chain-c-h1"}, nil},
+		&depGroup{fakeGroup{
+			tag: "chain-c-h2", kind: C.TypeSelector,
+			all: []string{"chain-c-h2-x", "chain-c-h2-y"},
+			now: "chain-c-h2-x",
+		}, []string{"chain-c-h2-x", "chain-c-h2-y"}},
+		&depGroup{fakeGroup{
+			tag: "chain-c-h3", kind: C.TypeSelector,
+			all: []string{"chain-c-h3-a", "chain-c-h3-b"},
+			now: "chain-c-h3-a",
+		}, []string{"chain-c-h3-a", "chain-c-h3-b"}},
+		// Noise the projection must ignore: a member copy is not a hop, another
+		// chain's wrapper is not this chain's.
+		&depOutbound{failingOutbound{tag: "chain-c-h2-x"}, nil},
+		&depOutbound{failingOutbound{tag: "chain-other-h1"}, nil},
+	}
+
+	hops := chainHopHealthOf(pool, "c", newHealthView(hist, healthTTLFloor, now))
+	if len(hops) != 3 {
+		t.Fatalf("got %d hops, want 3: %+v", len(hops), hops)
+	}
+
+	// Ordered by index, exit on the largest.
+	for i, wantIdx := range []int{1, 2, 3} {
+		if hops[i].Index != wantIdx {
+			t.Fatalf("hops out of order: %+v", hops)
+		}
+		if got, want := hops[i].Exit, wantIdx == 3; got != want {
+			t.Errorf("hop %d Exit = %v, want %v", wantIdx, got, want)
+		}
+	}
+
+	h1, h2, h3 := hops[0], hops[1], hops[2]
+
+	// L1: one measurement, its own numbers, no selection.
+	if h1.Kind != "node" || h1.State != HealthAlive || h1.Total != 1 || h1.Alive != 1 ||
+		h1.DelayMs != 40 || h1.AgeSeconds != 5 || h1.Selected != "" {
+		t.Errorf("h1 = %+v, want an alive node hop with its own 40ms/5s and no selection", h1)
+	}
+
+	// L2: the dead hop. Both members tested, none alive => a POSITIVE dead
+	// finding on exactly this index. The selected member (x) is dead, and a
+	// dead observation carries delay 0 with the failure's age.
+	if h2.Kind != "group" || h2.State != HealthDead {
+		t.Fatalf("h2 = %+v, want the DEAD group hop — this is the answer to 'which hop died'", h2)
+	}
+	if h2.Total != 2 || h2.Tested != 2 || h2.Alive != 0 || h2.Dead != 2 || h2.Untested != 0 {
+		t.Errorf("h2 counters = %+v, want 2 tested / 2 dead", h2)
+	}
+	if h2.Selected != "x" {
+		t.Errorf("h2 Selected = %q, want the node NAME x", h2.Selected)
+	}
+	if h2.DelayMs != 0 || h2.AgeSeconds != 3 {
+		t.Errorf("h2 delay/age = %d/%d, want 0/3 (the selected member's failure observation)", h2.DelayMs, h2.AgeSeconds)
+	}
+
+	// L3: BEHIND the break. Its board still holds an alive record for member "a"
+	// (probed before L2 died), but the prober has not dialled this hop since, so
+	// publishing that record would be a verdict nobody currently holds — and the
+	// symmetric case, a stale FAILURE, would send the operator to fix a hop that
+	// is fine. It reads untested, counters collapsed into Untested, with L2 named.
+	if h3.Kind != "group" || h3.State != HealthUntested {
+		t.Fatalf("h3 = %+v, want an UNTESTED hop: the walk stopped at L2 and never dialled it", h3)
+	}
+	if h3.BlockedBy == nil {
+		t.Fatalf("h3 = %+v, want BlockedBy naming L2 — 'why is this empty' must be answered on the row", h3)
+	}
+	if h3.BlockedBy.Index != 2 || h3.BlockedBy.Tag != "chain-c-h2" {
+		t.Errorf("h3.BlockedBy = %+v, want the dead hop 2 / chain-c-h2", *h3.BlockedBy)
+	}
+	if h3.Total != 2 || h3.Tested != 0 || h3.Alive != 0 || h3.Dead != 0 || h3.Untested != 2 {
+		t.Errorf("h3 counters = %+v, want every member untested (nothing was measured at this hop)", h3)
+	}
+	if h3.DelayMs != 0 || h3.AgeSeconds != -1 {
+		t.Errorf("h3 delay/age = %d/%d, want 0/-1 — there is no observation to age", h3.DelayMs, h3.AgeSeconds)
+	}
+	// The selection is not a measurement: the wrapper really would route through
+	// "a", and that stays true whether or not anything reached this hop.
+	if h3.Selected != "a" {
+		t.Errorf("h3 Selected = %q, want a — a block hides measurements, not configuration", h3.Selected)
+	}
+	// Nothing in FRONT of the break may be touched by it.
+	if h1.BlockedBy != nil || h2.BlockedBy != nil {
+		t.Errorf("BlockedBy leaked onto a measured hop: h1=%+v h2=%+v", h1.BlockedBy, h2.BlockedBy)
+	}
+
+	// The invariants, on every hop, exactly as GroupHealth promises.
+	for _, h := range hops {
+		if h.Tested != h.Alive+h.Dead {
+			t.Errorf("hop %d: Tested = %d, want alive+dead = %d", h.Index, h.Tested, h.Alive+h.Dead)
+		}
+		if h.Alive+h.Dead+h.Untested != h.Total {
+			t.Errorf("hop %d: alive+dead+untested = %d, want total = %d",
+				h.Index, h.Alive+h.Dead+h.Untested, h.Total)
+		}
+	}
+}
+
+// A group hop whose SELECTION has no measurement shows the freshest ALIVE
+// member's numbers instead — the shown number must always be an observation
+// somebody took.
+func TestChainHopSelectionWithoutMeasurementFallsBack(t *testing.T) {
+	hist := urltest.NewHistoryStorage()
+	now := time.Now()
+	hist.StoreURLTestHistory("chain-c-h1-a", &adapter.URLTestHistory{LastOK: now.Add(-30 * time.Second), Delay: 90})
+	hist.StoreURLTestHistory("chain-c-h1-b", &adapter.URLTestHistory{LastOK: now.Add(-4 * time.Second), Delay: 150})
+	// The selection points at c, which has no observation at all.
+	pool := []adapter.Outbound{
+		&depGroup{fakeGroup{
+			tag: "chain-c-h1", kind: C.TypeSelector,
+			all: []string{"chain-c-h1-a", "chain-c-h1-b", "chain-c-h1-c"},
+			now: "chain-c-h1-c",
+		}, nil},
+	}
+	hops := chainHopHealthOf(pool, "c", newHealthView(hist, healthTTLFloor, now))
+	if len(hops) != 1 {
+		t.Fatalf("got %d hops, want 1", len(hops))
+	}
+	h := hops[0]
+	if h.Selected != "c" {
+		t.Errorf("Selected = %q, want c (the selection is reported even unmeasured)", h.Selected)
+	}
+	if h.DelayMs != 150 || h.AgeSeconds != 4 {
+		t.Errorf("delay/age = %d/%d, want the freshest ALIVE member's 150/4", h.DelayMs, h.AgeSeconds)
+	}
+	if h.State != HealthAlive || h.Alive != 2 || h.Untested != 1 {
+		t.Errorf("hop = %+v, want alive with 2 alive / 1 untested", h)
 	}
 }

shater/engine/grouptest.go

@@ -12,33 +12,46 @@ import (
 	"time"
 
 	"github.com/sagernet/sing-box/adapter"
-	"github.com/sagernet/sing-box/common/urltest"
 	C "github.com/sagernet/sing-box/constant"
 )
 
-// Exit test — "what am I actually going out through, and how fast" (F2, plan §5.E).
+// Group/chain test — "ask the observatory to refresh, then report what it
+// measured, plus the exit address" (F2, plan §5.E; reworked under the one-probe
+// rule).
 //
-// # Why this is not just another node probe
+// # This file no longer measures latency. On purpose.
 //
-// The observatory (observatory.go) answers "which nodes are alive". It cannot
-// answer the question an operator actually asks after switching a group: *through
-// which address am I leaving the country right now*. A group is an indirection —
-// a selector or urltest over many members — so the delay of the group and the
-// public address it exits from are properties of the CURRENT selection, not of
-// any node the panel can point at. A CHAIN is the same question over a longer
-// path: its exit wrapper "chain-<name>-hN" tunnels through every hop, so dialling
-// it measures the whole L1..Ln path end to end.
+// It used to: one urltest.URLTest per target, dialled right here. That was the
+// defect, not a feature. The observatory (observatory.go) probes every path the
+// routing rules actually use — per-chain member copies, egress-bound group
+// copies, chain exits — and it is the ONLY thing allowed to dial for health.
+// A second dial path from this file measured the WRONG thing: pressing "Test"
+// dialled the base groups directly from the router over the default WAN, a path
+// no rule routes through, and (worse) URLTest's DialContext touched the group,
+// arming its own 30-minute probing ticker, which kept writing those false
+// direct measurements under the members' base tags. For a node that is blocked
+// on the direct WAN and alive only behind a tunnel, that reading is not stale —
+// it is FALSE, and it poisoned selection and the panel alike.
 //
-// So one test per target (group or chain) measures two things:
+// So a manual test is now a READ with a refresh request in front of it:
 //
-//	delay_ms  — reusing urltest.URLTest, the SAME primitive the observatory uses
-//	            for a node. There is deliberately no second latency mechanism
-//	            here: the target is just an outbound, and probing it exercises
-//	            exactly the path traffic will take.
-//	exit_ip   — a real HTTP request THROUGH the target to a service that echoes
-//	            the client address back. Nothing else can produce this number: the
-//	            router cannot know its own public address, and the proxy protocol
-//	            does not report it.
+//	delay_ms / ok — the observatory's OWN measurement of the target's dial
+//	                path. TestGroups rewinds the observatory (RefreshObservatory)
+//	                so the numbers are fresh — taken AFTER the button press —
+//	                then each target waits for an observation newer than the
+//	                run's start instant and reports it verbatim. tested_unix is
+//	                the instant that observation was taken, not "now".
+//	exit_ip       — a real HTTP request THROUGH the target to a service that
+//	                echoes the client address back. This is the ONE connection
+//	                this file still opens, and it stays because no probe can
+//	                answer it: the router cannot know its own public address,
+//	                and the proxy protocol does not report it. It is NOT a
+//	                second health probe — it travels the target's own routed
+//	                path (the same outbound object the rules dial), and it is
+//	                only ever issued for a target that (a) the observatory's
+//	                used-set covers and (b) just resolved ALIVE. A target
+//	                outside the rules, or one whose path is down, gets an empty
+//	                exit_ip — never a connection.
 //
 // # The direct-egress trap
 //
@@ -51,16 +64,15 @@ import (
 // exit_ip instead of somebody else's number.
 
 const (
-	// groupTestDelayTimeout bounds the latency probe of one group.
-	groupTestDelayTimeout = 5 * time.Second
 	// groupTestExitTimeout bounds the whole exit-address lookup for one group
 	// (connect through the tunnel + TLS + response). Short on purpose: this runs
 	// while a human waits on a panel button, and a slow answer is worth less than a
 	// prompt "could not determine".
 	groupTestExitTimeout = 6 * time.Second
-	// groupTestConcurrency bounds how many groups are tested in parallel. Small:
-	// every one of these opens a real tunnelled connection, and a router with a
-	// handful of groups is the normal case.
+	// groupTestConcurrency bounds how many EXIT-ADDRESS lookups run in parallel.
+	// Small: each one opens a real tunnelled connection, and a router with a
+	// handful of groups is the normal case. The board polling itself is not
+	// bounded by this — it is sleep-and-read, no I/O.
 	groupTestConcurrency = 4
 	// exitBodyLimit caps what is read from the exit-address service. These responses
 	// are a few hundred bytes; anything larger is a hijacked/captive-portal answer
@@ -68,19 +80,66 @@ const (
 	exitBodyLimit = 4 << 10
 )
 
+// groupTestWaitDeadline / groupTestPollEvery pace the wait for the observatory:
+// each covered target polls the health board once a second until its measured
+// tag carries an observation newer than the run's start, giving up after the
+// deadline. 120s covers a forced pass of a large plan (batches chain
+// back-to-back during a forced pass, see observatoryTickOnce) with room for the
+// probe timeouts of a mostly-dead population. Package variables, not constants,
+// for exactly one reason: the timeout tests must not take two minutes.
+var (
+	groupTestWaitDeadline = 120 * time.Second
+	groupTestPollEvery    = time.Second
+)
+
+// The fixed result texts for the targets that are never (or not yet) measured.
+// They are contract, not decoration — the panel shows them verbatim.
+const (
+	// groupTestErrNotRouted: the target is outside the observatory's used-set,
+	// so no rule routes through it and nothing measures it. Dialling it anyway
+	// would recreate the false direct measurement this rework removed.
+	groupTestErrNotRouted = "not routed by any enabled rule, so nothing measures it — the observatory only probes paths the rules use"
+	// groupTestErrNotReached: the deadline passed without a fresh observation.
+	groupTestErrNotReached = "the observatory has not reached this target yet — it refreshes on the global probe interval"
+	// groupTestErrProbingOff: the observatory is disabled (the GroupHealth
+	// master switch), so a refresh request has nothing to wake and waiting for
+	// the deadline would just delay the same answer by two minutes.
+	groupTestErrProbingOff = "background probing is disabled, so there is nothing to measure this target with"
+	// groupTestErrPathDead: the fresh observation exists and it is a FAILURE —
+	// the observatory probed the target's path after the button press and the
+	// path did not answer. An honest negative, not a missing measurement.
+	groupTestErrPathDead = "the observatory's probe through this path failed"
+	// groupTestErrChainBlockedFmt: a CHAIN whose exit was never dialled because
+	// an earlier hop was probed and did not answer. The prober walks a chain in
+	// order and stops at the first dead hop, so there is no end-to-end
+	// measurement to wait for and there never will be while that hop is down.
+	// Reported at once, naming the hop, instead of spending the full deadline to
+	// answer "not reached yet" about a path that is known to be broken — and
+	// naming it is the whole value: the operator's next action is at that hop.
+	groupTestErrChainBlockedFmt = "hop %d of this chain was probed and did not answer, so nothing reaches the exit through it — fix that hop first"
+)
+
 // GroupTestResult is one target's test outcome — a group's or a chain's (Group
 // then carries the chain's model name). The JSON tags are the panel contract —
-// see the shater API docs for /api/groups/test.
+// see the shater API docs for /api/groups/test. The field names and types are
+// FROZEN; what changed in the rework is where the numbers come from.
+//
+// DelayMs and OK are a READ of the observatory's measurement of the target's
+// dial path — not a fresh dial performed by this file. OK is true exactly when
+// the health board's state for the measured tag is alive; DelayMs is that
+// observation's RTT; TestedUnix is the instant the OBSERVATION was taken (now
+// minus its age), so a result honestly says how old its number is instead of
+// stamping the poll time over it.
 //
 // Selected is the group's current pick (OutboundGroup.Now()); for a chain it is
 // the node NAME the chain's last group hop currently selects, "" when the chain
 // has no group hop (a fixed path selects nothing).
 //
-// OK reports whether the LATENCY measurement succeeded, which is the test's primary
-// question. A failed exit-address lookup deliberately does NOT clear it: knowing the
-// target is up and fast is useful on its own, and a probe service being unreachable
-// says nothing about the tunnel. In that case OK stays true and ExitIP is empty —
-// "not determined", never a guess and never somebody else's address.
+// A failed exit-address lookup deliberately does NOT clear OK: knowing the
+// target is up and fast is useful on its own, and a probe service being
+// unreachable says nothing about the tunnel. In that case OK stays true and
+// ExitIP is empty — "not determined", never a guess and never somebody else's
+// address.
 type GroupTestResult struct {
 	Group       string `json:"group"`
 	Selected    string `json:"selected"`
@@ -117,6 +176,13 @@ type groupTestTarget struct {
 	name string
 	ob   adapter.Outbound
 	sel  func() string
+	// isChain marks a CHAIN target. A chain is the one target whose measurement
+	// can be legitimately absent while the target is known to be broken: the
+	// prober walks a chain in order and stops at the first dead hop, so a chain
+	// whose hop 2 died never has its exit dialled at all. Without this flag a run
+	// would sit out its whole deadline and then report "not reached yet" about a
+	// path it knows is down — see testOneTarget.
+	isChain bool
 }
 
 // chainTargetsFrom discovers each materialised chain in the running
@@ -161,7 +227,7 @@ func chainTargetsFrom(pool []adapter.Outbound) []groupTestTarget {
 		if !ok {
 			continue
 		}
-		t := groupTestTarget{name: name, ob: ob}
+		t := groupTestTarget{name: name, ob: ob, isChain: true}
 		if hop := chainLastGroupHop(byTag, name); hop != nil {
 			t.sel = func() string { return chainMemberName(hop.Now(), name) }
 		}
@@ -175,18 +241,38 @@ func chainTargetsFrom(pool []adapter.Outbound) []groupTestTarget {
 // "chain-<name>-h<digits>". Member copies ("…-h<i>-<member>") and non-chain tags
 // parse false.
 func parseChainExitTag(tag string) (string, bool) {
+	name, _, ok := parseChainHopTag(tag)
+	return name, ok
+}
+
+// parseChainHopTag is parseChainExitTag plus the number: it recovers the chain
+// NAME and the 1-based HOP INDEX from a hop wrapper tag "chain-<name>-h<digits>"
+// (generate/chain.go buildHopWrapper). Member copies ("…-h<i>-<member>") and
+// non-chain tags parse false.
+//
+// The index is what lets the probe plan ORDER a chain's measurements. A chain is
+// a series path — hop i is dialled through hops 1..i-1 — so the difference
+// between "this hop failed" and "the hop in front of it failed" is the whole
+// diagnosis, and it cannot be recovered from an unordered set of tags.
+//
+// The split takes the LAST "-h<digits>", so a chain literally named "a-h2" is
+// read as chain "a-h2" hop 1 rather than chain "a" hop 2 — the same documented
+// reserved-namespace edge case parseGroupCopyTag accepts, pinned by
+// TestParseChainExitTag.
+func parseChainHopTag(tag string) (name string, hop int, ok bool) {
 	rest, ok := strings.CutPrefix(tag, "chain-")
 	if !ok {
-		return "", false
+		return "", 0, false
 	}
 	i := strings.LastIndex(rest, "-h")
 	if i <= 0 {
-		return "", false
+		return "", 0, false
 	}
-	if _, ok := parseAllDigits(rest[i+2:]); !ok {
-		return "", false
+	hop, ok = parseAllDigits(rest[i+2:])
+	if !ok {
+		return "", 0, false
 	}
-	return rest[:i], true
+	return rest[:i], hop, true
 }
 
 // chainLastGroupHop finds the chain's LAST group hop — the wrapper selector
@@ -259,19 +345,30 @@ func parseAllDigits(s string) (int, bool) {
 // and chains (empty/nil = every group and every chain in the running box),
 // returning started=false when a run is already in flight.
 //
-// It is a SINGLETON: a second request while a run is in flight is refused rather
-// than queued or run in parallel, because these runs open real tunnelled
-// connections and a panel that double-fires a button must not multiply the load
-// on the uplink. The observatory checks the same guard and skips its tick while
-// a run is in flight (observatory.go), so a manual test never competes with
-// background probing for the uplink.
+// It does NOT probe. It records the run's start instant, asks the observatory
+// for an out-of-turn full pass (RefreshObservatory), and then each target waits
+// for the health board to carry an observation NEWER than that instant on the
+// tag the observatory actually measures for it — see testOneTarget. The only
+// connection a run may still open is the exit-address lookup, and only for a
+// target that resolved alive on a rule-routed path.
 //
-// probeURL is the latency-probe URL; "" falls back to urltest's gstatic default.
+// It is a SINGLETON: a second request while a run is in flight is refused
+// rather than queued, because two overlapping runs would each rewind the
+// observatory's cursor and neither pass would ever complete — and the panel's
+// progress contract assumes one run's counters at a time anyway.
 //
-// Apply-swap safety: the target outbounds are snapshotted up front, so a config swap
-// mid-run cannot change what is being tested. A target torn down mid-run simply fails
-// its probe and is reported not-ok.
+// probeURL is accepted and IGNORED. The probe URL is a global observatory
+// setting now (ObservatoryConfig.ProbeURL, set at apply time); a per-run URL
+// would mean this run measures something different from what the board holds,
+// which is exactly the two-instruments split the rework removed. The parameter
+// stays so the callers (shater/apply, shater/panel) keep compiling and the
+// control-plane API shape does not churn.
+//
+// Apply-swap safety: the target outbounds are snapshotted up front, so a config
+// swap mid-run cannot change what is being tested. A target torn down mid-run
+// simply never receives a fresh observation and resolves on the deadline.
 func (e *Engine) TestGroups(names []string, probeURL string) (started bool) {
+	_ = probeURL // ignored — see the doc comment above
 	if !e.groupTestRunning.CompareAndSwap(false, true) {
 		return false
 	}
@@ -285,6 +382,13 @@ func (e *Engine) TestGroups(names []string, probeURL string) (started bool) {
 	// GroupTestStatus for why the scope is a set of names.
 	e.setGroupTestScope(scopeOf(targets, missing))
 
+	// The freshness watermark: only an observation taken AFTER this instant may
+	// answer this run. Recorded BEFORE the refresh request so a probe that lands
+	// between the two can never be missed, only double-counted as fresh — the
+	// harmless direction.
+	t0 := time.Now()
+	e.RefreshObservatory()
+
 	go func() {
 		defer e.groupTestRunning.Store(false)
 
@@ -303,15 +407,22 @@ func (e *Engine) TestGroups(names []string, probeURL string) (started bool) {
 		e.setGroupTestResults(results)
 		e.groupTestDone.Add(int64(len(missing)))
 
+		// The used-set and the enabled bit are snapshotted once for the whole
+		// run: they only change on an apply, and a run that straddles an apply
+		// is already best-effort (see the swap-safety note above).
+		used := e.observatoryUsed()
+		obsEnabled, _, _ := e.ObservatoryStatus()
+
+		// One goroutine per target: they spend their life sleeping on the board
+		// poll, so there is nothing to bound — the semaphore below bounds the
+		// exit-address lookups, the only real connections left in a run.
 		sem := make(chan struct{}, groupTestConcurrency)
 		var wg sync.WaitGroup
 		for i, tgt := range targets {
 			wg.Add(1)
-			sem <- struct{}{}
 			go func(i int, tgt groupTestTarget) {
 				defer wg.Done()
-				defer func() { <-sem }()
-				res := e.testOneTarget(tgt, probeURL)
+				res := e.testOneTarget(tgt, used, obsEnabled, t0, sem)
 				e.storeGroupTestResult(i, res)
 				e.groupTestDone.Add(1)
 			}(i, tgt)
@@ -393,29 +504,185 @@ func (e *Engine) groupTargets(names []string) (targets []groupTestTarget, missin
 	return targets, missing
 }
 
-// testOneTarget measures one target: what it currently selects, the latency
-// through it, and the public address it exits from. For a chain the dialled
-// outbound is the exit wrapper, so the delay and the exit address are end-to-end
-// properties of the whole L1..Ln path.
-func (e *Engine) testOneTarget(t groupTestTarget, probeURL string) GroupTestResult {
+// testOneTarget resolves one target WITHOUT probing it: it decides whether the
+// observatory measures this target at all, and if so waits for a fresh
+// observation and reports it. The three ways out, in order:
+//
+//  1. the used-set does not cover the target — no enabled rule routes through
+//     it, so nothing measures it and nothing SHOULD: resolved immediately with
+//     groupTestErrNotRouted, never dialled, empty exit address. A nil used-set
+//     means "unknown" (observatory not yet configured) and is treated as
+//     covered — no refusal is better than a wrong one;
+//  2. the observatory is disabled — the refresh request went nowhere, so the
+//     wait below could only ever end on its deadline: resolved immediately
+//     with groupTestErrProbingOff instead of stalling the panel for two
+//     minutes to say the same thing;
+//  3. covered and enabled — poll the health board once a second until the
+//     MEASURED TAG carries an observation newer than since, then report that
+//     observation verbatim (readFreshObservation). On the deadline:
+//     groupTestErrNotReached.
+//
+// The exit-address lookup runs ONLY on the alive path of (3) — a rule-routed
+// target whose path just answered a probe — and through sem, so a run never
+// opens more than groupTestConcurrency tunnelled connections at once.
+func (e *Engine) testOneTarget(t groupTestTarget, used map[string]bool, obsEnabled bool, since time.Time, sem chan struct{}) GroupTestResult {
 	res := GroupTestResult{Group: t.name, TestedUnix: time.Now().Unix()}
 	if t.sel != nil {
 		res.Selected = t.sel()
 	}
 
-	ctx, cancel := context.WithTimeout(context.Background(), groupTestDelayTimeout)
-	delay, err := urltest.URLTest(ctx, probeURL, t.ob)
-	cancel()
-	if err != nil {
-		res.Error = err.Error()
+	if used != nil && !used[t.ob.Tag()] {
+		res.Error = groupTestErrNotRouted
+		return res
+	}
+	if !obsEnabled {
+		res.Error = groupTestErrProbingOff
 		return res
 	}
-	res.OK = true
-	res.DelayMs = int(delay)
 
-	// The exit address is best-effort by design: see GroupTestResult.OK.
-	res.ExitIP, res.ExitCountry = e.exitAddress(t.ob)
-	return res
+	deadline := time.Now().Add(groupTestWaitDeadline)
+	for {
+		// Asked BEFORE the board read, and only for a chain. The exit of a blocked
+		// chain does carry a fresh dead verdict now — the observatory derives it
+		// from the broken hop (markChainExitDead) so selection cannot be tempted
+		// down a path that provably does not work — but "the probe through this
+		// path failed" would be the wrong sentence to show a person: no probe of
+		// the exit ran, and the actionable fact is WHICH hop stopped it. Same
+		// verdict, better answer.
+		if idx, ok := e.blockedChainHop(t, since); ok {
+			res.OK = false
+			res.Error = fmt.Sprintf(groupTestErrChainBlockedFmt, idx)
+			return res
+		}
+		if e.readFreshObservation(&res, t, since) {
+			if res.OK {
+				// The exit address is best-effort by design: see GroupTestResult.
+				sem <- struct{}{}
+				res.ExitIP, res.ExitCountry = e.exitAddress(t.ob)
+				<-sem
+			}
+			return res
+		}
+		if !time.Now().Before(deadline) {
+			res.Error = groupTestErrNotReached
+			return res
+		}
+		time.Sleep(groupTestPollEvery)
+	}
+}
+
+// blockedChainHop reports the hop that stopped this CHAIN target's ordered walk
+// before its exit, and only when the finding is FRESH enough to answer this run.
+//
+// Both halves matter. The projection (chainHopHealthOf) already computes the
+// break exactly once, and reading it here rather than re-deriving it keeps the
+// run and the Targets card from ever naming different hops. And the freshness
+// check is the same watermark readFreshObservation uses: a run rewinds the
+// observatory and then reports what it measures AFTERWARDS, so a break left over
+// from before the button press must not short-circuit the wait — the forced pass
+// may be about to find that hop alive again.
+//
+// Non-chain targets, unmaterialised chains and chains with no break all report
+// false, and the caller goes on waiting exactly as before.
+func (e *Engine) blockedChainHop(t groupTestTarget, since time.Time) (int, bool) {
+	if !t.isChain {
+		return 0, false
+	}
+	return blockingHopOf(chainHopHealthOf(e.runningPool(), t.name, e.HealthView()), time.Now(), since)
+}
+
+// blockingHopOf is the decision itself, split out so it is testable without a
+// box: given one chain's hop readout, the hop whose failure kept the EXIT from
+// being dialled, and only when that failure was observed at or after since.
+//
+// The exit's own BlockedBy is what is consulted, not "any dead hop": a chain
+// whose exit was itself probed and failed has a real end-to-end measurement, and
+// that measurement is the honest answer (groupTestErrPathDead). Only when the
+// exit was never reached is there nothing to wait for.
+func blockingHopOf(hops []ChainHopHealth, now, since time.Time) (int, bool) {
+	if len(hops) == 0 {
+		return 0, false
+	}
+	blocker := hops[len(hops)-1].BlockedBy
+	if blocker == nil {
+		return 0, false
+	}
+	for _, h := range hops {
+		if h.Index != blocker.Index {
+			continue
+		}
+		if h.AgeSeconds < 0 {
+			return 0, false
+		}
+		if now.Add(-time.Duration(h.AgeSeconds) * time.Second).Before(since) {
+			return 0, false // a break observed before this run started; keep waiting
+		}
+		return h.Index, true
+	}
+	return 0, false
+}
+
+// measuredTagOf is the tag the observatory actually probes for this target's
+// dial path — the tag whose board entry answers "how is this target doing":
+//
+//   - a GROUP target dials whatever it currently selects, so the group's health
+//     IS its selection's health: OutboundGroup.Now(). For an egress-bound group
+//     that is a per-group copy tag, which is exactly what the plan probes;
+//   - a CHAIN whose exit wrapper is a PLAIN outbound is probed end-to-end under
+//     that wrapper tag (probeplan.go: a chain exit is its own measurement);
+//   - a CHAIN whose exit wrapper is a GROUP (the last hop is a group) has its
+//     member copies probed instead of the wrapper, so the wrapper's Now() — the
+//     member copy the chain currently dials through — is the measured tag. The
+//     wrapper here IS the last group hop, so this is chainLastGroupHop's Now()
+//     without a second lookup;
+//   - fallback: the target's own tag, for a group that has not selected yet
+//     (cold selector mid-swap). Its board entry is almost certainly empty, and
+//     the caller then honestly reports "not reached" rather than inventing one.
+func measuredTagOf(t groupTestTarget) string {
+	if g, ok := t.ob.(adapter.OutboundGroup); ok {
+		if now := g.Now(); now != "" {
+			return now
+		}
+	}
+	return t.ob.Tag()
+}
+
+// readFreshObservation reads the board once: if the target's measured tag holds
+// an observation taken at or after since, it is written into res (state, delay,
+// the observation's own timestamp, and the current selection so Selected and
+// the measurement describe the same pick) and true is returned. Otherwise res
+// is left for the next poll.
+//
+// Precision note: the board reports ages in whole seconds, so "at or after
+// since" is accurate to one second — an observation taken up to a second
+// BEFORE the refresh can slip through as fresh. That is the acceptable
+// direction: it is still a real measurement of the same path, at most a second
+// older than requested; the strict direction (discarding genuinely fresh
+// observations) would make every run one probe interval slower for nothing.
+func (e *Engine) readFreshObservation(res *GroupTestResult, t groupTestTarget, since time.Time) bool {
+	// Re-read the selection at every poll: a forced observatory pass is exactly
+	// the kind of event that makes a urltest group switch members, and the
+	// measurement below is taken against the CURRENT pick.
+	if t.sel != nil {
+		res.Selected = t.sel()
+	}
+	tag := measuredTagOf(t)
+	now := time.Now()
+	state, delayMs, age := e.HealthView().State(tag)
+	if age < 0 {
+		return false // no observation at all (untested)
+	}
+	observedAt := now.Add(-time.Duration(age) * time.Second)
+	if observedAt.Before(since) {
+		return false // an old reading; the refresh has not reached this tag yet
+	}
+	res.OK = state == HealthAlive
+	res.DelayMs = delayMs
+	res.TestedUnix = observedAt.Unix()
+	if !res.OK {
+		res.Error = groupTestErrPathDead
+	}
+	return true
 }
 
 // exitProbe is one exit-address service: a URL and the parser for its body.

shater/engine/grouptest_test.go

@@ -3,6 +3,7 @@ package engine
 import (
 	"context"
 	"crypto/tls"
+	"fmt"
 	"net"
 	"net/http"
 	"net/http/httptest"
@@ -143,10 +144,10 @@ func TestChainMemberName(t *testing.T) {
 	}
 }
 
-// dialableOutbound routes every dial to a fixed local address — a stand-in for a
-// chain exit whose whole path is up. urltest.URLTest dials the probe URL's host
-// through the outbound, so pointing every dial at a local HTTP server makes the
-// latency probe succeed without any network.
+// dialableOutbound routes every dial to a fixed local address — the sink the
+// exit-address lookup lands in during tests. Since the rework nothing else in
+// this file dials at all; the local server refuses to be an exit-address
+// service (wrong status, no TLS), so the lookup honestly comes back empty.
 type dialableOutbound struct {
 	failingOutbound
 	addr string
@@ -158,20 +159,42 @@ func (d *dialableOutbound) DialContext(ctx context.Context, network string, _ M.
 	return (&net.Dialer{}).DialContext(ctx, network, d.addr)
 }
 
-// TestChainExitTestMeasuresEndToEnd is the §6-S4 acceptance path for chains: the
-// exit test dials the chain's EXIT TAG, returns a measured delay, carries the
-// chain's model name (not the wrapper tag) as the result's Group, and reports the
-// last group hop's pick as Selected. The exit address is measured through the
-// same outbound (unreachable from a test => empty, never a guess).
-func TestChainExitTestMeasuresEndToEnd(t *testing.T) {
-	probe := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+// countingOutbound counts every DialContext/ListenPacket. It is the tripwire of
+// the rework: a target the observatory does not cover must NEVER be dialled,
+// and the counter is the proof.
+type countingOutbound struct {
+	failingOutbound
+	dials int
+}
+
+func (c *countingOutbound) DialContext(ctx context.Context, network string, dst M.Socksaddr) (net.Conn, error) {
+	c.dials++
+	return nil, fmt.Errorf("dial refused (test)")
+}
+
+func (c *countingOutbound) ListenPacket(ctx context.Context, dst M.Socksaddr) (net.PacketConn, error) {
+	c.dials++
+	return nil, fmt.Errorf("dial refused (test)")
+}
+
+// groupTestSem is a fresh exit-lookup semaphore for direct testOneTarget calls.
+func groupTestSem() chan struct{} { return make(chan struct{}, groupTestConcurrency) }
+
+// TestChainTestReportsObservatoryMeasurement is the §6-S4 acceptance path for
+// chains under the one-probe rule: the manual test does NOT dial the exit — it
+// reads the observatory's board entry for the exit tag, reports its delay and
+// ITS timestamp, carries the chain's model name as Group and the last group
+// hop's pick as Selected. The exit-address lookup still travels the exit
+// outbound (unreachable from a test => empty, never a guess).
+func TestChainTestReportsObservatoryMeasurement(t *testing.T) {
+	sink := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		w.WriteHeader(http.StatusNoContent)
 	}))
-	defer probe.Close()
+	defer sink.Close()
 
 	exit := &dialableOutbound{
 		failingOutbound: failingOutbound{tag: "chain-x-h2"},
-		addr:            probe.Listener.Addr().String(),
+		addr:            sink.Listener.Addr().String(),
 		deps:            []string{"chain-x-h1"},
 	}
 	pool := []adapter.Outbound{
@@ -187,14 +210,31 @@ func TestChainExitTestMeasuresEndToEnd(t *testing.T) {
 	if len(targets) != 1 || targets[0].ob.Tag() != "chain-x-h2" {
 		t.Fatalf("targets = %+v, want the chain dialled via its exit tag", targets)
 	}
+	if got := measuredTagOf(targets[0]); got != "chain-x-h2" {
+		t.Fatalf("measuredTagOf = %q, want the plain exit wrapper itself", got)
+	}
 
 	e := New()
-	res := e.testOneTarget(targets[0], probe.URL)
+	// The observatory measured the exit 10 seconds ago, AFTER the run's start
+	// instant below: that observation — delay and timestamp both — is the answer.
+	observedAt := time.Now().Add(-10 * time.Second)
+	e.URLTestHistory().StoreURLTestHistory("chain-x-h2", &adapter.URLTestHistory{LastOK: observedAt, Delay: 77})
+	since := time.Now().Add(-time.Minute)
+
+	res := e.testOneTarget(targets[0], map[string]bool{"chain-x-h2": true}, true, since, groupTestSem())
 	if res.Group != "x" {
 		t.Errorf("Group = %q, want the chain's model name x", res.Group)
 	}
 	if !res.OK || res.Error != "" {
-		t.Fatalf("result = %+v, want a successful measurement through the exit tag (a local roundtrip may legitimately read 0ms)", res)
+		t.Fatalf("result = %+v, want the board's alive observation reported", res)
+	}
+	if res.DelayMs != 77 {
+		t.Errorf("DelayMs = %d, want the observatory's 77 — this file measures nothing itself", res.DelayMs)
+	}
+	// tested_unix is the OBSERVATION's instant (whole-second precision), never
+	// the poll's.
+	if got, want := res.TestedUnix, observedAt.Unix(); got < want-1 || got > want+1 {
+		t.Errorf("TestedUnix = %d, want the observation's instant ~%d", got, want)
 	}
 	if res.Selected != "relay" {
 		t.Errorf("Selected = %q, want the last group hop's pick relay", res.Selected)
@@ -204,6 +244,93 @@ func TestChainExitTestMeasuresEndToEnd(t *testing.T) {
 	}
 }
 
+// TestTestOneTargetUnroutedNeverDialled is the rework's core promise: a target
+// outside the observatory's used-set is resolved immediately with the
+// documented explanation and ZERO dials — no latency probe, no exit-address
+// lookup, nothing. Dialling it would manufacture exactly the false direct
+// measurement the rework removed.
+func TestTestOneTargetUnroutedNeverDialled(t *testing.T) {
+	e := New()
+	ob := &countingOutbound{failingOutbound: failingOutbound{tag: "idle"}}
+	tgt := groupTestTarget{name: "idle", ob: ob}
+
+	res := e.testOneTarget(tgt, map[string]bool{"something-else": true}, true, time.Now(), groupTestSem())
+	if ob.dials != 0 {
+		t.Fatalf("an unrouted target was dialled %d time(s); it must never be", ob.dials)
+	}
+	if res.OK {
+		t.Fatal("an unrouted target reported ok=true")
+	}
+	if res.Error != groupTestErrNotRouted {
+		t.Fatalf("Error = %q, want the not-routed explanation", res.Error)
+	}
+	if res.ExitIP != "" || res.ExitCountry != "" {
+		t.Fatalf("an unrouted target must carry no exit address: %+v", res)
+	}
+}
+
+// A disabled observatory resolves covered targets immediately too: the refresh
+// request went nowhere, so waiting out the deadline would only delay the same
+// honest answer — and still, nothing is dialled.
+func TestTestOneTargetProbingOffNeverDialled(t *testing.T) {
+	e := New()
+	ob := &countingOutbound{failingOutbound: failingOutbound{tag: "auto"}}
+	tgt := groupTestTarget{name: "auto", ob: ob}
+
+	res := e.testOneTarget(tgt, nil, false, time.Now(), groupTestSem())
+	if ob.dials != 0 {
+		t.Fatalf("target dialled %d time(s) with probing off; want 0", ob.dials)
+	}
+	if res.OK || res.Error != groupTestErrProbingOff {
+		t.Fatalf("result = %+v, want ok=false with the probing-off explanation", res)
+	}
+}
+
+// The deadline path: a covered target whose measured tag never receives a fresh
+// observation resolves with the not-reached explanation — and, again, without a
+// single dial of its own.
+func TestTestOneTargetDeadlineWithoutObservation(t *testing.T) {
+	// Shrink the wait so the test answers in milliseconds; restore afterwards.
+	oldDeadline, oldPoll := groupTestWaitDeadline, groupTestPollEvery
+	groupTestWaitDeadline, groupTestPollEvery = 30*time.Millisecond, 5*time.Millisecond
+	defer func() { groupTestWaitDeadline, groupTestPollEvery = oldDeadline, oldPoll }()
+
+	e := New()
+	ob := &countingOutbound{failingOutbound: failingOutbound{tag: "auto"}}
+	tgt := groupTestTarget{name: "auto", ob: ob}
+
+	res := e.testOneTarget(tgt, map[string]bool{"auto": true}, true, time.Now(), groupTestSem())
+	if ob.dials != 0 {
+		t.Fatalf("target dialled %d time(s) while waiting on the board; want 0", ob.dials)
+	}
+	if res.OK || res.Error != groupTestErrNotReached {
+		t.Fatalf("result = %+v, want ok=false with the not-reached explanation", res)
+	}
+}
+
+// A fresh DEAD observation is an answer, not a timeout: ok=false with the
+// path-dead explanation, delay 0, and no exit-address connection for a path
+// that just failed its probe.
+func TestTestOneTargetReportsFreshDeath(t *testing.T) {
+	e := New()
+	ob := &countingOutbound{failingOutbound: failingOutbound{tag: "auto"}}
+	tgt := groupTestTarget{name: "auto", ob: ob}
+
+	since := time.Now().Add(-time.Minute)
+	e.URLTestHistory().MarkFailed("auto")
+
+	res := e.testOneTarget(tgt, map[string]bool{"auto": true}, true, since, groupTestSem())
+	if ob.dials != 0 {
+		t.Fatalf("a dead target was dialled %d time(s); the exit lookup is for alive paths only", ob.dials)
+	}
+	if res.OK || res.Error != groupTestErrPathDead {
+		t.Fatalf("result = %+v, want ok=false with the path-dead explanation", res)
+	}
+	if res.DelayMs != 0 || res.ExitIP != "" {
+		t.Fatalf("a dead path must carry no delay and no exit address: %+v", res)
+	}
+}
+
 // TestGroupTestSingleton is the "no parallel runs" invariant: while a run holds the
 // guard, a second TestGroups must be refused rather than starting a concurrent run
 // (each of these opens real tunnelled connections).
@@ -460,3 +587,57 @@ func TestGroupTestScopeDedupedAndSorted(t *testing.T) {
 		t.Fatalf("scope = %v, want [alpha zeta]", scope)
 	}
 }
+
+// A manual run against a chain whose walk is blocked must say so, at once and by
+// hop number. The prober stops at the first dead hop, so such a chain's exit is
+// never dialled and no end-to-end observation is ever coming: waiting for one
+// would burn the full 120s deadline and then answer "the observatory has not
+// reached this target yet" — which reads like a timing artefact about a chain
+// that is, in fact, down and whose broken hop is already known.
+//
+// The freshness rule is the same one readFreshObservation uses: a run rewinds
+// the observatory and reports what it measures AFTERWARDS, so a break left over
+// from before the button press must not short-circuit the wait — the forced pass
+// may be about to find that hop alive again.
+func TestBlockingHopOfChainWalk(t *testing.T) {
+	since := time.Now()
+	// A 3-hop readout as the projection builds it: hop 2 dead, hop 3 not reached.
+	blocked := func(ageOfBreak int64) []ChainHopHealth {
+		blocker := &ChainHopBlock{Index: 2, Tag: "chain-c-h2"}
+		return []ChainHopHealth{
+			{Index: 1, Tag: "chain-c-h1", State: HealthAlive, AgeSeconds: 1},
+			{Index: 2, Tag: "chain-c-h2", State: HealthDead, AgeSeconds: ageOfBreak},
+			{Index: 3, Tag: "chain-c-h3", State: HealthUntested, AgeSeconds: -1, Exit: true, BlockedBy: blocker},
+		}
+	}
+
+	// Measured after the run started: answer now, naming hop 2.
+	if idx, ok := blockingHopOf(blocked(0), since.Add(time.Second), since); !ok || idx != 2 {
+		t.Errorf("fresh break = (%d,%v), want (2,true) — the run must name the hop instead of timing out", idx, ok)
+	}
+
+	// Measured BEFORE the run started: keep waiting. The forced pass has not
+	// re-probed that hop yet, and it may be about to answer.
+	if idx, ok := blockingHopOf(blocked(30), since.Add(time.Second), since); ok {
+		t.Errorf("stale break reported as this run's finding (hop %d); a run answers with what IT measured", idx)
+	}
+
+	// A chain whose exit WAS reached has a real end-to-end reading; the ordinary
+	// path reports it (alive, or groupTestErrPathDead). Nothing to short-circuit.
+	reached := []ChainHopHealth{
+		{Index: 1, Tag: "chain-c-h1", State: HealthAlive, AgeSeconds: 1},
+		{Index: 2, Tag: "chain-c-h2", State: HealthDead, AgeSeconds: 0, Exit: true},
+	}
+	if _, ok := blockingHopOf(reached, since.Add(time.Second), since); ok {
+		t.Error("a dead EXIT was reported as a block; its own probe is the answer")
+	}
+	if _, ok := blockingHopOf(nil, since, since); ok {
+		t.Error("an unmaterialised chain reported a block")
+	}
+
+	// A non-chain target never consults any of this.
+	e := New()
+	if _, ok := e.blockedChainHop(groupTestTarget{name: "auto"}, since); ok {
+		t.Error("a GROUP target was treated as a blocked chain")
+	}
+}

shater/engine/observatory.go

@@ -2,6 +2,7 @@ package engine
 
 import (
 	"context"
+	"sort"
 	"sync"
 	"time"
 
@@ -23,12 +24,40 @@ import (
 // MarkFailed. The strategies read the same board, so a death found here makes the
 // node unselectable immediately — no private overlay, no panel-only truth.
 //
-// It never replaces a group's own probing. An ACTIVE urltest group is its own
-// fast failure detector (failover switches on its 30s probe tick), and the
-// freshness gate below skips any tag whose newest observation is younger than the
-// global probe interval — which is precisely the set of tags an active group is
-// already keeping current. The observatory's budget lands on what nobody else
-// probes: idle groups' members, selector candidates, chain exits.
+// # ONE DIALLER PER TARGET
+//
+// It does not probe what a urltest group already probes — not "less often",
+// not at all. A urltest group dials its own members on its own ticker
+// (protocol/group/urltest.go testNodes), and for a chain's group hop that
+// checker is strictly the better instrument: each member copy carries the
+// previous hop as its Detour, so the group's probe travels the chain prefix by
+// construction, which is the path the traffic takes. Two probers on one target
+// is one measurement too many however carefully they take turns, so the plan
+// marks those measurements SelfChecked (probeplan.go) and this loop skips them.
+//
+// What is left is what no group covers, and it is the observatory's whole job:
+//
+//   - targets with no group behind them — a chain's NODE hop wrapper, the
+//     AmneziaWG endpoint at L1, a chain exit that is a plain outbound. Nothing
+//     else in the process will ever dial these;
+//   - members of a SELECTOR group, which has no checker at all
+//     (protocol/group/selector.go), and of a urltest group whose self-check was
+//     stood down;
+//   - egress-bound group copies, whose measurement carries the base-tag alias
+//     that feeds the per-node stats and that no group checker writes;
+//   - the GATE — who is allowed to dial at all (the used-set drives SelfCheck,
+//     the ordered walk drives ProbeAllowed) — and the reading of the board back
+//     out for the panel.
+//
+// The freshness gate below still skips any tag whose newest observation is
+// younger than the global probe interval, which now matters for the remaining
+// population rather than as the mechanism that kept two probers apart.
+//
+// The cost of the split: a used urltest group that carries no traffic runs its
+// warm-up sweep at box start and then, once its idle timeout expires, stops —
+// and the observatory no longer fills in behind it, so its members age out to
+// "untested". That is honest (nothing measured them) rather than wrong, and it
+// is the price of one dialler per target.
 //
 // # Schedule and cost
 //
@@ -41,6 +70,21 @@ import (
 // first pass starts IMMEDIATELY after an apply (cold boot: used paths validated
 // in seconds).
 //
+// # A chain is walked in order, and the walk stops at the first dead hop
+//
+// Most of the plan is independent measurements and is fired concurrently. A
+// CHAIN is not: hop i is dialled through hops 1..i-1, so a probe of hop 3 across
+// a hop 2 with no live member measures hop 2's failure and learns nothing about
+// hop 3. Each chain's jobs therefore run hop by hop (probeChainOrdered), and the
+// walk returns at the first hop the board reads dead — every hop behind it is
+// left undialled for that pass and reported untested with the blocker named,
+// which is both the honest answer and the cheap one.
+//
+// Nothing about a block is remembered. The gate is a fresh read of the health
+// board at every hop of every batch, and each cycle restarts at the top of the
+// plan, so the blocking hop is always re-probed and a recovery opens the rest of
+// the chain in the same pass.
+//
 // # Reconfiguration must not restart the walk
 //
 // ConfigureObservatory runs on every successful apply, and cron reconciles every
@@ -98,8 +142,46 @@ type observatoryState struct {
 	used   map[string]bool
 	cursor int
 	cycles uint64
+	// keepWarm is the set of tags the routing config REACHES, and it answers
+	// ProbeWhenIdle: a group in it keeps its own probing ticker running with no
+	// traffic flowing.
+	//
+	// It is deliberately NOT the same field as used above, even though both hold
+	// a used-set. used is published only after a SUCCESSFUL apply, because the
+	// panel's "unused" badge must describe the config that is actually running;
+	// this one is published BEFORE the new box is built, because a group asks the
+	// question during its own PostStart. A failed apply can therefore leave this
+	// describing a config that never started — worst case a group keeps a ticker
+	// it did not need until the next apply, which nobody can see and which costs
+	// one probe interval. Merging the two would force one of those two timings on
+	// the other, and each is right for its own reader.
+	keepWarm map[string]bool
+	// hops indexes the plan's chain measurements — chain name -> hops in wire
+	// order -> the dial tags measuring each hop. Rebuilt from the plan whenever
+	// the plan is, and read (never mutated) by a tick to decide whether a hop's
+	// jobs may run at all. Derived state, kept rather than recomputed per tick
+	// only because it is derived from something that changes once per apply.
+	hops chainPlanIndex
+	// probeFn overrides how ONE planned measurement is executed. nil — the
+	// production value — means probeJob: resolve the dial tag against the running
+	// box and dial it.
+	//
+	// It exists for one assertion that cannot be made any other way. The ordered
+	// walk's contract is that hops below a dead one are never DIALLED; the
+	// resulting health readout cannot prove that, because "not probed" and
+	// "probed, failed, then overridden" are the same untested row. Only counting
+	// attempts distinguishes them, so the tests substitute a counter here.
+	probeFn func(ProbeJob)
 	// busy guards against a slow tick overlapping the next one.
 	busy bool
+	// force is the ONE-SHOT "check now" flag raised by RefreshObservatory: while
+	// it is up the freshness gate is bypassed, so a manual refresh really
+	// RE-MEASURES everything on the plan instead of skipping whatever happens to
+	// be fresh, and each finished batch immediately nudges the next one so the
+	// pass runs back-to-back rather than on the 10s tick. It is cleared the
+	// moment the forced pass schedules its last batch (cursor reaches the end of
+	// the plan) — one full walk, then back to the polite steady state.
+	force bool
 }
 
 // ConfigureObservatory installs (or re-tunes, or stops) the observatory. It is
@@ -119,7 +201,11 @@ func (e *Engine) ConfigureObservatory(cfg ObservatoryConfig) {
 	e.obs.interval = interval
 
 	if !cfg.Enabled {
-		e.obs.plan, e.obs.used, e.obs.cursor = nil, nil, 0
+		e.obs.plan, e.obs.used, e.obs.cursor, e.obs.hops = nil, nil, 0, nil
+		// The master switch is off: nothing probes in the background, so no group
+		// keeps a ticker alive on the background prober's behalf either.
+		e.obs.keepWarm = nil
+		e.obs.force = false
 		if e.obs.stop != nil {
 			close(e.obs.stop)
 			e.obs.stop, e.obs.done, e.obs.nudge = nil, nil, nil
@@ -134,6 +220,10 @@ func (e *Engine) ConfigureObservatory(cfg ObservatoryConfig) {
 		e.obs.plan = jobs
 		e.obs.cursor = 0
 	}
+	// Derived from whichever plan is now installed — the kept one when the
+	// reconcile was a no-op, so the index can never describe a plan the cursor is
+	// not walking.
+	e.obs.hops = indexChainHops(e.obs.plan)
 
 	if e.obs.stop == nil {
 		stop, done := make(chan struct{}), make(chan struct{})
@@ -162,7 +252,9 @@ func (e *Engine) StopObservatory() {
 	stop, done := e.obs.stop, e.obs.done
 	e.obs.stop, e.obs.done, e.obs.nudge = nil, nil, nil
 	e.obs.enabled = false
-	e.obs.plan, e.obs.used, e.obs.cursor = nil, nil, 0
+	e.obs.plan, e.obs.used, e.obs.cursor, e.obs.hops = nil, nil, 0, nil
+	e.obs.keepWarm = nil
+	e.obs.force = false
 	if stop != nil {
 		close(stop)
 	}
@@ -172,6 +264,49 @@ func (e *Engine) StopObservatory() {
 	}
 }
 
+// RefreshObservatory asks for an OUT-OF-TURN full pass of the probe plan: the
+// cursor is rewound to the top, the one-shot force flag is raised so the pass
+// bypasses the freshness gate (a "check now" that skipped everything fresh
+// would measure nothing and answer with yesterday's numbers), and the loop is
+// nudged so the first batch starts immediately instead of on the next 10s tick.
+//
+// This is the ONLY way the panel's "Test" button touches the network now: the
+// manual group test (grouptest.go) no longer dials anything itself — it calls
+// this, then watches the health board for observations newer than its start
+// instant. One prober, one set of dial paths, one truth.
+//
+// No-op when the observatory is disabled or has no plan: there is nothing to
+// walk, and raising force with no loop to consume it would only arm a stale
+// flag for a future enable. The caller (TestGroups) reports that situation
+// through its own results; this method stays silent about it on purpose —
+// it is a request, not a query.
+func (e *Engine) RefreshObservatory() {
+	e.obsMu.Lock()
+	defer e.obsMu.Unlock()
+	if !e.obs.enabled || len(e.obs.plan) == 0 {
+		return
+	}
+	e.obs.cursor = 0
+	e.obs.force = true
+	if e.obs.nudge != nil {
+		select {
+		case e.obs.nudge <- struct{}{}:
+		default:
+		}
+	}
+}
+
+// setProbeFn installs a stand-in for the per-job dial (observatoryState.probeFn).
+// It exists for the tests: the ordered walk promises that hops behind a dead one
+// are never DIALLED, and only an attempt counter can show that — the health
+// readout afterwards looks the same whether a job was skipped or run and
+// discarded. Passing nil restores the production path.
+func (e *Engine) setProbeFn(f func(ProbeJob)) {
+	e.obsMu.Lock()
+	defer e.obsMu.Unlock()
+	e.obs.probeFn = f
+}
+
 // ObservatoryStatus reports whether the observatory is on, how many measurements
 // the current plan holds, and how many full passes have completed. Cheap; used by
 // tests and available for diagnostics.
@@ -240,17 +375,18 @@ func (e *Engine) observatoryLoop(stop <-chan struct{}, done chan<- struct{}, nud
 // observatoryTickOnce runs one batch. It is deliberately conservative about when
 // it does nothing:
 //
-//   - a manual exit test (grouptest.go) is in flight => SKIP. That run is the
-//     human's explicit request; the observatory defers rather than competing for
-//     the uplink. It checks the guard but never CLAIMS it, so a pressed Test
-//     button can never be answered "already running" because of background work;
 //   - the previous tick has not finished => SKIP, so slow probes never stack;
 //   - the engine is stopped => jobs no longer resolve and are skipped (probeJob).
+//
+// There is deliberately NO deferral to a manual group test any more. The guard
+// that used to sit here ("skip the tick while e.groupTestRunning is up") made
+// sense when the manual run dialled the uplink itself and the two would have
+// competed for it; that run no longer probes ANYTHING — it asks for a forced
+// pass via RefreshObservatory and then reads the board. Keeping the guard would
+// therefore be worse than pointless: a manual run now DEPENDS on the
+// observatory ticking, and skipping ticks for its whole duration would deadlock
+// the very refresh it is waiting on (up to its full 120s deadline).
 func (e *Engine) observatoryTickOnce() {
-	if e.groupTestRunning.Load() {
-		return
-	}
-
 	e.obsMu.Lock()
 	if e.obs.busy || !e.obs.enabled || len(e.obs.plan) == 0 {
 		e.obsMu.Unlock()
@@ -258,7 +394,8 @@ func (e *Engine) observatoryTickOnce() {
 	}
 	if e.obs.cursor >= len(e.obs.plan) {
 		// Cycle complete: count it and start the next pass from the top. This is
-		// the ONE place the cursor is deliberately rewound.
+		// the ONE place the cursor is rewound on the observatory's own schedule
+		// (RefreshObservatory rewinds it too, but that is the caller's request).
 		e.obs.cycles++
 		e.obs.cursor = 0
 	}
@@ -270,12 +407,37 @@ func (e *Engine) observatoryTickOnce() {
 	batch := append([]ProbeJob(nil), e.obs.plan[start:end]...)
 	e.obs.cursor = end
 	interval := e.obs.interval
+	hops := e.obs.hops
+	probe := e.obs.probeFn
+	if probe == nil {
+		probe = e.probeJob
+	}
+	// A forced pass (RefreshObservatory) bypasses the freshness gate for its one
+	// walk of the plan. The flag is captured for THIS batch and cleared the
+	// moment the walk's last batch is scheduled: the remaining jobs of this very
+	// batch still run unfiltered off the captured copy, and the next pass is an
+	// ordinary polite one again.
+	force := e.obs.force
+	if force && end >= len(e.obs.plan) {
+		e.obs.force = false
+	}
 	e.obs.busy = true
 	e.obsMu.Unlock()
 
 	defer func() {
 		e.obsMu.Lock()
 		e.obs.busy = false
+		// Mid-forced-pass: chain straight into the next batch instead of waiting
+		// out the 10s tick. A human pressed "check now"; walking a subscription-
+		// sized plan at one batch per tick would stretch the answer past the
+		// manual run's deadline for no reason — the batch size still bounds how
+		// much is in flight at once, which is the limit that actually matters.
+		if e.obs.force && e.obs.nudge != nil {
+			select {
+			case e.obs.nudge <- struct{}{}:
+			default:
+			}
+		}
 		e.obsMu.Unlock()
 	}()
 
@@ -286,8 +448,13 @@ func (e *Engine) observatoryTickOnce() {
 	now := time.Now()
 	sem := make(chan struct{}, observatoryConcurrency)
 	var wg sync.WaitGroup
-	for _, j := range batch {
-		if !observatoryShouldProbe(hist, j, interval, now) {
+
+	// Two populations, two disciplines. A job that measures nothing on a chain is
+	// independent of every other job and runs as it always did: fire the lot,
+	// bounded by the semaphore. A chain's jobs are a SERIES and run hop by hop.
+	free, chains := splitChainBatch(batch)
+	for _, j := range free {
+		if j.SelfChecked || (!force && !observatoryShouldProbe(hist, j, interval, now)) {
 			continue
 		}
 		wg.Add(1)
@@ -295,12 +462,393 @@ func (e *Engine) observatoryTickOnce() {
 		go func(j ProbeJob) {
 			defer wg.Done()
 			defer func() { <-sem }()
-			e.probeJob(j)
+			probe(j)
 		}(j)
 	}
+	for _, walk := range chains {
+		wg.Add(1)
+		go func(walk chainBatch) {
+			defer wg.Done()
+			e.probeChainOrdered(walk, hops, hist, interval, now, force, sem, probe)
+		}(walk)
+	}
 	wg.Wait()
 }
 
+// probeChainOrdered runs one chain's share of a batch hop by hop, in wire order,
+// and RETURNS the moment a hop below a dead one comes up — leaving every
+// remaining job of that chain undialled for this pass.
+//
+// # Why the walk stops rather than filtering
+//
+// A chain is a series path: the plan's job for hop i dials through hops 1..i-1.
+// If hop 2 has no live member, the hop-3 probe fails INSIDE hop 2, learns nothing
+// about hop 3, and costs a full probe timeout to learn it. Recording that as
+// "hop 3 is dead" is the exact class of lie this codebase keeps removing: dead is
+// a positive finding about the thing that was probed, and hop 3 was never
+// reached. So the walk stops, the skipped hops keep no fresh observation, and
+// they surface as untested WITH the blocking hop named (grouphealth.go
+// ChainHopHealth.BlockedBy) rather than as faults of their own.
+//
+// # Why it cannot get stuck
+//
+// Nothing is remembered between passes. The gate is re-read from the health board
+// at every hop of every batch (blockedBefore), and the cursor rewinds to the top
+// of the plan each cycle, so the FIRST dead hop is always re-probed — it is never
+// itself below a break. The moment it answers again the gate opens and the rest
+// of the chain is probed in the same pass. There is no "blocked" flag anywhere to
+// clear, which is the point: a cached block would outlive the failure that caused
+// it.
+//
+// # Cost
+//
+// Hops within one batch are serialised, so a batch holding several hops of one
+// chain can take hops x probeTimeout instead of one probeTimeout. The busy guard
+// in observatoryTickOnce absorbs that (a slow tick makes the next a no-op), and
+// the case it exists for — a dead hop — now costs LESS than before, because
+// everything behind it is skipped entirely.
+func (e *Engine) probeChainOrdered(
+	walk chainBatch,
+	idx chainPlanIndex,
+	hist *urltest.HistoryStorage,
+	interval time.Duration,
+	now time.Time,
+	force bool,
+	sem chan struct{},
+	probe func(ProbeJob),
+) {
+	for _, hop := range walk.hops {
+		// Re-read the board here, not once up front: the hop before this one may
+		// have been probed moments ago by the loop's previous iteration, and its
+		// verdict is exactly what decides this one.
+		if blocker, blocked := idx.blockedBefore(walk.chain, hop.index, e.HealthView()); blocked {
+			e.markChainExitDead(walk.chain, idx, blocker)
+			return // hops are ascending, so everything left is behind the same break
+		}
+		var wg sync.WaitGroup
+		for _, j := range hop.jobs {
+			// A self-checked job is measured by its own group's checker, which is
+			// gated by ProbeAllowed on the same rule this loop applies. It is
+			// present here only so the hop shows up in the map the gate reads;
+			// dialling it would be the duplicate the flag exists to remove.
+			if j.SelfChecked || (!force && !observatoryShouldProbe(hist, j, interval, now)) {
+				continue
+			}
+			wg.Add(1)
+			sem <- struct{}{}
+			go func(j ProbeJob) {
+				defer wg.Done()
+				defer func() { <-sem }()
+				probe(j)
+			}(j)
+		}
+		// The hop's verdict is only complete when every member copy has answered,
+		// so the next hop's gate must not be read until they have.
+		wg.Wait()
+	}
+}
+
+// markChainExitDead records a DEAD verdict for a chain's EXIT on the shared
+// health board when the ordered walk stopped short of it.
+//
+// # Two questions, two answers, and why they differ
+//
+// "Is this hop's own node alive?" and "can this chain carry traffic?" are not the
+// same question, and behind a break they have different answers:
+//
+//   - the HOP's own health is genuinely UNKNOWN. Nothing dialled it, so its card
+//     reads untested with BlockedBy naming the blocker (markBlockedHops), and
+//     that stays exactly as it is;
+//   - the CHAIN's health is KNOWN, and the answer is no. A chain is a series
+//     path; hop k was probed and did not answer — a positive finding — and every
+//     dial through the chain crosses hop k. The exit cannot carry traffic. That
+//     is not absence of knowledge, it is a conclusion drawn from a measurement
+//     somebody took.
+//
+// # Why the board must say so
+//
+// The board is not only the panel's data source; the strategies select on it, and
+// they rank UNTESTED ABOVE DEAD on purpose (protocol/group/urltest_health_lx.go
+// selectExcluding: "an untested one is a better bet than a known-dead one"). So
+// leaving a provably-broken path untested is not a neutral silence — it is an
+// invitation to route traffic down it, which is worse than the mislabelling this
+// whole change set exists to remove. Marking it keeps
+// TestChainDeadExitMarkedByObservatory's older invariant intact too: a dead chain
+// is marked dead, and a dead entry stays distinguishable from a never-measured
+// one.
+//
+// # Why this is not a fabricated verdict
+//
+// Nothing here claims a probe happened. It records the CONSEQUENCE of a probe
+// that did happen, one hop earlier, and it is re-derived from the board every
+// pass — the instant the blocking hop answers again, the walk proceeds and the
+// exit's next verdict is a real measurement of its own. The flip is logged with
+// its cause so the trail never reads as an unexplained death.
+//
+// The exit's tags are the LAST hop's measurement tags: the wrapper itself for a
+// node exit, the member copies for a group exit — in both cases exactly what a
+// reader consults to ask whether the chain works.
+func (e *Engine) markChainExitDead(chain string, idx chainPlanIndex, blocker chainPlanHop) {
+	hops := idx[chain]
+	if len(hops) == 0 {
+		return
+	}
+	exit := hops[len(hops)-1]
+	if exit.index <= blocker.index {
+		return // the blocker IS the exit: its own probe already said so
+	}
+	hist := e.URLTestHistory()
+	if hist == nil {
+		return
+	}
+	ttl := e.healthTTL()
+	for _, tag := range exit.tags {
+		if e.log != nil && hist.Verdict(tag, ttl) != urltest.VerdictDead {
+			e.log.Info("observatory: ", tag, ": -> dead (chain hop ", blocker.index,
+				" was probed and did not answer, so nothing reaches the exit through it)")
+		}
+		hist.MarkFailed(tag)
+	}
+}
+
+// chainBatch is one chain's share of one batch, grouped into hops in wire order.
+type chainBatch struct {
+	chain string
+	hops  []chainBatchHop
+}
+
+// chainBatchHop is the jobs of ONE hop that fell inside the current batch. It is
+// a subset: a hop with more member copies than a batch holds is split across
+// consecutive batches, which is harmless — the gate is computed from the whole
+// plan (chainPlanIndex), never from what happens to be in hand.
+type chainBatchHop struct {
+	index int
+	jobs  []ProbeJob
+}
+
+// splitChainBatch separates a batch into the chain-less jobs and the per-chain
+// series. The plan is already sorted so a chain's jobs are contiguous and
+// ascending, but this does not rely on that: it groups explicitly and sorts each
+// chain's hops, so a batch that arrived out of order still runs in path order.
+func splitChainBatch(batch []ProbeJob) (free []ProbeJob, chains []chainBatch) {
+	at := map[string]int{}
+	for _, j := range batch {
+		if j.Chain == "" {
+			free = append(free, j)
+			continue
+		}
+		i, ok := at[j.Chain]
+		if !ok {
+			i = len(chains)
+			at[j.Chain] = i
+			chains = append(chains, chainBatch{chain: j.Chain})
+		}
+		c := &chains[i]
+		h := -1
+		for k := range c.hops {
+			if c.hops[k].index == j.Hop {
+				h = k
+				break
+			}
+		}
+		if h < 0 {
+			c.hops = append(c.hops, chainBatchHop{index: j.Hop})
+			h = len(c.hops) - 1
+		}
+		c.hops[h].jobs = append(c.hops[h].jobs, j)
+	}
+	for i := range chains {
+		hops := chains[i].hops
+		sort.Slice(hops, func(a, b int) bool { return hops[a].index < hops[b].index })
+	}
+	return free, chains
+}
+
+// chainPlanIndex is the whole plan's chain measurements: chain name -> hops in
+// wire order, each carrying every DIAL TAG that measures it.
+//
+// It is the gate's source of truth, and it must come from the PLAN rather than
+// from the running box, for the same reason probeJob resolves its dial tag at
+// dial time: the plan is what this pass is walking. A hop's verdict rolled up
+// from a set of tags the pass does not probe would gate the pass on somebody
+// else's measurements.
+type chainPlanIndex map[string][]chainPlanHop
+
+type chainPlanHop struct {
+	index int
+	tags  []string
+}
+
+// indexChainHops builds the index from a plan. nil for a plan with no chain in it
+// — the overwhelmingly common case, and a nil map answers every lookup with "no
+// hops, nothing blocks" at no cost.
+func indexChainHops(plan []ProbeJob) chainPlanIndex {
+	var out chainPlanIndex
+	for _, j := range plan {
+		if j.Chain == "" {
+			continue
+		}
+		if out == nil {
+			out = chainPlanIndex{}
+		}
+		hops := out[j.Chain]
+		at := -1
+		for k := range hops {
+			if hops[k].index == j.Hop {
+				at = k
+				break
+			}
+		}
+		if at < 0 {
+			hops = append(hops, chainPlanHop{index: j.Hop})
+			at = len(hops) - 1
+		}
+		hops[at].tags = append(hops[at].tags, j.Dial)
+		out[j.Chain] = hops
+	}
+	for name, hops := range out {
+		sort.Slice(hops, func(a, b int) bool { return hops[a].index < hops[b].index })
+		out[name] = hops
+	}
+	return out
+}
+
+// blockedBefore reports the LOWEST hop of chain, strictly before hop, that the
+// health board currently reads dead — the hop that stops the walk. Only a
+// positive dead finding blocks: an untested hop in front means nothing is known
+// yet, and refusing to probe on "nothing is known" would make a cold start
+// permanent.
+func (idx chainPlanIndex) blockedBefore(chain string, hop int, view HealthView) (chainPlanHop, bool) {
+	for _, h := range idx[chain] {
+		if h.index >= hop {
+			break
+		}
+		if hopBoardState(h.tags, view) == HealthDead {
+			return h, true
+		}
+	}
+	return chainPlanHop{}, false
+}
+
+// ProbeAllowed implements urltest.ProbeGate: it answers, for one outbound tag,
+// whether that outbound's OWN probing schedule may dial right now.
+//
+// # What it is for
+//
+// The observatory is not the only prober. A urltest group probes its own
+// members — a warm-up sweep at box start and a ticker while traffic touches it
+// — and a chain's GROUP HOP is materialised as exactly such a group
+// ("chain-<n>-h<i>", generate/chain.go). Every member of that group dials
+// through the hop in front of it, so when THAT hop is dead the group's sweep
+// spends one probe timeout per member to rediscover the same broken hop, and
+// files each result against a hop nothing reached. The projection already
+// refuses to publish those readings (markBlockedHops); this stops them being
+// taken.
+//
+// # Why this is not the SelfCheck flag
+//
+// standDownUnusedSelfCheck answers a different question — "does any enabled
+// rule reach this group at all" — decided once, written into the config that
+// reaches box.New, and true for the whole life of that box. A chain hop wrapper
+// IS reached by the rules and must keep its flag (TestStandDownNeverTouchesChainHopWrappers
+// pins that). This answers "is the path in front of it working AT THIS MOMENT",
+// which changes on its own and must therefore be asked, never stored. Merging
+// the two would either put a live question in a static flag or take a used
+// chain permanently off the board.
+//
+// # Why it cannot get stuck
+//
+// It holds nothing. Every call recomputes the block from the same plan index
+// and the same health board the observatory's own walk uses, so the moment the
+// blocking hop answers a probe this returns true again and the wrapper's next
+// tick dials normally. The blocking hop is itself never behind a break, so the
+// observatory re-probes it every cycle: there is always something re-asking the
+// question that produced the block.
+//
+// It answers TRUE whenever it does not know — no plan, observatory disabled,
+// not a chain hop tag, chain absent from the plan. Refusing on missing
+// information would silence probing exactly when least is known, and nothing
+// would ever measure its way back out.
+func (e *Engine) ProbeAllowed(tag string) bool {
+	chain, hop, ok := parseChainHopTag(tag)
+	if !ok {
+		return true // not a chain hop wrapper: nothing sits in front of it
+	}
+	e.obsMu.Lock()
+	enabled, idx := e.obs.enabled, e.obs.hops
+	e.obsMu.Unlock()
+	if !enabled || idx == nil {
+		return true
+	}
+	// HealthView takes obsMu itself (healthTTL), so it must be called with the
+	// lock released.
+	_, blocked := idx.blockedBefore(chain, hop, e.HealthView())
+	return !blocked
+}
+
+// ProbeWhenIdle implements urltest.ProbeGate: it reports whether an outbound
+// must keep measuring itself with no traffic flowing through it.
+//
+// TRUE for anything the routing configuration reaches. Such a group's health is
+// a live question at all times — a rule matching a narrow domain list is in
+// force whether or not it fired in the last half hour — and without this its own
+// ticker would retire on the idle timeout and NOTHING would replace it: the
+// observatory stands off a urltest group's members by design (ProbeJob.SelfChecked).
+// The panel would then report "untested" about an armed rule, and the first
+// request through it would pay a cold probe instead of picking a member already
+// known to work. Keeping the group's own ticker running restores exactly what
+// the observatory used to spend on those same targets, at the same interval, and
+// keeps it to ONE dialler.
+//
+// FALSE whenever nothing is known — no config applied yet, unknown tag, master
+// switch off. This method adds work rather than withholding it, so the direction
+// of "don't know" is the opposite of ProbeAllowed's: claiming keep-warm on
+// missing information would leave groups probing forever for no stated reason.
+//
+// It does NOT bypass any other refusal. A stood-down group never asks (the
+// group's own keepWarm short-circuits on selfCheckDisabled), and a kept-warm
+// group behind a dead hop still ticks WITHOUT dialling, because every tick goes
+// through scheduledCheck -> ProbeAllowed. Warm means measured on schedule, never
+// "hammering through a broken path".
+func (e *Engine) ProbeWhenIdle(tag string) bool {
+	e.obsMu.Lock()
+	defer e.obsMu.Unlock()
+	return e.obs.keepWarm[tag]
+}
+
+// publishKeepWarm installs the set ProbeWhenIdle answers from. Called from
+// applyLocked with the used-set of the config being applied, and cleared when
+// background probing is switched off — see the keepWarm field for why this is
+// published on a different schedule from the observatory's own used-set.
+func (e *Engine) publishKeepWarm(used map[string]bool) {
+	e.obsMu.Lock()
+	e.obs.keepWarm = used
+	e.obsMu.Unlock()
+}
+
+// hopBoardState rolls a hop's measurement tags into one verdict, by exactly the
+// rule chainGroupHop (grouphealth.go) uses for the same question — "can this hop
+// carry the chain": alive as soon as anything answers, dead only when something
+// was tested and nothing answers, untested when nothing was tested at all. The
+// prober and the panel must agree about which hop broke a chain, so this is that
+// rule written once more against tags instead of against a materialised group;
+// TestChainHopGateMatchesProjection pins the two together.
+func hopBoardState(tags []string, view HealthView) string {
+	tested := 0
+	for _, tag := range tags {
+		switch state, _, _ := view.State(tag); state {
+		case HealthAlive:
+			return HealthAlive
+		case HealthDead:
+			tested++
+		}
+	}
+	if tested > 0 {
+		return HealthDead
+	}
+	return HealthUntested
+}
+
 // observatoryShouldProbe is the freshness gate: a job runs when ANY tag it
 // covers has no observation at all, or its newest observation — success OR
 // failure — is older than the global probe interval.

shater/engine/probeplan.go

@@ -27,8 +27,12 @@ import (
 //   - a CHAIN entry tag "chain-<n>-hN" (the exit wrapper, generate/chain.go) is
 //     probed ITSELF: dialling the copy pulls the whole L1→…→Ln path through its
 //     Detour links, so one probe is the chain's end-to-end health. Intermediate
-//     NODE hops are not probed separately — their death is visible in the exit
-//     probe and no selection depends on them;
+//     NODE hop wrappers ("chain-<n>-h<i>" that are plain outbounds) are probed
+//     TOO — not because selection needs them (it does not), but because an
+//     operator does: dialling "chain-<n>-h1" measures exactly the prefix of the
+//     path up to and including hop 1, so when the exit probe dies the per-hop
+//     probes say WHICH hop died instead of only that the chain did (the
+//     ChainHealth hops surface, grouphealth.go);
 //   - a GROUP hop inside a chain ("chain-<n>-h<i>", a wrapper selector reached by
 //     walking the exit's Detour links) additionally has its member copies
 //     "chain-<n>-h<i>-<member>" probed: they are the wrapper's selection
@@ -47,6 +51,28 @@ import (
 // ever being dialled by the observatory. Chain copies are recorded only under
 // themselves: a chain prefix is nobody else's path.
 //
+// # Who dials
+//
+// Reaching a target does not make it OURS to dial. A member of a urltest group
+// already has a prober — that group's own checker — and for a chain hop that
+// checker measures the better path: the member copy carries the previous hop as
+// its Detour, so its probe travels the chain prefix without anything having to
+// be arranged. Those measurements are emitted with SelfChecked set and the
+// observatory never dials them. What stays ours is what no group probes: node
+// hop wrappers, endpoints, selector members, stood-down groups' members, and
+// egress copies (whose base-tag alias no checker writes).
+//
+// # Order
+//
+// The plan is a LIST, not a set, and for a chain the order is load-bearing. Each
+// job that measures a chain carries the chain name and its 1-based hop index
+// (ProbeJob.Chain/Hop, threaded through the walk as planPos), and the emitted
+// plan is sorted so a chain's jobs come out grouped and ascending by hop. The
+// observatory walks them in that order and stops at the first dead hop —
+// probing hop 3 through a hop 2 with no live member measures hop 2's failure and
+// nothing else, so recording it against hop 3 names the wrong suspect and burns a
+// probe timeout doing it.
+//
 // # Dedup
 //
 // The probe URL is global (plan §5.D), so the dedup key is the dial path alone:
@@ -65,6 +91,42 @@ type ProbeJob struct {
 	Dial  string
 	URL   string
 	Store []string
+	// Chain and Hop place this measurement ON a chain's path: the chain NAME and
+	// the 1-based hop index it measures. Both zero ("" / 0) for every job that is
+	// not part of a chain, which is most of them.
+	//
+	// This is what makes the walk ORDERED (observatory.go). A chain is a series
+	// path, so the probe of hop 3 dials THROUGH hops 1 and 2; if hop 2 has no live
+	// member, that probe fails at hop 2 and says nothing whatsoever about hop 3.
+	// Recording it as a verdict about hop 3 was the defect: it pointed the
+	// operator at the wrong hop and spent a full probe timeout to do it. The
+	// prober therefore walks a chain h1 -> hN and stops at the first dead hop, and
+	// these two fields are the only thing that tells it which jobs form which hop
+	// of which path.
+	//
+	// A hop is usually SEVERAL jobs — a group hop's member copies are one
+	// measurement each — so Hop is a grouping key, never a job's identity.
+	Chain string
+	Hop   int
+	// SelfChecked marks a measurement whose target already has a dialler: it is a
+	// member of a URLTEST group, and that group's own checker probes exactly this
+	// outbound, on exactly this dial path, on the same global interval
+	// (protocol/group/urltest.go testNodes). The observatory therefore does NOT
+	// dial it. One target, one dialler.
+	//
+	// The group's checker is the RIGHT dialler for these, not merely an equal
+	// one: a chain hop member carries the Detour of the hop in front of it, so
+	// the checker's probe travels the chain prefix by construction, which is the
+	// path the traffic takes. Nothing had to be arranged for that to be true.
+	//
+	// The job STAYS in the plan rather than being dropped, because the plan is
+	// two things at once: the observatory's dial list AND the map of which tags
+	// measure which hop of which chain. The ordered walk's gate reads the second
+	// (indexChainHops -> blockedBefore), and it must see every measurement of a
+	// hop, not only the ones this prober performs — a hop whose members are all
+	// self-checked would otherwise vanish from the map and stop being able to
+	// block anything.
+	SelfChecked bool
 }
 
 // BuildObservatoryPlan derives the observatory's probe plan from the applied
@@ -77,7 +139,7 @@ type ProbeJob struct {
 // probeURL is the global probe URL; "" lets urltest fall back to its gstatic
 // default, exactly as the generator does.
 //
-// The result is deterministic (sorted by Dial) so a cursor walking it across
+// The result is deterministic (see probeJobLess) so a cursor walking it across
 // ticks is stable, and plansEqual can tell a no-op reconcile from real change.
 func BuildObservatoryPlan(opts option.Options, probeURL string) ([]ProbeJob, map[string]bool) {
 	w := &planWalk{
@@ -88,7 +150,9 @@ func BuildObservatoryPlan(opts option.Options, probeURL string) ([]ProbeJob, map
 		targets: map[string]*planTarget{},
 	}
 	for _, root := range planRoots(opts) {
-		w.visit(root)
+		// A root comes from a routing rule, not from a group, so nothing is
+		// checking it on our behalf.
+		w.visit(root, planPos{}, false)
 	}
 
 	jobs := make([]ProbeJob, 0, len(w.targets))
@@ -98,18 +162,49 @@ func BuildObservatoryPlan(opts option.Options, probeURL string) ([]ProbeJob, map
 			store = append(store, tag)
 		}
 		sort.Strings(store)
-		jobs = append(jobs, ProbeJob{Dial: t.dial, URL: probeURL, Store: store})
+		jobs = append(jobs, ProbeJob{
+			Dial: t.dial, URL: probeURL, Store: store,
+			Chain: t.pos.chain, Hop: t.pos.hop, SelfChecked: t.selfChecked,
+		})
 	}
-	sort.Slice(jobs, func(i, j int) bool { return jobs[i].Dial < jobs[j].Dial })
+	sort.Slice(jobs, func(i, j int) bool { return probeJobLess(jobs[i], jobs[j]) })
 	return jobs, w.used
 }
 
+// probeJobLess is the plan's order, and it is a CONTRACT rather than a
+// presentation detail: a chain's jobs come out grouped by chain and ascending by
+// hop, so a cursor walking the plan meets h1 before h2 before h3. The observatory
+// relies on that to stop at the first dead hop (see probeChainOrdered); a plan
+// sorted by dial tag alone would interleave the hops of one path and there would
+// be no "first" to stop at.
+//
+// Chain-less jobs (Chain "") sort first, among themselves by dial tag, exactly as
+// the whole plan used to. Ties inside one hop break on the dial tag, so the order
+// is total and two builds of one config are byte-identical — the property
+// ConfigureObservatory's cursor keeping depends on.
+func probeJobLess(a, b ProbeJob) bool {
+	if a.Chain != b.Chain {
+		return a.Chain < b.Chain
+	}
+	if a.Hop != b.Hop {
+		return a.Hop < b.Hop
+	}
+	return a.Dial < b.Dial
+}
+
 // planEntry is the slice of one configured outbound/endpoint the planner needs.
 type planEntry struct {
 	typ     string
 	group   bool     // selector/urltest — expanded, never dialled
 	members []string // group members, in config order
 	detour  string   // DialerOptions.Detour ("" when none)
+	// selfChecks is true for a group that PROBES ITS OWN MEMBERS: a urltest
+	// group whose self-check has not been stood down. A selector never does (it
+	// has no checker at all — protocol/group/selector.go), and a urltest group
+	// with SelfCheck=false has had its schedule taken away deliberately
+	// (standDownUnusedSelfCheck), so in both of those cases the observatory is
+	// the only possible dialler and must remain one.
+	selfChecks bool
 }
 
 // planIndex flattens opts into tag -> planEntry. Endpoints (wireguard/AmneziaWG)
@@ -119,10 +214,11 @@ func planIndex(opts option.Options) map[string]planEntry {
 	byTag := make(map[string]planEntry, len(opts.Outbounds)+len(opts.Endpoints))
 	for _, ob := range opts.Outbounds {
 		byTag[ob.Tag] = planEntry{
-			typ:     ob.Type,
-			group:   ob.Type == C.TypeSelector || ob.Type == C.TypeURLTest,
-			members: groupMembersOf(ob.Options),
-			detour:  detourOf(ob.Options),
+			typ:        ob.Type,
+			group:      ob.Type == C.TypeSelector || ob.Type == C.TypeURLTest,
+			members:    groupMembersOf(ob.Options),
+			detour:     detourOf(ob.Options),
+			selfChecks: ob.Type == C.TypeURLTest && urltestSelfChecks(ob.Options),
 		}
 	}
 	for _, ep := range opts.Endpoints {
@@ -188,6 +284,23 @@ func groupMembersOf(options any) []string {
 	return nil
 }
 
+// urltestSelfChecks reports whether a urltest group still runs its OWN probing
+// schedule. nil/absent SelfCheck means on — the documented compatibility default
+// (option.URLTestOutboundOptions.SelfCheck) — and only an explicit false stands
+// it down. An options struct of an unexpected shape is read as "on", the same
+// direction standDownUnusedSelfCheck refuses to guess in: assuming a checker
+// exists when it does not would leave a target with NO dialler, which is worse
+// than one dialled twice.
+func urltestSelfChecks(options any) bool {
+	switch o := options.(type) {
+	case *option.URLTestOutboundOptions:
+		return o.SelfCheck == nil || *o.SelfCheck
+	case option.URLTestOutboundOptions:
+		return o.SelfCheck == nil || *o.SelfCheck
+	}
+	return true
+}
+
 // detourOf reads DialerOptions.Detour out of any outbound/endpoint/dns-server
 // option struct that carries dialer options (they all embed option.DialerOptions
 // and therefore satisfy DialerOptionsWrapper); "" for everything else.
@@ -211,11 +324,40 @@ func probeablePlanTag(typ, tag string) bool {
 	return !strings.HasPrefix(tag, "egress-")
 }
 
+// planPos is where a measurement sits on a chain: the chain name and the 1-based
+// hop index. The zero value means "not on any chain", which is what every job
+// outside a chain carries.
+//
+// It is threaded through the walk rather than parsed back out of each tag,
+// because only the walk knows the STRUCTURE. A hop wrapper's tag names its own
+// position ("chain-<n>-h<i>"), but a group hop's member copies
+// ("chain-<n>-h<i>-<member>") end in free-form operator text: a node named
+// "h2-eu" would make the tag ambiguous to any parser. Inheriting the position
+// from the wrapper the walk arrived through has no such failure mode.
+type planPos struct {
+	chain string
+	hop   int
+}
+
+// chainPosOf reads a hop WRAPPER tag's own position, or the zero position for
+// anything else.
+func chainPosOf(tag string) planPos {
+	if name, hop, ok := parseChainHopTag(tag); ok {
+		return planPos{chain: name, hop: hop}
+	}
+	return planPos{}
+}
+
 // planTarget accumulates one dial path's job: the tag to dial (the smallest
-// dialable tag, for determinism) and every tag the measurement is recorded under.
+// dialable tag, for determinism), every tag the measurement is recorded under,
+// and where on a chain it sits (planPos).
 type planTarget struct {
 	dial  string
 	store map[string]bool
+	pos   planPos
+	// selfChecked: some group's own checker already dials this path, so the
+	// observatory must not. See ProbeJob.SelfChecked.
+	selfChecked bool
 }
 
 // planWalk is the reachability expansion state.
@@ -230,13 +372,30 @@ type planWalk struct {
 // addTarget registers tag as a measurement under the dial-path key, recording it
 // under itself plus aliases. When several dialable tags share one path (copies of
 // one node through one egress across groups) the smallest dials, deterministically.
-func (w *planWalk) addTarget(key, tag string, aliases ...string) {
+//
+// pos is the chain position and is taken from the FIRST arrival only. Two
+// arrivals at one dial-path key are copies of the same node through the same
+// egress (see the file header), which live outside every chain and therefore
+// carry the zero position anyway; a chain tag's dial key is its own tag, so it is
+// reached once by construction. Never overwriting means a shared key cannot have
+// its position depend on walk order.
+//
+// selfChecked, by contrast, is ANDed across arrivals, and the asymmetry is
+// deliberate: it claims "somebody else already dials this", and one path that
+// reaches the target WITHOUT a checker in front of it (a selector member, a
+// stood-down group) falsifies the claim outright. Merging the pessimistic way
+// can only cost a duplicate probe; merging the optimistic way would leave a
+// target with no dialler at all.
+func (w *planWalk) addTarget(key string, pos planPos, selfChecked bool, tag string, aliases ...string) {
 	t := w.targets[key]
 	if t == nil {
-		t = &planTarget{dial: tag, store: map[string]bool{}}
+		t = &planTarget{dial: tag, store: map[string]bool{}, pos: pos, selfChecked: selfChecked}
 		w.targets[key] = t
-	} else if tag < t.dial {
-		t.dial = tag
+	} else {
+		if tag < t.dial {
+			t.dial = tag
+		}
+		t.selfChecked = t.selfChecked && selfChecked
 	}
 	t.store[tag] = true
 	for _, a := range aliases {
@@ -247,7 +406,15 @@ func (w *planWalk) addTarget(key, tag string, aliases ...string) {
 // visit expands one root/member reference. Groups recurse into their members;
 // leaves become measurements; a leaf's Detour link is then walked to surface the
 // group hops of a chain (walkDetour).
-func (w *planWalk) visit(tag string) {
+//
+// pos is the chain position INHERITED from the group this reference was expanded
+// out of; a tag that names its own hop position (a chain hop wrapper, typically
+// the exit reached straight from a rule) overrides it.
+//
+// selfChecked says the group this reference came out of probes its own members,
+// so the leaf reached here already has a dialler. A ROOT is reached from a
+// routing rule, which has no checker behind it, so roots start false.
+func (w *planWalk) visit(tag string, pos planPos, selfChecked bool) {
 	if tag == "" || w.visited[tag] {
 		return
 	}
@@ -257,14 +424,17 @@ func (w *planWalk) visit(tag string) {
 		return // dangling reference; box.New would have refused it anyway
 	}
 	w.used[tag] = true
+	if own := chainPosOf(tag); own.chain != "" {
+		pos = own
+	}
 	if ent.group {
 		for _, m := range ent.members {
-			w.visitMember(tag, m)
+			w.visitMember(tag, m, pos, ent.selfChecks)
 		}
 		return
 	}
 	if probeablePlanTag(ent.typ, tag) {
-		w.addTarget(dialKeyTag(tag), tag)
+		w.addTarget(dialKeyTag(tag), pos, selfChecked, tag)
 	}
 	w.walkDetour(ent.detour)
 }
@@ -273,26 +443,49 @@ func (w *planWalk) visit(tag string) {
 // ("group-<group>-m<i>-<member>") is keyed by its (node, egress) dial path — so
 // two groups bound to one egress share the measurement — and recorded under the
 // copy AND the base node tag (the store alias). Every other member is expanded
-// exactly like a root.
-func (w *planWalk) visitMember(group, member string) {
+// exactly like a root, carrying the owning group's chain position down with it:
+// the members of a chain's exit group hop are measurements OF that hop.
+//
+// The EGRESS COPY branch is deliberately never marked self-checked, even when
+// the owning group runs its own checker. Such a job carries a second store tag —
+// the base node tag, the alias that keeps the per-node stats surface filled in
+// without the base outbound (a different dial path) ever being dialled. The
+// group's checker records under the copy tag ALONE (protocol/group/urltest.go
+// stores under RealTag), so handing this measurement over would silently drop
+// the alias and empty /api/stats node_health for every egress-bound node. That
+// is a real duplicate probe knowingly kept, and the alternative is losing data
+// no other writer produces.
+func (w *planWalk) visitMember(group, member string, pos planPos, ownerSelfChecks bool) {
 	ent, ok := w.byTag[member]
 	if !ok {
 		return
 	}
 	if node, isCopy := parseGroupCopyTag(group, member); isCopy && !ent.group {
 		w.used[member] = true
-		w.addTarget(dialKeyCopy(node, ent.detour), member, node)
+		w.addTarget(dialKeyCopy(node, ent.detour), pos, false, member, node)
 		w.walkDetour(ent.detour)
 		return
 	}
-	w.visit(member)
+	w.visit(member, pos, ownerSelfChecks)
 }
 
 // walkDetour follows a probed tag's Detour links toward the router. A group met
 // on the way is a chain's GROUP HOP wrapper: its member copies are selection
 // candidates and are probed, each through its own prefix of the path. A plain
-// outbound met on the way is an intermediate node hop — marked used, never probed
-// on its own — and the walk continues through it.
+// outbound met on the way falls in two halves:
+//
+//   - a chain NODE HOP wrapper ("chain-<n>-h<i>", parseChainExitTag) is probed
+//     as a measurement of its own. Dialling it pulls exactly the prefix of the
+//     chain up to and including hop i through the Detour links, so its verdict
+//     LOCALISES a failure: the exit probe says the chain died, the hop probes
+//     say where. It used to be skipped here on the argument that its death
+//     shows up in the exit probe anyway — true for end-to-end health, useless
+//     for an operator staring at a dead 4-hop chain. The measurement is stored
+//     under the wrapper tag alone: a chain prefix is nobody else's dial path,
+//     so no alias may borrow its verdict;
+//   - anything else on the detour path — an "egress-…" interface outbound, an
+//     ordinary node's egress detour — keeps today's behaviour: marked used,
+//     never probed on its own, and the walk continues through it.
 func (w *planWalk) walkDetour(tag string) {
 	if tag == "" || w.walked[tag] {
 		return
@@ -303,25 +496,38 @@ func (w *planWalk) walkDetour(tag string) {
 		return
 	}
 	w.used[tag] = true
-	if ent.group {
-		next := ""
-		for _, m := range ent.members {
-			ment, ok := w.byTag[m]
-			if !ok {
-				continue
-			}
-			w.used[m] = true
-			if !ment.group && probeablePlanTag(ment.typ, m) {
-				w.addTarget(dialKeyTag(m), m)
-			}
-			if next == "" {
-				next = ment.detour // every member detours into the same previous hop
-			}
+	// A tag met on a detour path names its own position or has none: the walk
+	// arrived here from BELOW (a later hop detouring back), so there is nothing
+	// sensible to inherit.
+	pos := chainPosOf(tag)
+	if !ent.group {
+		// A NODE hop wrapper is nobody's member, so no checker exists for it and
+		// the observatory is its only possible dialler.
+		if pos.chain != "" && probeablePlanTag(ent.typ, tag) {
+			w.addTarget(dialKeyTag(tag), pos, false, tag)
 		}
-		w.walkDetour(next)
+		w.walkDetour(ent.detour)
 		return
 	}
-	w.walkDetour(ent.detour)
+	next := ""
+	for _, m := range ent.members {
+		ment, ok := w.byTag[m]
+		if !ok {
+			continue
+		}
+		w.used[m] = true
+		if !ment.group && probeablePlanTag(ment.typ, m) {
+			// A GROUP hop's members are that wrapper's members: when the wrapper
+			// is a urltest group it probes them itself, and it probes them along
+			// the chain prefix, because each copy carries the previous hop as its
+			// Detour. A selector wrapper has no checker, so those stay ours.
+			w.addTarget(dialKeyTag(m), pos, ent.selfChecks, m)
+		}
+		if next == "" {
+			next = ment.detour // every member detours into the same previous hop
+		}
+	}
+	w.walkDetour(next)
 }
 
 // dialKeyTag / dialKeyCopy build the dedup key for one measurement target.
@@ -342,8 +548,14 @@ func dialKeyCopy(node, egress string) string {
 func lenPrefixed(s string) string { return fmt.Sprintf("%d:%s", len(s), s) }
 
 // plansEqual reports whether two plans describe exactly the same measurements —
-// same jobs, same order, same dial tag, same URL, same store set. This is what
-// lets a no-op reconcile keep the observatory's cursor (see ConfigureObservatory).
+// same jobs, same order, same dial tag, same URL, same store set, same chain
+// position. This is what lets a no-op reconcile keep the observatory's cursor
+// (see ConfigureObservatory).
+//
+// The chain position is part of the comparison because it is part of the PLAN,
+// not an annotation on it: it decides the order jobs run in and which of them the
+// ordered walk may skip. A config that moves a node from hop 2 to hop 3 without
+// changing any dial tag is a different plan and must restart the cursor.
 func plansEqual(a, b []ProbeJob) bool {
 	if len(a) != len(b) {
 		return false
@@ -352,6 +564,9 @@ func plansEqual(a, b []ProbeJob) bool {
 		if a[i].Dial != b[i].Dial || a[i].URL != b[i].URL || len(a[i].Store) != len(b[i].Store) {
 			return false
 		}
+		if a[i].Chain != b[i].Chain || a[i].Hop != b[i].Hop || a[i].SelfChecked != b[i].SelfChecked {
+			return false
+		}
 		for j := range a[i].Store {
 			if a[i].Store[j] != b[i].Store[j] {
 				return false

shater/engine/probeplan_test.go

@@ -218,6 +218,64 @@ func TestObservatoryPlanChain(t *testing.T) {
 	}
 }
 
+// A chain with a plain NODE hop wrapper on the way to the exit: the wrapper IS
+// a measurement of its own now (walkDetour) — dialling "chain-<c>-h1" measures
+// exactly the path prefix up to hop 1, which is what localises a dead hop. And
+// the other half of the honesty contract: a base node that appears ONLY as a
+// chain group-hop member is never measured under its BASE tag — the member
+// copy's observation stays on the copy, because the copy's dial path (through
+// the chain prefix) is not the base outbound's dial path, and no job may write
+// a direct-looking verdict onto a tag it did not dial.
+func TestObservatoryPlanChainHopWrappersProbed(t *testing.T) {
+	opts := option.Options{
+		Outbounds: []option.Outbound{
+			// L1: a plain node hop wrapper.
+			fixNode("chain-c2-h1", ""),
+			// L2: a group hop over one member copy of base node N.
+			fixNode("chain-c2-h2-N", "chain-c2-h1"),
+			fixGroup(C.TypeSelector, "chain-c2-h2", "chain-c2-h2-N"),
+			// L3: the exit, a plain node copy.
+			fixNode("chain-c2-h3", "chain-c2-h2"),
+			// The base node behind the L2 member copy: emitted, referenced by
+			// NOTHING but the copy's name.
+			fixNode("N", ""),
+		},
+		Route: fixRoute("", "chain-c2-h3"),
+	}
+	byDial, used := planOf(t, opts)
+
+	// The node hop wrapper is a job of its own, stored only under itself.
+	j := assertPlanHas(t, byDial, "chain-c2-h1")
+	if len(j.Store) != 1 || j.Store[0] != "chain-c2-h1" {
+		t.Errorf("node hop wrapper store = %v, want only itself (a chain prefix is nobody else's path)", j.Store)
+	}
+	// The exit and the group-hop member copy are jobs, as before.
+	assertPlanHas(t, byDial, "chain-c2-h3")
+	assertPlanHas(t, byDial, "chain-c2-h2-N")
+
+	// NO job may record anything under the base tag N: not as a dial, not as a
+	// store alias. A direct measurement can never land on it from this config.
+	if _, ok := byDial["N"]; ok {
+		t.Error("plan dials the base node N, which no rule reaches")
+	}
+	for dial, job := range byDial {
+		for _, tag := range job.Store {
+			if tag == "N" {
+				t.Errorf("job %q stores under the base tag N; a chain member copy must never alias its base", dial)
+			}
+		}
+	}
+	if used["N"] {
+		t.Error("used-set claims the base node N; only the chain copies are on the path")
+	}
+	// The wrappers themselves are used (they are the path).
+	for _, u := range []string{"chain-c2-h1", "chain-c2-h2", "chain-c2-h3", "chain-c2-h2-N"} {
+		if !used[u] {
+			t.Errorf("used-set is missing %q", u)
+		}
+	}
+}
+
 // DNS server detours and route.Final are roots too — a group referenced only as a
 // resolver's detour is still a used, probed path.
 func TestObservatoryPlanDNSDetourRoot(t *testing.T) {

shater/engine/selfcheck.go

@@ -0,0 +1,99 @@
+package engine
+
+import (
+	C "github.com/sagernet/sing-box/constant"
+	"github.com/sagernet/sing-box/option"
+)
+
+// Standing down the self-check of unused urltest groups (health board §5.C).
+//
+// # Why the engine does this, and not the generator
+//
+// A urltest group probes its own members: a warm-up sweep at PostStart and a
+// ticker for as long as traffic touches it (protocol/group/urltest.go). For a
+// group the routing rules actually use, that probing travels the same path the
+// traffic does and is welcome. For a group NO rule reaches, it dials the
+// members' base outbounds directly from the router — a path nothing uses — and
+// stores the results under the base tags, where every health consumer reads
+// them as "the node's health". A node that only works behind a tunnel then
+// reads dead on the shared board because an idle group measured it over the
+// blocked direct WAN. The observatory's plan (probeplan.go) is the single
+// statement of what gets probed and along which paths; this file makes the
+// config that reaches box.New SAY so, by flipping SelfCheck off on every
+// urltest group outside the plan's used-set.
+//
+// The generator cannot do this: whether a group is used is a property of the
+// EMITTED rules and DNS detours as a whole, and BuildObservatoryPlan is the one
+// place that reachability is computed. Recomputing it in the generator would be
+// a second copy of the same walk, and second copies drift.
+//
+// # NOT the same thing as Engine.ProbeAllowed — do not merge them
+//
+// There are two reasons a group's own schedule stands down, and they are
+// answered in two different places on purpose:
+//
+//	THIS FILE — "no enabled rule reaches this group AT ALL". A property of the
+//	  CONFIG. Decided once per apply, written into the option struct that
+//	  reaches box.New, and unchangeable for the life of that box, because the
+//	  rules cannot change while the box runs.
+//
+//	Engine.ProbeAllowed — "the path in front of this group is dead RIGHT NOW".
+//	  A property of the WORLD. It changes on its own, so it is asked at every
+//	  scheduled check and never stored; that is the only reason a chain hop
+//	  resumes probing by itself when the hop in front of it recovers.
+//
+// A CHAIN HOP WRAPPER is precisely where the difference bites. It is reached by
+// the rules, so it is in the used-set and this function must leave its flag
+// alone — TestStandDownNeverTouchesChainHopWrappers pins that, and it is not a
+// technicality: standing a hop wrapper down here would take a working chain off
+// its own board permanently, with nothing to turn it back on. If a hop wrapper
+// should not probe because the hop in front of it is down, that is the gate's
+// answer, not this one's, and it lasts exactly as long as the failure does.
+
+// standDownUnusedSelfCheck flips SelfCheck to false on every urltest group in
+// opts that the observatory's used-set does not cover, and returns how many it
+// stood down (for the apply log and the tests). Selector groups are left alone
+// — they have no self-check to stand down — and a group any rule, route.Final
+// or DNS detour reaches keeps its default (nil == on).
+//
+// It MUTATES the option structs the caller handed in: opts.Outbounds carries
+// the generator's option values as pointers behind an `any` (the generator
+// always emits *option.URLTestOutboundOptions), so writing through them changes
+// the caller's config. That is deliberate, not an accident to guard against:
+// the plan is the single source of what gets probed, and the config that
+// reaches box.New must already say so — a copy-on-write here would build a box
+// whose groups probe paths the hash and the plan claim nobody probes. It runs
+// BEFORE hashOptions in applyLocked for the same reason: the hash must describe
+// what is really built, so a rule change that flips a group used<->unused is a
+// real config change and triggers a swap.
+//
+// A urltest outbound whose Options is not the expected pointer type (a value,
+// or something foreign) is skipped rather than guessed at: it did not come from
+// our generator, and silently rebuilding somebody else's option struct is worse
+// than leaving one group's self-check up.
+// It also RETURNS the used-set it computed. That set is needed before the new
+// box starts — a group's PostStart asks whether it must keep measuring with no
+// traffic (Engine.ProbeWhenIdle), and the observatory does not publish its own
+// copy until after the swap has succeeded. Handing back the walk that has
+// already run here is cheaper than a second one and, more to the point, cannot
+// disagree with the flags just written from it.
+func standDownUnusedSelfCheck(opts option.Options) (int, map[string]bool) {
+	_, used := BuildObservatoryPlan(opts, "")
+	stood := 0
+	for i := range opts.Outbounds {
+		ob := &opts.Outbounds[i]
+		if ob.Type != C.TypeURLTest || used[ob.Tag] {
+			continue
+		}
+		utOpts, ok := ob.Options.(*option.URLTestOutboundOptions)
+		if !ok {
+			continue
+		}
+		// One fresh pointer per group, so no two option structs alias a shared
+		// bool that a later caller could flip for both at once.
+		off := false
+		utOpts.SelfCheck = &off
+		stood++
+	}
+	return stood, used
+}

shater/engine/selfcheck_test.go

@@ -0,0 +1,224 @@
+package engine
+
+import (
+	"testing"
+
+	C "github.com/sagernet/sing-box/constant"
+	"github.com/sagernet/sing-box/option"
+)
+
+// standDownUnusedSelfCheck is the enforcement half of the one-probe rule: a
+// urltest group no rule reaches gets SelfCheck=false written into its option
+// struct (in place — the struct the box will be built from), a rule-reachable
+// one keeps its nil default, and selectors are never touched at all (they have
+// no self-check to stand down). The count it returns is what applyLocked logs.
+func TestStandDownUnusedSelfCheck(t *testing.T) {
+	opts := option.Options{
+		Outbounds: []option.Outbound{
+			fixNode("n1", ""),
+			fixNode("idle1", ""),
+			fixNode("pin1", ""),
+			// Reached by a rule: must keep probing itself.
+			fixGroup(C.TypeURLTest, "auto", "n1"),
+			// Reached by nothing: its self-check dials a path nobody uses and
+			// must be stood down.
+			fixGroup(C.TypeURLTest, "idle", "idle1"),
+			// A selector reached by nothing: left alone — no self-check exists.
+			fixGroup(C.TypeSelector, "pins", "pin1"),
+		},
+		Route: fixRoute("auto"),
+	}
+
+	stood, used := standDownUnusedSelfCheck(opts)
+	if stood != 1 {
+		t.Fatalf("stood down %d group(s), want exactly 1 (idle)", stood)
+	}
+	// The returned used-set is the SAME walk the flags were written from — the
+	// engine publishes it for ProbeWhenIdle before the new box is built, so the
+	// two statements about one group ("keeps its self-check", "keeps it warm")
+	// can never be computed from two different reachability answers.
+	if !used["auto"] || used["idle"] {
+		t.Errorf("used-set = %v, want the reached group in and the unreached one out", used)
+	}
+
+	find := func(tag string) option.Outbound {
+		t.Helper()
+		for _, ob := range opts.Outbounds {
+			if ob.Tag == tag {
+				return ob
+			}
+		}
+		t.Fatalf("outbound %q missing from opts", tag)
+		return option.Outbound{}
+	}
+
+	// The unused urltest group: SelfCheck is now an explicit false in the very
+	// struct the caller handed in (the mutation is the point — the config that
+	// reaches box.New must say what the plan says).
+	idle := find("idle").Options.(*option.URLTestOutboundOptions)
+	if idle.SelfCheck == nil || *idle.SelfCheck {
+		t.Fatalf("idle group SelfCheck = %v, want an explicit false", idle.SelfCheck)
+	}
+
+	// The used urltest group keeps the nil default (self-check on): its probing
+	// travels the path traffic actually takes and is welcome.
+	auto := find("auto").Options.(*option.URLTestOutboundOptions)
+	if auto.SelfCheck != nil {
+		t.Fatalf("used group SelfCheck = %v, want nil (untouched default)", *auto.SelfCheck)
+	}
+
+	// The selector's option struct has no SelfCheck field at all; what is
+	// pinned here is that stand-down neither counted it nor mangled its type.
+	if _, ok := find("pins").Options.(*option.SelectorOutboundOptions); !ok {
+		t.Fatal("selector options were rebuilt; stand-down must leave selectors alone")
+	}
+
+	// Idempotence: a second pass finds the same one group (already-false is
+	// still counted — the function reports plan membership, not novelty) and
+	// changes nothing further. This is what keeps the apply hash stable across
+	// the every-minute reconcile.
+	if again, _ := standDownUnusedSelfCheck(opts); again != 1 {
+		t.Fatalf("second pass stood down %d, want 1 (deterministic on identical opts)", again)
+	}
+	if idle.SelfCheck == nil || *idle.SelfCheck {
+		t.Fatal("second pass flipped the idle group's SelfCheck back")
+	}
+}
+
+// TestStandDownNeverTouchesChainHopWrappers pins the property the production
+// config lives or dies by, DIRECTLY rather than transitively through the plan
+// tests: a CHAIN HOP WRAPPER group ("chain-<name>-h<i>", type urltest) must
+// NEVER be stood down.
+//
+// The failure mode this guards against is subtle and silent. The owner's live
+// rule routes through egress:ewan -> node:awgout -> group:sub0 -> group:sub1
+// -> group:sub2 — a chain whose group hops are rebuilt as urltest wrappers
+// over per-chain member copies. Those wrappers are the ONLY probing that
+// travels the real path: their own schedule is the on-path measurement, and
+// the selection inside the chain (which member each hop dials) depends on the
+// verdicts it writes. Reachability of a wrapper is established by walkDetour
+// (probeplan.go) walking the exit's Detour links — a DIFFERENT code path from
+// the root/member expansion the other tests exercise. If a future change to
+// that walk dropped the wrappers from the used-set, standDownUnusedSelfCheck
+// would obediently flip their SelfCheck off, the chain would stop measuring
+// itself, and nothing else would ever measure it: we would have replaced the
+// false reading this rework removed with NO reading at all. Both existing
+// tests could stay green while that happened — the plan test asserts the
+// used-set, not the stand-down; the stand-down test above never mentions
+// chains. This one closes that gap by asserting the stand-down's OUTPUT on
+// the production topology.
+//
+// The fixture mirrors what the generator actually emits for that config, not
+// a toy: an exit wrapper chain-p-h4 (urltest over member copies) whose copies
+// detour into chain-p-h3 (urltest), whose copies detour into chain-p-h2
+// (urltest), whose copies detour into chain-p-h1 (the plain AmneziaWG hop
+// copy), which detours into egress-ewan — the detour lives on the member
+// COPIES, not on the wrappers, exactly as generate/chain.go builds it. The
+// base groups sub0/sub1/sub2 are ALSO emitted, over the base node tags,
+// reached by nothing — which is precisely what buildGroups does today and
+// precisely the false direct prober the stand-down exists to silence.
+func TestStandDownNeverTouchesChainHopWrappers(t *testing.T) {
+	opts := option.Options{
+		Outbounds: []option.Outbound{
+			// The egress the whole chain leaves through.
+			fixUtility(C.TypeDirect, "egress-ewan"),
+			// L1: the plain AWG hop copy (in production an endpoint; a plain
+			// outbound here exercises the same walk — walkDetour treats both as
+			// non-group entries).
+			fixNode("chain-p-h1", "egress-ewan"),
+			// L2..L4: urltest group hop wrappers over per-chain member copies;
+			// each COPY detours into the previous hop.
+			fixNode("chain-p-h2-a", "chain-p-h1"),
+			fixNode("chain-p-h2-b", "chain-p-h1"),
+			fixGroup(C.TypeURLTest, "chain-p-h2", "chain-p-h2-a", "chain-p-h2-b"),
+			fixNode("chain-p-h3-a", "chain-p-h2"),
+			fixNode("chain-p-h3-b", "chain-p-h2"),
+			fixGroup(C.TypeURLTest, "chain-p-h3", "chain-p-h3-a", "chain-p-h3-b"),
+			fixNode("chain-p-h4-a", "chain-p-h3"),
+			fixNode("chain-p-h4-b", "chain-p-h3"),
+			fixGroup(C.TypeURLTest, "chain-p-h4", "chain-p-h4-a", "chain-p-h4-b"),
+			// The base nodes and the base groups over them: emitted alongside
+			// the chain, reached by no rule — the generator keeps emitting them
+			// today, and they are exactly the direct-WAN probers that poisoned
+			// the board.
+			fixNode("a", ""),
+			fixNode("b", ""),
+			fixGroup(C.TypeURLTest, "sub0", "a", "b"),
+			fixGroup(C.TypeURLTest, "sub1", "a", "b"),
+			fixGroup(C.TypeURLTest, "sub2", "a", "b"),
+		},
+		// One rule, targeting the chain's exit wrapper — the production shape.
+		Route: fixRoute("", "chain-p-h4"),
+	}
+
+	stood, used := standDownUnusedSelfCheck(opts)
+	if stood != 3 {
+		t.Fatalf("stood down %d group(s), want exactly the 3 base groups sub0/sub1/sub2", stood)
+	}
+	// The hop wrappers are also what ProbeWhenIdle must say yes to: the chain is
+	// reached by a rule, so its hops keep measuring whether or not traffic is
+	// crossing them. The base groups behind them are not, and must not.
+	for _, tag := range []string{"chain-p-h2", "chain-p-h3", "chain-p-h4"} {
+		if !used[tag] {
+			t.Errorf("used-set is missing hop wrapper %q; it would idle out with nothing to replace it", tag)
+		}
+	}
+	for _, tag := range []string{"sub0", "sub1", "sub2"} {
+		if used[tag] {
+			t.Errorf("used-set claims base group %q, which no rule reaches", tag)
+		}
+	}
+
+	utOptions := func(tag string) *option.URLTestOutboundOptions {
+		t.Helper()
+		for _, ob := range opts.Outbounds {
+			if ob.Tag == tag {
+				o, ok := ob.Options.(*option.URLTestOutboundOptions)
+				if !ok {
+					t.Fatalf("outbound %q is not a urltest group", tag)
+				}
+				return o
+			}
+		}
+		t.Fatalf("outbound %q missing from opts", tag)
+		return nil
+	}
+
+	// Every hop wrapper keeps its self-check: nil, the untouched default. An
+	// explicit false on ANY of these is the chain going blind.
+	for _, wrapper := range []string{"chain-p-h2", "chain-p-h3", "chain-p-h4"} {
+		if sc := utOptions(wrapper).SelfCheck; sc != nil {
+			t.Errorf("hop wrapper %q SelfCheck = %v, want nil — its own schedule IS the on-path measurement", wrapper, *sc)
+		}
+	}
+	// Every base group is stood down: nothing routes through them, and their
+	// probing would dial the base nodes over the direct WAN.
+	for _, base := range []string{"sub0", "sub1", "sub2"} {
+		if sc := utOptions(base).SelfCheck; sc == nil || *sc {
+			t.Errorf("base group %q SelfCheck = %v, want an explicit false", base, sc)
+		}
+	}
+
+	// And the reason the stand-down of sub0/sub1/sub2 is SAFE in this config:
+	// with the base groups silenced, the base node tags a/b have no measurement
+	// path at all — no job dials them and no job stores under them (the chain's
+	// member copies record only under themselves). Their board entries simply
+	// stop being written, honestly untested, instead of carrying a false
+	// direct-WAN verdict.
+	jobs, used := BuildObservatoryPlan(opts, "")
+	for _, baseTag := range []string{"a", "b"} {
+		if used[baseTag] {
+			t.Errorf("used-set claims base node %q; only the chain copies are on the path", baseTag)
+		}
+		for _, j := range jobs {
+			if j.Dial == baseTag {
+				t.Errorf("plan dials base node %q, which no rule reaches", baseTag)
+			}
+			for _, s := range j.Store {
+				if s == baseTag {
+					t.Errorf("job %q stores under base node %q; a chain copy must never alias its base", j.Dial, baseTag)
+				}
+			}
+		}
+	}
+}

shater/generate/observatory_reach_integration_test.go

@@ -162,14 +162,23 @@ func TestObservatoryPlanFromGeneratedConfig(t *testing.T) {
 			t.Errorf("chain exit %q store = %v, want only itself (a chain prefix is nobody else's path)", chainExit, store)
 		}
 	}
-	// The chain's intermediate node hop n1 is used (the exit detours through it) but
-	// NOT probed separately — its death is visible in the exit probe, and no
-	// selection depends on it. n1 IS probed via the plain "auto" group, so the
-	// assertion is "no SEPARATE chain-hop-h1 job", not "n1 never dialled".
-	if jobDials(jobs, "chain-hop-h1") {
-		t.Errorf("plan dials intermediate chain hop chain-hop-h1 — only the exit is probed end-to-end")
+	// The chain's intermediate NODE hop wrapper IS probed now, as a measurement
+	// of its own: dialling chain-hop-h1 measures exactly the prefix of the path
+	// up to and including hop 1, which is what lets an operator see WHICH hop
+	// died instead of only that the chain did (engine/probeplan.go walkDetour,
+	// surfaced through ChainHealth.Hops). Its store is only itself — a chain
+	// prefix is nobody else's dial path, so the base node n1 must never inherit
+	// a verdict measured through the chain.
+	chainHop := "chain-hop-h1"
+	if !jobDials(jobs, chainHop) {
+		t.Errorf("plan does not probe intermediate chain hop %q; jobs=%v", chainHop, dialsOfJobs(jobs))
+	} else {
+		store := jobStore(jobs, chainHop)
+		if len(store) != 1 || store[0] != chainHop {
+			t.Errorf("chain hop %q store = %v, want only itself (no base alias for a chain prefix)", chainHop, store)
+		}
 	}
-	if !used[chainExit] || !used["chain-hop-h1"] {
+	if !used[chainExit] || !used[chainHop] {
 		t.Errorf("used-set is missing chain hop tags; used=%v", used)
 	}
 	// The used groups themselves are in the used-set but never dialled (a group is

shater/panel/api.go

@@ -1359,9 +1359,24 @@ type groupTestStartResponse struct {
 	Reason  string `json:"reason,omitempty"`
 }
 
-// handleGroupsTest → /api/groups/test: measure each target's (group's or chain's)
-// latency and the public address it currently exits from. A chain is dialled via
-// its exit wrapper, so the numbers are end-to-end properties of the whole path.
+// handleGroupsTest → /api/groups/test: report each target's (group's or chain's)
+// health as the OBSERVATORY just measured it, plus the public address it
+// currently exits from.
+//
+// POST no longer dials anything. It asks the engine's observatory for an
+// out-of-turn pass of its probe plan and then reports what that pass measured:
+// each target resolves off the health board the moment its measured path
+// carries an observation newer than the button press. The consequences the
+// frontend should expect (the response SHAPE is unchanged):
+//   - delay_ms/ok/tested_unix describe the observatory's measurement of the
+//     target's real routed path — tested_unix is when that observation was
+//     taken, which may be a second or two before the poll that delivered it;
+//   - a target no enabled rule routes through is never measured and comes back
+//     ok=false with an error saying so, immediately — the observatory only
+//     probes paths the rules use, and the panel should render that as a
+//     routing fact, not a failure;
+//   - a run can take up to the engine's wait deadline (120s) when the
+//     observatory has a large plan to walk; poll GET for progress as before.
 //
 // Contract for the frontend:
 //   - POST {"name":"auto"} → test that group or chain; an empty/absent name tests
@@ -1395,9 +1410,11 @@ func (s *Server) handleGroupsTest(w http.ResponseWriter, r *http.Request) {
 		if n := strings.TrimSpace(req.Name); n != "" {
 			names = []string{n}
 		}
-		// Probe URL from the model (best-effort) — the global one, the only probe
-		// instrument left; a UCI read failure falls back to the engine's built-in
-		// default.
+		// Probe URL from the model (best-effort), passed through for signature
+		// stability only: the engine IGNORES it now. The probe URL is a global
+		// observatory setting installed at apply time, and the manual test reads
+		// the observatory's measurements rather than dialling with a URL of its
+		// own — see engine.TestGroups.
 		var probeURL string
 		if m, err := model.ReadUCI(); err == nil {
 			probeURL = strings.TrimSpace(m.Globals.ProbeURL)
@@ -1469,6 +1486,40 @@ type groupHealthResponse struct {
 //     was measured THROUGH the group's egress, so it is not comparable with the global
 //     per-node numbers in /api/stats node_health. That is the entire point of this
 //     endpoint — the same node in two groups with two egresses has two health states.
+//
+// chains[] additionally carries the PER-HOP readout for every chain the running
+// box materialised:
+//
+//	chains[].hops = [{index,tag,kind,exit,state,delay_ms,age_seconds,selected,
+//	                  total,tested,alive,dead,untested,blocked_by?}...]
+//
+//   - hops is L1..Ln in wire order (index is 1-based); the entry with exit=true
+//     is the last hop, where traffic leaves to the internet. Each hop's numbers
+//     measure the chain PREFIX up to and including that hop — the observatory
+//     dials the hop wrappers — so a dead hop N with alive hops 1..N-1 localises
+//     the failure to hop N's own leg.
+//   - the walk is ORDERED and stops at the first dead hop. Hop N is dialled
+//     THROUGH hops 1..N-1, so probing it across a dead hop 2 would measure hop 2
+//     again and say nothing about hop N. Everything behind the first dead hop is
+//     therefore left undialled and reported state="untested" with
+//     blocked_by={index,tag} naming the hop that stopped the walk; its counters
+//     are 0/0/0/total and its delay_ms/age_seconds are 0/-1. blocked_by is
+//     OMITTED on every hop that was actually measured, and a hop carrying it is
+//     never a fault of its own — the fault is at blocked_by.index. It follows
+//     that a dead hop with a LIVE hop behind it can no longer be reported.
+//   - kind is "node" (one measurement: total=1, selected empty) or "group" (a
+//     roll-up of the hop's member copies, with the same counter invariants as a
+//     group: tested == alive+dead, alive+dead+untested == total; selected is
+//     the node NAME the hop currently picks; delay_ms/age_seconds are the
+//     selected member's observation, or the freshest alive member's when the
+//     selection has none).
+//   - state follows the same closed set and the same honesty rule as groups:
+//     "dead" only on a positive finding, "untested" for no fresh data — and for
+//     a group hop, "alive" as long as ANY member answers.
+//   - an ABSENT/empty hops key means the running box never materialised the
+//     chain (unused, or a 1-hop chain that resolves straight to its target) —
+//     it must NOT be read as "this chain has no hops"; the config, not this
+//     endpoint, knows the configured hop count.
 func (s *Server) handleGroupsHealth(w http.ResponseWriter, r *http.Request) {
 	if r.Method != http.MethodGet {
 		writeError(w, http.StatusMethodNotAllowed, "method not allowed")