diff --git a/src/client/link-tunnel.ts b/src/client/link-tunnel.ts
index 40165273c95..e4c2e46f22a 100644
--- a/src/client/link-tunnel.ts
+++ b/src/client/link-tunnel.ts
@@ -23,6 +23,7 @@ import {
 import { isLinkPort } from "../link/ports";
 import { inspectIpv4LoopbackListener, type LoopbackOwnerLookupResult } from "../server/port-reclaim";
 import type { LinkDiagnosticSink } from "../link/diagnostics";
+import { createTunnelHistoryBook, exitCause, type TunnelCause } from "../link/tunnel-history";
 import { isLinkConnection, readClientConnectionState } from "./state";
 import { clientLinkStatePath, readClientLinkState, type ClientLinkState } from "./link-state";
 import type { LinkTunnelGate, LinkAdmissionResult } from "./link-relay";
@@ -167,6 +168,8 @@ export interface ClientTunnelPidfile {
 }
 
 const STOP_TIMEOUT_MS = 5_000;
+/** A Child supervises one tunnel; its history needs one fixed key, never the link id. */
+const CHILD_TUNNEL_HISTORY_KEY = "child";
 const REAP_POLL_MS = 100;
 const TIMER_MS = 1_000;
 /** Consecutive unreadable reads before the supervisor acts on them. */
@@ -607,6 +610,20 @@ export function createClientLinkSupervisor(deps: ClientLinkSupervisorDeps = {}):
   /** Minted on every transition into connected from the process-wide sequence (relay-auth.ts). */
   let connection: string | undefined;
   let state: TunnelState = IDLE;
+  /**
+   * Child tunnel transitions go to the same bounded diagnostics file as the Home's, with the same
+   * fixed causes (#7297). The port is remembered past stop, which clears the live port first.
+   */
+  let lastTunnelPort: number | undefined;
+  // The record is built at the transition but delivered after the current synchronous step, so a
+  // sink that re-enters the supervisor (stop, status) never runs in the middle of a transition.
+  const transitionSink = deps.diagnostic;
+  const history = createTunnelHistoryBook({
+    diagnostic: transitionSink
+      ? record => queueMicrotask(() => { try { transitionSink(record); } catch { /* best effort */ } })
+      : undefined,
+    tunnelPort: () => lastTunnelPort,
+  });
   let linkId: string | null = null;
   let failure: ClientLinkSupervisorStatus | undefined;
   let tickFlight: Promise<void> | undefined;
@@ -689,9 +706,12 @@ export function createClientLinkSupervisor(deps: ClientLinkSupervisorDeps = {}):
     for (const settle of [...waiters]) settle(ready);
   };
 
-  const setState = (next: TunnelState): void => {
+  const setState = (next: TunnelState, cause: TunnelCause | null = null): void => {
+    const before = state;
     if (next.kind === "connected" && state.kind !== "connected") connection = nextLinkConnectionGeneration();
     state = next;
+    // Records only a real change and swallows sink errors; diagnostics never steer the lifecycle.
+    history.observe(CHILD_TUNNEL_HISTORY_KEY, before, next, cause, now());
     settleWaiters();
   };
 
@@ -750,7 +770,7 @@ export function createClientLinkSupervisor(deps: ClientLinkSupervisorDeps = {}):
   };
 
   const onExit = (stderrClass: StderrClass): void => {
-    setState(reduceTunnel(state, { type: "exit", now: now(), stderrClass }, random, policy));
+    setState(reduceTunnel(state, { type: "exit", now: now(), stderrClass }, random, policy), exitCause(stderrClass));
   };
 
   const spawn = (sidecar: ClientLinkState): void => {
@@ -766,13 +786,16 @@ export function createClientLinkSupervisor(deps: ClientLinkSupervisorDeps = {}):
       }, deps);
     } catch {
       // A due retry that cannot start counts as a failed retry, so the backoff keeps its step.
-      setState(failedSpawnTunnel(state, "forward", timestamp, policy));
+      lastTunnelPort = sidecar.tunnelPort;
+      setState(failedSpawnTunnel(state, "forward", timestamp, policy), "spawn_failed");
       deps.warn?.("client link tunnel could not be started");
       return;
     }
     invalidateOwnerProof();
     linkId = sidecar.linkId;
     tunnelPort = sidecar.tunnelPort;
+    lastTunnelPort = sidecar.tunnelPort;
+    history.spawned(CHILD_TUNNEL_HISTORY_KEY);
     probe = null;
     probeFailures = 0;
     nextProbeAt = timestamp;
@@ -871,6 +894,7 @@ export function createClientLinkSupervisor(deps: ClientLinkSupervisorDeps = {}):
       invalidateOwnerProof();
       linkId = afterReap.sidecar.linkId;
       tunnelPort = afterReap.sidecar.tunnelPort;
+      lastTunnelPort = afterReap.sidecar.tunnelPort;
       nextProbeAt = now();
       setState({ kind: "connecting", since: now() });
       return;
@@ -919,7 +943,8 @@ export function createClientLinkSupervisor(deps: ClientLinkSupervisorDeps = {}):
       probe = null;
       setState(IDLE);
     }
-    setState(reduceTunnel(state, { type: "tick", now: now() }, random, policy));
+    // A tick changes state only when the outage limit fails the link, so its cause is the timeout.
+    setState(reduceTunnel(state, { type: "tick", now: now() }, random, policy), "timeout");
     // An adopted tunnel is never killed. Failed identity or socket-owner proof keeps it
     // unprobed and unable to promote, even when the process stays alive after a timeout.
     if (state.kind === "failed" && !state.inFlight && adopted !== null) setState({ ...state, inFlight: true });
diff --git a/structure/remote-link.md b/structure/remote-link.md
index bfd7574e84e..8d385629650 100644
--- a/structure/remote-link.md
+++ b/structure/remote-link.md
@@ -104,7 +104,7 @@ The `diagnostics.ts` module in `src/link/` owns best-effort local evidence in `<
 
 `owner_lookup` records only refused ownership checks; routine successful checks are silent. `owner_lookup.elapsedMs` measures the owner lookup itself. For `auth_timeout`, `relay_retry` and `relay_failure` report the actual transport-attempt duration in `elapsedMs` (normally about five seconds). A `relay_recovered` event with outcome `recovered` is emitted only after a retry receives an upstream response with `upstream.ok` (HTTP 2xx); it does not prove the streamed body completed. Rate limiting makes request-ID correlation sampled evidence, not a guaranteed trace of every failed request.
 
-The Home writes `tunnel_transition` records into its own `<configDir>/link/diagnostics.jsonl` through the same sink, wired in `src/server/index/optional-listeners.ts`: `previousState`, `state`, the cause as `outcome` (the history's causes, or `connected`, `spawned` and `stopped` when nothing failed), `attempt`, the link's `tunnelPort` and, when a retry is scheduled, `retryAfterMs`. The supervisor's `stop()` drains the sink for at most one second. The one-per-second limit per event/outcome applies across links.
+The Home writes `tunnel_transition` records into its own `<configDir>/link/diagnostics.jsonl` through the same sink, wired in `src/server/index/optional-listeners.ts`: `previousState`, `state`, the cause as `outcome` (the history's causes, or `connected`, `spawned` and `stopped` when nothing failed), `attempt`, the link's `tunnelPort` and, when a retry is scheduled, `retryAfterMs`. The supervisor's `stop()` drains the sink for at most one second. The one-per-second limit per event/outcome applies across links. The Child records its own `-L` tunnel the same way (#7297): `src/client/link-tunnel.ts` feeds every state change through a `createTunnelHistoryBook` keyed by one fixed key, with `spawn_failed` for a spawn that cannot start and the classified ssh stderr cause for an exit; the port is remembered past stop. Records carry the same fixed fields only, never argv, stderr, alias or link id, and reach the sink in a microtask after the transition, so a sink that re-enters the supervisor cannot interrupt one (`tests/clients/client-link-tunnel.test.ts`).
 
 `src/client/link-relay.ts` shares one preflight recovery attempt per request between an owner-lookup `deadline` and the typed `LinkRelayAuthTimeoutError` from `src/client/link-relay-transport.ts`. Only expiry of the credential-free authentication deadline before data dispatch produces that type; arbitrary aborts, authentication failures and transport errors do not qualify. Recovery obtains fresh owner proof where required and opens a fresh physical socket, authenticating it before sending any link credential or body. It never caches owner approval, bypasses security, restarts SSH or OpenCodex, or replays a dispatched body. The owner-lookup and authentication deadlines remain two and five seconds respectively, per attempt. Existing reconnect holding remains separate.
 
diff --git a/tests/clients/client-link-tunnel.test.ts b/tests/clients/client-link-tunnel.test.ts
index 4ccbf37c0cc..db7c4fa3833 100644
--- a/tests/clients/client-link-tunnel.test.ts
+++ b/tests/clients/client-link-tunnel.test.ts
@@ -1,4 +1,4 @@
-import { expect, test } from "bun:test";
+import { describe, expect, test } from "bun:test";
 import { existsSync, mkdirSync, readFileSync, renameSync, statSync, writeFileSync, mkdtempSync, rmSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
@@ -17,6 +17,8 @@ import type { ClientLinkState } from "../../src/client/link-state";
 import type { SshChild, SshRunner } from "../../src/link/ssh-runner";
 import { buildTunnelArgv } from "../../src/link/ssh-argv";
 import { relayLinkDataRequest } from "../../src/client/link-relay";
+import type { LinkDiagnosticRecord } from "../../src/link/diagnostics";
+import { FAILED_AFTER_MS } from "../../src/link/tunnel-state";
 
 function deferred<T>() {
   let resolve!: (value: T) => void;
@@ -1327,7 +1329,8 @@ test("a stop triggered by failure diagnostics leaves admission closed", async ()
   let stopping: Promise<void> | undefined;
   const h = supervisorHarness({
     inspectLoopbackListener: async () => ({ kind: result, elapsedMs: 1 }),
-    diagnostic: () => { stopping = h.supervisor.stop(); },
+    // Tunnel transitions reach the same sink (#7297); this case is about the owner-lookup refusal.
+    diagnostic: record => { if (record.event === "owner_lookup") stopping = h.supervisor.stop(); },
   });
   try {
     await h.start();
@@ -1389,3 +1392,86 @@ test("a supervisor rebuilt by a restart rollback in the same process never repea
   expect(seen[1]).toEqual(expect.any(String));
   expect(seen[1]).not.toBe(seen[0]);
 });
+
+describe("Child tunnel transitions in the diagnostics file (#7297)", () => {
+  const ALLOWED = new Set(["event", "outcome", "state", "previousState", "attempt", "tunnelPort", "retryAfterMs"]);
+  const transitions = (records: LinkDiagnosticRecord[]) => records.filter(record => record.event === "tunnel_transition");
+
+  test("spawn, connect, a classified exit and stop each leave one fixed-cause record", async () => {
+    const records: LinkDiagnosticRecord[] = [];
+    const h = supervisorHarness({ diagnostic: record => { records.push(record); } });
+    try {
+      await h.start();
+      expect(h.state()).toBe("connecting");
+      await h.step();
+      expect(h.state()).toBe("connected");
+      h.ssh.live()[0]!.exit(255, HOST_KEY_CHANGED);
+      await h.settle();
+    } finally { await h.close(); }
+    const seen = transitions(records);
+    expect(seen.map(record => [record.previousState, record.state, record.outcome])).toEqual([
+      ["idle", "connecting", "spawned"],
+      ["connecting", "connected", "connected"],
+      [seen[2]!.previousState, seen[2]!.state, "hostkey"],
+      [seen[3]!.previousState, "idle", "stopped"],
+    ]);
+    expect(seen[2]!.previousState).toBe("connected");
+    expect(seen[0]!.attempt).toBe(1);
+    expect(seen[1]!.attempt).toBe(0);
+    // The port survives stop, which clears the live port before the transition is recorded.
+    for (const record of seen) expect(record.tunnelPort).toBe(sidecar().tunnelPort);
+    // Fixed metadata only: no argv, stderr, alias or link id ever reaches the sink.
+    for (const record of seen) for (const key of Object.keys(record)) expect(ALLOWED.has(key)).toBe(true);
+  });
+
+  test("a link that stays down past the outage limit records timeout", async () => {
+    // ssh stays up but the keyed probe never succeeds (the Home is down): the tick that fails the
+    // link at FAILED_AFTER_MS must say timeout, as the Home does, not fall back to spawned.
+    const records: LinkDiagnosticRecord[] = [];
+    const h = supervisorHarness({ diagnostic: record => { records.push(record); } });
+    h.view.readyz = "refused";
+    try {
+      await h.start();
+      expect(h.state()).toBe("connecting");
+      await h.step(FAILED_AFTER_MS);
+      expect(h.state()).toBe("failed");
+    } finally { await h.close(); }
+    expect(transitions(records).find(record => record.state === "failed")).toMatchObject({
+      outcome: "timeout", previousState: "connecting", state: "failed",
+    });
+  });
+
+  test("a spawn that cannot start records spawn_failed", async () => {
+    const records: LinkDiagnosticRecord[] = [];
+    const h = supervisorHarness({
+      diagnostic: record => { records.push(record); },
+      runner: { async run() { return { code: 0, stdout: "", stderr: "" }; }, spawnTunnel() { throw new Error("ssh missing"); } },
+      warn: () => {},
+    });
+    try { await h.start(); } finally { await h.close(); }
+    expect(transitions(records)[0]).toMatchObject({ outcome: "spawn_failed", previousState: "idle", state: "failed" });
+  });
+
+  test("a sink that stops the supervisor on a transition never interrupts that transition", async () => {
+    let stopping: Promise<void> | undefined;
+    const h = supervisorHarness({
+      diagnostic: record => { if (record.event === "tunnel_transition" && record.outcome === "spawned") stopping = h.supervisor.stop(); },
+    });
+    try {
+      await h.start();
+      expect(stopping).toBeDefined();
+      await stopping;
+      expect(h.state()).toBe("idle");
+      expect(h.ssh.live()).toEqual([]);
+    } finally { await stopping; await h.close(); }
+  });
+
+  test("a throwing sink never changes the tunnel lifecycle", async () => {
+    const h = supervisorHarness({ diagnostic: () => { throw new Error("disk full"); } });
+    try {
+      await h.start();
+      await h.step();
+      expect(h.state()).toBe("connected");
+    } finally { await h.close(); }
+  });
+});