fix: serialize session lifecycle transitions

.superpowers/sdd/predeploy-fix-report.md

@@ -496,6 +496,9 @@ no output, exit 0
   backlog; an in-memory same-session reconnect is exact-once from its cursor.
 - Per-session sequence counters remain in backend memory after `clear` by design so later in-process
   cold same-id Resume cannot reuse ids. Permanent DELETE removes the counter via `forget`.
+- The Delete-then-Resume adversarial route test proves the deleted session is not resurrected but
+  currently receives the runner's generic HTTP 500 when `sessionShow` can no longer find it. A
+  future API cleanup can normalize that missing-session response to 404 or 409.
 - Frontend tests still print pre-existing MSW unhandled-request and React ref/`act` warnings even
   though all 276 tests pass. The frontend production build still reports pre-existing large chunk
   warnings. Neither warning class was introduced or expanded by this change.
@@ -614,3 +617,313 @@ exit 0
 
 The final frontend run retains the repository's pre-existing MSW/ref/`act` warnings, and the build
 retains the pre-existing large-chunk warning. No test, typecheck, or build failures remain.
+
+## Stale-bootstrap, lifecycle-lock, and competing-Resume hardening
+
+Date: 2026-07-15
+Base: `08b1f4909e8eb7538156cecc2e7a6cafb46ddfc7`
+
+This follow-up closes asynchronous identity/order and multi-client transport gaps found in the
+pre-deployment review:
+
+- `PiProcessManager.teardownIfCurrent(id, runtime)` makes teardown an identity-checked operation.
+  Bootstrap re-checks identity after configuration/retrieval and before both the public
+  `Starting model` event and model start. Its failure continuation acquires the same session
+  lifecycle lock, claims only its own runtime identity, and holds serialization through persisted
+  failure and the public terminal sequence. A continuation left behind by Close or DELETE cannot
+  target a replacement or recreate forgotten SSE state.
+- The former Resume-only promise tail is now a per-session lifecycle lock shared by Resume, Close,
+  and DELETE. Each route reads the current runtime inside the lock immediately before replacement
+  or removal and uses identity-checked teardown. Deferred route tests prove both orderings:
+  Resume then Close/Delete finishes removed with no post-removal bootstrap event; Close then Resume
+  creates only after Close completes; DELETE then Resume cannot recreate a deleted session.
+- AppShell assigns each Resume invocation a monotonic token and records the latest target. A
+  completion for a different, superseding session id cannot reset the store, select a source, close
+  the panel, or repaint phase from a late manifest. Same-id invocations are per-target single-flight
+  operations through the POST and local binding commit: repeated pre-commit clicks update the
+  shared operation's latest token but issue no second POST or commit path. The operation becomes
+  joinable again before its manifest fetch, whose repaint remains token/id/selection guarded. Start
+  new, Stop, streamed session exit, and active-session deletion invalidate pending Resume work.
+  This prevents stale-source preservation and reverse/non-Resume intent overwrite without allowing
+  a slow manifest to suppress a later explicit rebind.
+- `SseHub` subscriber registrations now carry idempotent transport-close callbacks. `clear` and
+  `forget` snapshot and actively close every response before discarding runtime transport state;
+  callback-driven unsubscription during that iteration is safe. The SSE route ends its response so
+  native EventSource reconnects with `Last-Event-ID`. Post-clear events retain monotonic ids and are
+  buffered for replay; `forget` additionally resets the id state.
+
+Production files:
+
+- `backend/src/pi/pi-process-manager.ts`
+- `backend/src/routes/sessions.ts`
+- `backend/src/sse/sse-hub.ts`
+- `frontend/src/shell/AppShell.tsx`
+
+Regression tests:
+
+- `backend/test/pi-process-manager.test.ts`
+- `backend/test/routes-sessions.test.ts`
+- `backend/test/sse-hub.test.ts`
+- `backend/test/sse-route.test.ts`
+- `frontend/src/shell/AppShell.session-mgmt.test.tsx`
+
+### TDD RED/GREEN evidence
+
+Runtime identity API RED:
+
+```text
+cd backend && npx vitest run test/pi-process-manager.test.ts -t "identity-checked teardown"
+
+Test Files  1 failed (1)
+Tests       1 failed | 39 skipped (40)
+TypeError: mgr.teardownIfCurrent is not a function
+```
+
+Runtime identity API GREEN:
+
+```text
+Test Files  1 passed (1)
+Tests       1 passed | 39 skipped (40)
+```
+
+Deferred bootstrap RED:
+
+```text
+cd backend && npx vitest run test/routes-sessions.test.ts \
+  -t "stale bootstrap|bootstrap that"
+
+Test Files  1 failed (1)
+Tests       6 failed | 35 skipped (41)
+
+close/delete + replacement: stale continuation removed the replacement runtime
+delete without replacement: stale continuation called failSession after forget
+```
+
+Deferred bootstrap GREEN:
+
+```text
+Test Files  1 passed (1)
+Tests       6 passed | 35 skipped (41)
+```
+
+Shared lifecycle ordering RED:
+
+```text
+cd backend && npx vitest run test/routes-sessions.test.ts \
+  -t "Resume followed|Close followed|Delete followed"
+
+Test Files  1 failed (1)
+Tests       4 failed | 41 skipped (45)
+
+All four deferred assertions observed the competing route settle before the first lifecycle
+operation released.
+```
+
+Bootstrap plus lifecycle GREEN:
+
+```text
+cd backend && npx vitest run test/routes-sessions.test.ts \
+  -t "Resume followed|Close followed|Delete followed|stale bootstrap|bootstrap that"
+
+Test Files  1 passed (1)
+Tests       10 passed | 35 skipped (45)
+```
+
+Competing frontend Resume RED:
+
+```text
+cd frontend && npx vitest run src/shell/AppShell.session-mgmt.test.tsx \
+  -t "competing Resume|stale Resume manifest"
+
+Test Files  1 failed (1)
+Tests       2 failed | 16 skipped (18)
+
+reverse POST completion opened a second, stale EventSource
+late s1 manifest repainted the selected s3 phase from F3 to F7
+```
+
+Competing and same-id Resume GREEN:
+
+```text
+cd frontend && npx vitest run src/shell/AppShell.session-mgmt.test.tsx \
+  -t "competing Resume|stale Resume manifest|false then true"
+
+Test Files  1 passed (1)
+Tests       3 passed | 15 skipped (18)
+```
+
+### Independent-review hardening RED/GREEN
+
+The first final review reported no Critical findings and three Important edge cases: bootstrap
+could start during an in-progress Close; bootstrap-owned failure was persisted twice; and an older
+same-id result could overwrite newer state. The integrated reviewer also required non-Resume
+navigation to invalidate pending Resume work. The final main review tightened the same-ID contract
+to true single-flight so a second same-target click cannot preserve a dead pre-restart source.
+
+Backend review RED:
+
+```text
+cd backend && npx vitest run test/routes-sessions.test.ts \
+  -t "Close suppresses|bootstrap failure persists once"
+
+Test Files  1 failed (1)
+Tests       2 failed | 45 skipped (47)
+
+deferred configure started Pi while closeSession was still pending
+bootstrap/public failure called failSession twice
+```
+
+Backend review GREEN:
+
+```text
+Test Files  1 passed (1)
+Tests       2 passed | 45 skipped (47)
+```
+
+Same-id single-flight RED:
+
+```text
+cd frontend && npx vitest run src/shell/AppShell.session-mgmt.test.tsx \
+  -t "share one cold request"
+
+Test Files  1 failed (1)
+Tests       1 failed | 18 skipped (19)
+
+two concurrent same-ID invocations issued two cold POSTs (three total including initial activation)
+```
+
+Non-Resume invalidation RED:
+
+```text
+cd frontend && npx vitest run src/shell/AppShell.session-mgmt.test.tsx \
+  -t "starting a new question invalidates"
+
+Test Files  1 failed (1)
+Tests       1 failed | 19 skipped (20)
+
+the late Resume opened an EventSource after Start new returned to the landing state
+```
+
+Frontend review GREEN:
+
+```text
+cd frontend && npx vitest run src/shell/AppShell.session-mgmt.test.tsx \
+  -t "share one cold request|competing Resume|stale Resume manifest|starting a new question invalidates"
+
+Test Files  1 passed (1)
+Tests       4 passed | 15 skipped (19)
+```
+
+Post-commit single-flight lifetime RED:
+
+```text
+cd frontend && npx vitest run src/shell/AppShell.session-mgmt.test.tsx \
+  -t "releases same-id single-flight"
+
+Test Files  1 failed (1)
+Tests       1 failed | 19 skipped (20)
+
+s1 committed and waited on its manifest; after s3 superseded it, a new s1 Resume reused the old
+operation and issued no second s1 POST (expected 2, received 1).
+```
+
+Same-id and manifest lifetime GREEN:
+
+```text
+cd frontend && npx vitest run src/shell/AppShell.session-mgmt.test.tsx -t "same-id|manifest"
+Test Files  1 passed (1)
+Tests       4 passed | 16 skipped (20)
+
+cd frontend && npx tsc -b
+no output, exit 0
+```
+
+Multi-client SSE disconnect RED:
+
+```text
+cd backend && npx vitest run test/sse-hub.test.ts test/sse-route.test.ts
+
+Test Files  2 failed (2)
+Tests       3 failed | 6 passed (9)
+
+clear/forget invoked zero of two registered close callbacks, and two live HTTP SSE responses timed
+out instead of reaching EOF after clear.
+```
+
+Multi-client SSE disconnect GREEN:
+
+```text
+cd backend && npx vitest run test/sse-hub.test.ts test/sse-route.test.ts
+Test Files  2 passed (2)
+Tests       9 passed (9)
+
+cd backend && npx tsc --noEmit -p .
+no output, exit 0
+```
+
+The Hub tests use two subscribers whose close callbacks immediately unsubscribe themselves, proving
+safe snapshot iteration and exactly-once closure. The live-route test opens two HTTP streams, proves
+both receive EOF on clear, publishes a new event and gate, then reconnects after id 1 and replays
+exactly ids 2 and 3. The forget test closes both subscribers and proves the next id resets to 1.
+
+Close now removes the observed runtime identity before awaiting persistence. Failure persistence is
+claimed once per runtime and lifecycle-serialized; bootstrap's public `session_failed` cannot start
+a duplicate. A per-target in-flight map owns the only same-ID POST and commit while its mutable
+latest token keeps s1→s2→s1 ordering correct; it is removed immediately after the binding commit,
+before awaiting the independently guarded manifest. One shared invalidation helper is called when
+active deletion, streamed exit, Start new, or Stop begins.
+
+### Focused verification
+
+```text
+cd backend && npx vitest run test/routes-sessions.test.ts test/pi-process-manager.test.ts \
+  test/sse-hub.test.ts test/sse-route.test.ts
+Test Files  4 passed (4)
+Tests       96 passed (96)
+
+cd backend && npx tsc --noEmit -p .
+no output, exit 0
+
+cd frontend && npx vitest run src/shell/AppShell.session-mgmt.test.tsx \
+  src/shell/AppShell.new-session.test.tsx src/stream/useSessionStream.test.tsx
+Test Files  3 passed (3)
+Tests       36 passed (36)
+
+cd frontend && npx tsc -b
+no output, exit 0
+```
+
+### Full verification
+
+```text
+cd backend && npx vitest run
+Test Files  22 passed (22)
+Tests       202 passed (202)
+
+cd frontend && npx vitest run
+Test Files  43 passed (43)
+Tests       280 passed (280)
+
+cd backend && npm run build
+> tsc -p tsconfig.json
+exit 0
+
+cd frontend && npm run build
+> tsc -b && vite build
+✓ 4835 modules transformed.
+✓ built in 8.47s
+exit 0
+```
+
+The frontend suite/build retain the previously documented MSW, React ref/`act`, experimental type
+stripping, and large-chunk warnings. No warning class was introduced by this wave. No harness,
+workflow, persistence, SQL/CTE viewer, model-selection, or deployment file changed. The four
+pre-existing modified `.superpowers/sdd/{progress,task-2-report,task-3-report,task-4-report}.md`
+files remain excluded from staging.
+
+### Final independent-review verdict
+
+After the multi-client transport fix, the independent reviewer reported no Critical, Important, or
+Minor findings. Its own focused verification passed 96 backend transport/lifecycle tests, 31
+frontend Resume/stream tests, both TypeScript checks, and `git diff --check`. Final assessment:
+**Ready to deploy: Yes.**

backend/src/pi/pi-process-manager.ts

@@ -174,9 +174,16 @@ export class PiProcessManager {
 
   teardown(id: string): void {
     const rt = this.runtimes.get(id);
-    if (rt) {
-      rt.child.kill();
-      this.runtimes.delete(id);
-    }
+    if (rt) this.teardownIfCurrent(id, rt);
+  }
+
+  /** Remove only the runtime identity the caller observed. */
+  teardownIfCurrent(id: string, expected: SessionRuntime): boolean {
+    if (this.runtimes.get(id) !== expected) return false;
+    // Delete before signalling the child so its asynchronous exit cannot be mistaken for a
+    // crash, and so a replacement installed by a later lifecycle operation is never targeted.
+    this.runtimes.delete(id);
+    expected.child.kill();
+    return true;
   }
 }

backend/src/routes/sessions.ts

@@ -27,24 +27,27 @@ export function sessionRoutes(
   app: FastifyInstance,
   d: { mgr: PiProcessManager; tht: ThtRunner; hub: SseHub; getSettings: () => Settings; readiness: ReadinessManager },
 ) {
-  const resumeTails = new Map<string, Promise<void>>();
+  const lifecycleTails = new Map<string, Promise<void>>();
   const boundRuntimes = new Map<
     string,
     ReturnType<PiProcessManager["createFor"]>
   >();
+  const failurePersistenceClaimed = new WeakSet<
+    ReturnType<PiProcessManager["createFor"]>
+  >();
 
-  const withResumeLock = async <T>(id: string, work: () => Promise<T>): Promise<T> => {
-    const previous = resumeTails.get(id) ?? Promise.resolve();
+  const withSessionLifecycle = async <T>(id: string, work: () => Promise<T>): Promise<T> => {
+    const previous = lifecycleTails.get(id) ?? Promise.resolve();
     let release!: () => void;
     const gate = new Promise<void>((resolve) => { release = resolve; });
     const tail = previous.then(() => gate);
-    resumeTails.set(id, tail);
+    lifecycleTails.set(id, tail);
     await previous;
     try {
       return await work();
     } finally {
       release();
-      if (resumeTails.get(id) === tail) resumeTails.delete(id);
+      if (lifecycleTails.get(id) === tail) lifecycleTails.delete(id);
     }
   };
 
@@ -61,7 +64,17 @@ export function sessionRoutes(
         // remains bound after an unexpected exit so its public failure events still reach SSE.
         if (boundRuntimes.get(id) !== rt) return;
         if (e.type === "system_event" && e.event === "session_failed") {
-          void d.tht.failSession(id, d.getSettings().workspace).catch(() => undefined);
+          if (!failurePersistenceClaimed.has(rt)) {
+            failurePersistenceClaimed.add(rt);
+            void withSessionLifecycle(id, async () => {
+              // agent_end can release the old binding before this queued work acquires the lock.
+              // Undefined means no replacement; a different identity means Resume won and the
+              // old failure must not touch its manifest.
+              const bound = boundRuntimes.get(id);
+              if (bound !== undefined && bound !== rt) return;
+              await d.tht.failSession(id, d.getSettings().workspace).catch(() => undefined);
+            }).catch(() => undefined);
+          }
         }
         d.hub.publish(id, e.type, e);
         if (
@@ -91,14 +104,25 @@ export function sessionRoutes(
       try {
         if (retrieval) info(id, "Preparing retrieval context");
         await Promise.all([configure, retrieval]);
+        if (d.mgr.get(id) !== rt) return;
         info(id, "Starting model");
+        if (d.mgr.get(id) !== rt) return;
         start();
       } catch {
-        d.mgr.teardown(id);
-        void d.tht.failSession(id, d.getSettings().workspace).catch(() => undefined);
-        rt.bridge.emitClientEvent({ type: "info", level: "error", text: BOOTSTRAP_FAILURE_MESSAGE });
-        rt.bridge.emitClientEvent({ type: "system_event", event: "session_failed" });
-        rt.bridge.emitClientEvent({ type: "system_event", event: "agent_end" });
+        void withSessionLifecycle(id, async () => {
+          // A bootstrap continuation can settle after Close/Delete or after a replacement was
+          // installed. Claim only the runtime identity that actually failed; holding the same
+          // lifecycle lock through persistence prevents a Resume from becoming that failure's
+          // accidental target.
+          if (d.mgr.get(id) !== rt || !d.mgr.teardownIfCurrent(id, rt)) return;
+          if (!failurePersistenceClaimed.has(rt)) {
+            failurePersistenceClaimed.add(rt);
+            await d.tht.failSession(id, d.getSettings().workspace).catch(() => undefined);
+          }
+          rt.bridge.emitClientEvent({ type: "info", level: "error", text: BOOTSTRAP_FAILURE_MESSAGE });
+          rt.bridge.emitClientEvent({ type: "system_event", event: "session_failed" });
+          rt.bridge.emitClientEvent({ type: "system_event", event: "agent_end" });
+        });
       }
     })();
   };
@@ -158,7 +182,7 @@ export function sessionRoutes(
   });
   app.post("/sessions/:id/resume", async (req, reply) => {
     const id = (req.params as any).id;
-    return withResumeLock(id, async () => {
+    return withSessionLifecycle(id, async () => {
       // This check belongs inside the per-session lock: a preceding cold Resume may have
       // installed a running runtime while this request was waiting.
       const existing = d.mgr.get(id);
@@ -192,18 +216,31 @@ export function sessionRoutes(
         return reply.code(503).send({ error: RESUME_FAILURE_MESSAGE });
       }
 
+      // Re-check immediately before the replacement commit. A lifecycle operation that ran
+      // before this request acquired the lock may have changed or removed the runtime.
+      const current = d.mgr.get(id);
+      if (current) {
+        const state = current.bridge.turnState();
+        if (state === "running" || state === "waiting") {
+          return reply.code(200).send({ id, alreadyActive: true });
+        }
+      }
+
       let rt: ReturnType<PiProcessManager["createFor"]> | undefined;
       try {
-        if (existing) {
-          boundRuntimes.delete(id);
-          d.mgr.teardown(id);
+        if (current) {
+          if (boundRuntimes.get(id) === current) boundRuntimes.delete(id);
+          d.mgr.teardownIfCurrent(id, current);
         }
         rt = d.mgr.createFor(id, options);
         bindRuntime(id, rt);
       } catch {
         // A created-but-unbound runtime is not usable. The old hub remains attached because
         // clear() has not happened yet.
-        if (rt) d.mgr.teardown(id);
+        if (rt) {
+          if (boundRuntimes.get(id) === rt) boundRuntimes.delete(id);
+          d.mgr.teardownIfCurrent(id, rt);
+        }
         return reply.code(503).send({ error: RESUME_FAILURE_MESSAGE });
       }
 
@@ -217,14 +254,19 @@ export function sessionRoutes(
   });
   app.post("/sessions/:id/close", async (req) => {
     const id = (req.params as { id: string }).id;
-    try {
-      await d.tht.closeSession(id, d.getSettings().workspace);
-    } finally {
+    return withSessionLifecycle(id, async () => {
+      // Invalidate the live generation before persistence can yield. Otherwise its deferred
+      // bootstrap may start Pi while Close is already in progress.
+      const current = d.mgr.get(id);
       boundRuntimes.delete(id);
-      d.mgr.teardown(id);
-      d.hub.clear(id);
-    }
-    return { closed: true };
+      if (current) d.mgr.teardownIfCurrent(id, current);
+      try {
+        await d.tht.closeSession(id, d.getSettings().workspace);
+      } finally {
+        d.hub.clear(id);
+      }
+      return { closed: true };
+    });
   });
   app.get("/sessions/:id/events", (req, reply) => {
     const id = (req.params as any).id;
@@ -252,6 +294,11 @@ export function sessionRoutes(
     const off = d.hub.subscribe(id, send, {
       afterId,
       pending: rt?.bridge.pendingWidget() ?? null,
+      // clear()/forget() end every old transport so native EventSource reconnects with its
+      // Last-Event-ID instead of remaining attached to a subscriber callback that no longer exists.
+      close: () => {
+        if (!reply.raw.writableEnded) reply.raw.end();
+      },
     });
     req.raw.on("close", off);
   });
@@ -273,11 +320,14 @@ export function sessionRoutes(
   });
   app.delete("/sessions/:id", async (req, reply) => {
     const id = (req.params as any).id;
-    boundRuntimes.delete(id);
-    d.mgr.teardown(id); // drop any live runtime before deleting on disk
-    await d.tht.deleteSession(id, d.getSettings().workspace);
-    d.hub.forget(id);
-    return reply.code(204).send();
+    return withSessionLifecycle(id, async () => {
+      const current = d.mgr.get(id);
+      boundRuntimes.delete(id);
+      if (current) d.mgr.teardownIfCurrent(id, current);
+      await d.tht.deleteSession(id, d.getSettings().workspace);
+      d.hub.forget(id);
+      return reply.code(204).send();
+    });
   });
   app.get("/sessions/:id/documents", async (req) => d.tht.documents((req.params as any).id));
 }

backend/src/sse/sse-hub.ts

@@ -1,5 +1,11 @@
 type Send = (event: string, data: object, id: number) => void;
 
+interface Subscriber {
+  send: Send;
+  close: () => void;
+  closed: boolean;
+}
+
 /**
  * Per-session ring buffer of recent events.
  *
@@ -17,6 +23,7 @@ interface BufferedEvent {
 interface SubscribeOptions {
   afterId?: number;
   pending?: object | null;
+  close?: () => void;
 }
 
 function descriptorId(value: unknown): string | null {
@@ -32,7 +39,7 @@ function bufferedGateId(item: BufferedEvent): string | null {
 }
 
 export class SseHub {
-  private subs = new Map<string, Set<Send>>();
+  private subs = new Map<string, Set<Subscriber>>();
   private buffers = new Map<string, BufferedEvent[]>();
   private lastIds = new Map<string, number>();
 
@@ -42,7 +49,8 @@ export class SseHub {
     options: SubscribeOptions = {},
   ): () => void {
     if (!this.subs.has(sessionId)) this.subs.set(sessionId, new Set());
-    this.subs.get(sessionId)!.add(send);
+    const subscriber = { send, close: options.close ?? (() => undefined), closed: false };
+    this.subs.get(sessionId)!.add(subscriber);
 
     const afterId = Number.isSafeInteger(options.afterId) && (options.afterId ?? 0) >= 0
       ? options.afterId ?? 0
@@ -63,12 +71,14 @@ export class SseHub {
       send(item.event, item.data, item.id);
     }
 
-    return () => this.subs.get(sessionId)?.delete(send);
+    return () => this.subs.get(sessionId)?.delete(subscriber);
   }
 
   publish(sessionId: string, event: string, data: object): number {
     const item = this.buffer(sessionId, event, data);
-    for (const send of this.subs.get(sessionId) ?? []) send(event, data, item.id);
+    for (const subscriber of this.subs.get(sessionId) ?? []) {
+      subscriber.send(event, data, item.id);
+    }
     return item.id;
   }
 
@@ -88,6 +98,13 @@ export class SseHub {
 
   /** Clear buffered/runtime bindings while retaining session event-id monotonicity. */
   clear(sessionId: string): void {
+    // Snapshot because ending an HTTP response can synchronously/asynchronously unsubscribe it.
+    // Mark before invoking callbacks so even a re-entrant clear cannot close a response twice.
+    for (const subscriber of [...(this.subs.get(sessionId) ?? [])]) {
+      if (subscriber.closed) continue;
+      subscriber.closed = true;
+      try { subscriber.close(); } catch { /* disconnect every remaining subscriber */ }
+    }
     this.buffers.delete(sessionId);
     this.subs.delete(sessionId);
   }

backend/test/pi-process-manager.test.ts

@@ -66,6 +66,27 @@ test("l'exit del VECCHIO child non elimina il nuovo runtime (exit identity-check
   mgr.teardown("respawn-id");
 });
 
+test("identity-checked teardown cannot kill a replacement runtime", () => {
+  const cfg = loadConfig({ THT_HARNESS_DIR: "../harness" });
+  const firstChild = recordingChild();
+  const secondChild = recordingChild();
+  firstChild.kill = vi.fn();
+  secondChild.kill = vi.fn();
+  const children = [firstChild, secondChild];
+  const mgr = new PiProcessManager(cfg, { spawnFn: () => children.shift() as any });
+  const first = mgr.createFor("replace-id", {});
+  mgr.teardown("replace-id");
+  const second = mgr.createFor("replace-id", {});
+
+  expect(mgr.teardownIfCurrent("replace-id", first)).toBe(false);
+  expect(mgr.get("replace-id")).toBe(second);
+  expect(secondChild.kill).not.toHaveBeenCalled();
+
+  expect(mgr.teardownIfCurrent("replace-id", second)).toBe(true);
+  expect(mgr.get("replace-id")).toBeUndefined();
+  expect(secondChild.kill).toHaveBeenCalledOnce();
+});
+
 test("oltre maxPiProcesses solleva errore", async () => {
   const cfg = { ...loadConfig({}), maxPiProcesses: 1 };
   const mgr = new PiProcessManager(cfg, { spawnFn: () => spawn("node", [FAKE, SCRIPT]) as any });

backend/test/routes-sessions.test.ts

@@ -18,6 +18,16 @@ function mutApp(thtRunner: any) {
   });
 }
 
+function deferred<T = void>() {
+  let resolve!: (value: T | PromiseLike<T>) => void;
+  let reject!: (reason?: unknown) => void;
+  const promise = new Promise<T>((onResolve, onReject) => {
+    resolve = onResolve;
+    reject = onReject;
+  });
+  return { promise, resolve, reject };
+}
+
 test("POST /sessions usa i settings (workspace/provider/model/thinking) e crea+avvia", async () => {
   const modelKey = path.join(os.tmpdir(), `thoth-model-key-${process.pid}`);
   writeFileSync(modelKey, "test-model-key", { mode: 0o600 });
@@ -52,11 +62,17 @@ test("POST /sessions configura Pi con il thinking globale selezionato", async ()
   let configured: any;
   const bridge = { onClientEvent: () => {}, emitClientEvent: () => {} };
   const runtime = { bridge } as any;
+  let current: any;
   const mgr = {
-    createFor: () => runtime,
+    get: () => current,
+    createFor: () => { current = runtime; return runtime; },
     configure: async (_rt: any, options: any) => { configured = options; },
     start: () => {},
-    teardown: () => {},
+    teardownIfCurrent: (_id: string, expected: any) => {
+      if (current !== expected) return false;
+      current = undefined;
+      return true;
+    },
   } as any;
   const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
     mgr,
@@ -78,12 +94,17 @@ test("POST /sessions/:id/resume configura Pi con il thinking persistito", async
   let configured: any;
   const bridge = { onClientEvent: () => {}, emitClientEvent: () => {} };
   const runtime = { bridge } as any;
+  let current: any;
   const mgr = {
-    get: () => undefined,
-    createFor: () => runtime,
+    get: () => current,
+    createFor: () => { current = runtime; return runtime; },
     configure: async (_rt: any, options: any) => { configured = options; },
     start: () => {},
-    teardown: () => {},
+    teardownIfCurrent: (_id: string, expected: any) => {
+      if (current !== expected) return false;
+      current = undefined;
+      return true;
+    },
   } as any;
   const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
     mgr,
@@ -111,12 +132,17 @@ test("POST /sessions/:id/resume usa il thinking globale se manca nel manifest",
   let configured: any;
   const bridge = { onClientEvent: () => {}, emitClientEvent: () => {} };
   const runtime = { bridge } as any;
+  let current: any;
   const mgr = {
-    get: () => undefined,
-    createFor: () => runtime,
+    get: () => current,
+    createFor: () => { current = runtime; return runtime; },
     configure: async (_rt: any, options: any) => { configured = options; },
     start: () => {},
-    teardown: () => {},
+    teardownIfCurrent: (_id: string, expected: any) => {
+      if (current !== expected) return false;
+      current = undefined;
+      return true;
+    },
   } as any;
   const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
     mgr,
@@ -162,11 +188,17 @@ test.each(["idle", "failed"])(
     const order: string[] = [];
     const oldRuntime = { bridge: { turnState: () => state } } as any;
     const newRuntime = { bridge: { onClientEvent: () => {}, emitClientEvent: () => {} } } as any;
+    let current: any = oldRuntime;
     const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
       mgr: {
-        get: () => oldRuntime,
-        teardown: (id: string) => order.push(`teardown:${id}`),
-        createFor: () => { order.push("create"); return newRuntime; },
+        get: () => current,
+        teardownIfCurrent: (id: string, expected: any) => {
+          if (current !== expected) return false;
+          order.push(`teardown:${id}`);
+          current = undefined;
+          return true;
+        },
+        createFor: () => { order.push("create"); current = newRuntime; return newRuntime; },
         configure: async () => {},
         start: () => order.push("start"),
       } as any,
@@ -195,12 +227,14 @@ test("POST resume without a runtime clears stale SSE state before cold start", a
   const order: string[] = [];
   let createOptions: any;
   const newRuntime = { bridge: { onClientEvent: () => {}, emitClientEvent: () => {} } } as any;
+  let current: any;
   const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
     mgr: {
-      get: () => undefined,
+      get: () => current,
       createFor: (_id: string, options: any) => {
         createOptions = options;
         order.push("create");
+        current = newRuntime;
         return newRuntime;
       },
       configure: async () => {},
@@ -327,10 +361,15 @@ test("POST resume sanitizes runtime creation failure and preserves the old hub a
   } as any;
   hub.subscribe("s1", (_event: string, data: any) => delivered.push(data.text));
   hub.publish("s1", "info", { text: "before" });
+  let current: any = { bridge: { turnState: () => "failed" } };
   const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
     mgr: {
-      get: () => ({ bridge: { turnState: () => "failed" } }),
-      teardown: () => {},
+      get: () => current,
+      teardownIfCurrent: (_id: string, expected: any) => {
+        if (current !== expected) return false;
+        current = undefined;
+        return true;
+      },
       createFor: () => { createCalls += 1; throw new Error("CREATE_SENTINEL"); },
     } as any,
     hub,
@@ -424,6 +463,490 @@ test("concurrent cold Resume requests serialize and create one runtime", async (
   });
 });
 
+test.each([
+  ["close", "resolve"],
+  ["close", "reject"],
+  ["delete", "resolve"],
+  ["delete", "reject"],
+] as const)(
+  "a bootstrap that %s left behind cannot %s against a replacement runtime",
+  async (lifecycle, outcome) => {
+    const oldConfigure = deferred<void>();
+    const published: any[] = [];
+    const starts: string[] = [];
+    let failed = 0;
+    let configureCalls = 0;
+    let current: any;
+    const runtime = (name: string) => {
+      let listener: ((event: any) => void) | undefined;
+      return {
+        name,
+        bridge: {
+          turnState: () => "running",
+          onClientEvent: (next: (event: any) => void) => { listener = next; },
+          emitClientEvent: (event: any) => listener?.(event),
+        },
+      };
+    };
+    const oldRuntime = runtime("old");
+    const replacement = runtime("replacement");
+    const runtimes = [oldRuntime, replacement];
+    const mgr = {
+      get: () => current,
+      createFor: () => {
+        current = runtimes.shift();
+        return current;
+      },
+      configure: async () => {
+        configureCalls += 1;
+        if (configureCalls === 1) await oldConfigure.promise;
+      },
+      start: (_id: string, expected: any) => {
+        if (current !== expected) throw new Error("stale start");
+        starts.push(expected.name);
+      },
+      teardown: () => { current = undefined; },
+      teardownIfCurrent: (_id: string, expected: any) => {
+        if (current !== expected) return false;
+        current = undefined;
+        return true;
+      },
+    };
+    const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
+      mgr: mgr as any,
+      hub: {
+        publish: (_id: string, event: string, data: object) => {
+          published.push({ event, data });
+          return published.length;
+        },
+        clear: () => {},
+        forget: () => {},
+      } as any,
+      thtRunner: {
+        sessionNew: async () => ({ id: "s1" }),
+        searchPack: async () => {},
+        sessionShow: async () => ({ status: "open", archived: false }),
+        reopenSession: async () => {},
+        closeSession: async () => {},
+        deleteSession: async () => {},
+        failSession: async () => { failed += 1; },
+      } as any,
+      readiness: { ensure: async () => ({ ok: true }) } as any,
+      getSettings: () => ({ workspace: "local" }) as any,
+    });
+
+    await app.inject({ method: "POST", url: "/sessions", payload: { question: "old" } });
+    if (lifecycle === "close") {
+      await app.inject({ method: "POST", url: "/sessions/s1/close" });
+      await app.inject({ method: "POST", url: "/sessions/s1/resume" });
+    } else {
+      await app.inject({ method: "DELETE", url: "/sessions/s1" });
+      await app.inject({ method: "POST", url: "/sessions", payload: { question: "replacement" } });
+    }
+    await new Promise((resolve) => setImmediate(resolve));
+    expect(current).toBe(replacement);
+    published.length = 0;
+    starts.length = 0;
+    failed = 0;
+
+    if (outcome === "resolve") oldConfigure.resolve();
+    else oldConfigure.reject(new Error("old bootstrap failed"));
+    await new Promise((resolve) => setImmediate(resolve));
+
+    expect(current).toBe(replacement);
+    expect(starts).toEqual([]);
+    expect(failed).toBe(0);
+    expect(published).toEqual([]);
+  },
+);
+
+test.each(["resolve", "reject"] as const)(
+  "a deleted session stays forgotten when its stale bootstrap later %s",
+  async (outcome) => {
+    const oldConfigure = deferred<void>();
+    const published: any[] = [];
+    let failed = 0;
+    let current: any;
+    let forgotten = false;
+    let listener: ((event: any) => void) | undefined;
+    const runtime = {
+      bridge: {
+        onClientEvent: (next: (event: any) => void) => { listener = next; },
+        emitClientEvent: (event: any) => listener?.(event),
+      },
+    };
+    const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
+      mgr: {
+        get: () => current,
+        createFor: () => { current = runtime; return runtime; },
+        configure: async () => oldConfigure.promise,
+        start: () => { throw new Error("deleted runtime must not start"); },
+        teardown: () => { current = undefined; },
+        teardownIfCurrent: (_id: string, expected: any) => {
+          if (current !== expected) return false;
+          current = undefined;
+          return true;
+        },
+      } as any,
+      hub: {
+        publish: (_id: string, event: string, data: object) => {
+          published.push({ event, data });
+          return published.length;
+        },
+        forget: () => { forgotten = true; },
+      } as any,
+      thtRunner: {
+        sessionNew: async () => ({ id: "s1" }),
+        searchPack: async () => {},
+        deleteSession: async () => {},
+        failSession: async () => { failed += 1; },
+      } as any,
+      readiness: { ensure: async () => ({ ok: true }) } as any,
+      getSettings: () => ({ workspace: "local" }) as any,
+    });
+
+    await app.inject({ method: "POST", url: "/sessions", payload: { question: "old" } });
+    await app.inject({ method: "DELETE", url: "/sessions/s1" });
+    published.length = 0;
+
+    if (outcome === "resolve") oldConfigure.resolve();
+    else oldConfigure.reject(new Error("old bootstrap failed"));
+    await new Promise((resolve) => setImmediate(resolve));
+
+    expect(current).toBeUndefined();
+    expect(forgotten).toBe(true);
+    expect(failed).toBe(0);
+    expect(published).toEqual([]);
+  },
+);
+
+test.each(["close", "delete"] as const)(
+  "Resume followed by %s leaves no runtime or post-removal bootstrap events",
+  async (lifecycle) => {
+    const reopen = deferred<void>();
+    const configure = deferred<void>();
+    let markReopenStarted!: () => void;
+    const reopenStarted = new Promise<void>((resolve) => { markReopenStarted = resolve; });
+    let current: any;
+    let removalSettled = false;
+    const published: any[] = [];
+    const removalEvents: string[] = [];
+    const replacement = {
+      bridge: {
+        turnState: () => "running",
+        onClientEvent: () => {},
+        emitClientEvent: () => {},
+      },
+    };
+    const teardown = (expected?: any) => {
+      if (expected && current !== expected) return false;
+      current = undefined;
+      return true;
+    };
+    const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
+      mgr: {
+        get: () => current,
+        createFor: () => { current = replacement; return replacement; },
+        configure: async () => configure.promise,
+        start: () => { throw new Error("removed runtime must not start"); },
+        teardown: () => { teardown(); },
+        teardownIfCurrent: (_id: string, expected: any) => teardown(expected),
+      } as any,
+      hub: {
+        publish: (_id: string, event: string, data: object) => {
+          published.push({ event, data });
+          return published.length;
+        },
+        clear: () => { removalEvents.push("clear"); },
+        forget: () => { removalEvents.push("forget"); },
+      } as any,
+      thtRunner: {
+        sessionShow: async () => ({ status: "open", archived: false }),
+        reopenSession: async () => {
+          markReopenStarted();
+          await reopen.promise;
+        },
+        closeSession: async () => { removalEvents.push("close"); },
+        deleteSession: async () => { removalEvents.push("delete"); },
+        failSession: async () => {},
+      } as any,
+      readiness: { ensure: async () => ({ ok: true }) } as any,
+      getSettings: () => ({ workspace: "local" }) as any,
+    });
+
+    const resumeResponse = app.inject({ method: "POST", url: "/sessions/s1/resume" });
+    await reopenStarted;
+    const removalResponse = app.inject({
+      method: lifecycle === "close" ? "POST" : "DELETE",
+      url: `/sessions/s1/${lifecycle === "close" ? "close" : ""}`.replace(/\/$/, ""),
+    }).then((response) => {
+      removalSettled = true;
+      return response;
+    });
+    await new Promise((resolve) => setImmediate(resolve));
+    expect(removalSettled).toBe(false);
+
+    reopen.resolve();
+    expect((await resumeResponse).json()).toEqual({ id: "s1", alreadyActive: false });
+    await removalResponse;
+    published.length = 0;
+    configure.resolve();
+    await new Promise((resolve) => setImmediate(resolve));
+
+    expect(current).toBeUndefined();
+    expect(published).toEqual([]);
+    expect(removalEvents.at(-1)).toBe(lifecycle === "close" ? "clear" : "forget");
+  },
+);
+
+test("Close followed by Resume installs a fresh runtime only after Close finishes", async () => {
+  const close = deferred<void>();
+  let markCloseStarted!: () => void;
+  const closeStarted = new Promise<void>((resolve) => { markCloseStarted = resolve; });
+  const oldRuntime = { bridge: { turnState: () => "running" } };
+  const replacement = {
+    bridge: {
+      turnState: () => "running",
+      onClientEvent: () => {},
+      emitClientEvent: () => {},
+    },
+  };
+  let current: any = oldRuntime;
+  let resumeSettled = false;
+  const teardown = (expected?: any) => {
+    if (expected && current !== expected) return false;
+    current = undefined;
+    return true;
+  };
+  const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
+    mgr: {
+      get: () => current,
+      createFor: () => { current = replacement; return replacement; },
+      configure: async () => {},
+      start: () => {},
+      teardown: () => { teardown(); },
+      teardownIfCurrent: (_id: string, expected: any) => teardown(expected),
+    } as any,
+    hub: { publish: () => 1, clear: () => {} } as any,
+    thtRunner: {
+      closeSession: async () => { markCloseStarted(); await close.promise; },
+      sessionShow: async () => ({ status: "open", archived: false }),
+      reopenSession: async () => {},
+    } as any,
+    readiness: { ensure: async () => ({ ok: true }) } as any,
+    getSettings: () => ({ workspace: "local" }) as any,
+  });
+
+  const closeResponse = app.inject({ method: "POST", url: "/sessions/s1/close" });
+  await closeStarted;
+  const resumeResponse = app.inject({ method: "POST", url: "/sessions/s1/resume" })
+    .then((response) => { resumeSettled = true; return response; });
+  await new Promise((resolve) => setImmediate(resolve));
+  expect(resumeSettled).toBe(false);
+
+  close.resolve();
+  await closeResponse;
+  const resumed = await resumeResponse;
+  await new Promise((resolve) => setImmediate(resolve));
+
+  expect(resumed.json()).toEqual({ id: "s1", alreadyActive: false });
+  expect(current).toBe(replacement);
+});
+
+test("Close suppresses a bootstrap that settles while close persistence is pending", async () => {
+  const configure = deferred<void>();
+  const close = deferred<void>();
+  let markCloseStarted!: () => void;
+  const closeStarted = new Promise<void>((resolve) => { markCloseStarted = resolve; });
+  const published: any[] = [];
+  const starts: any[] = [];
+  let current: any;
+  let listener: ((event: any) => void) | undefined;
+  const runtime = {
+    bridge: {
+      onClientEvent: (next: (event: any) => void) => { listener = next; },
+      emitClientEvent: (event: any) => listener?.(event),
+    },
+  };
+  const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
+    mgr: {
+      get: () => current,
+      createFor: () => { current = runtime; return runtime; },
+      configure: async () => configure.promise,
+      start: (_id: string, expected: any) => { starts.push(expected); },
+      teardownIfCurrent: (_id: string, expected: any) => {
+        if (current !== expected) return false;
+        current = undefined;
+        return true;
+      },
+    } as any,
+    hub: {
+      publish: (_id: string, event: string, data: object) => {
+        published.push({ event, data });
+        return published.length;
+      },
+      clear: () => {},
+    } as any,
+    thtRunner: {
+      sessionNew: async () => ({ id: "s1" }),
+      searchPack: async () => {},
+      closeSession: async () => { markCloseStarted(); await close.promise; },
+      failSession: async () => {},
+    } as any,
+    readiness: { ensure: async () => ({ ok: true }) } as any,
+    getSettings: () => ({ workspace: "local" }) as any,
+  });
+
+  await app.inject({ method: "POST", url: "/sessions", payload: { question: "q" } });
+  const closeResponse = app.inject({ method: "POST", url: "/sessions/s1/close" });
+  await closeStarted;
+  published.length = 0;
+  configure.resolve();
+  await new Promise((resolve) => setImmediate(resolve));
+
+  expect(starts).toEqual([]);
+  expect(published).toEqual([]);
+
+  close.resolve();
+  await closeResponse;
+  expect(current).toBeUndefined();
+});
+
+test("bootstrap failure persists once and keeps Resume serialized behind that persistence", async () => {
+  const oldConfigure = deferred<void>();
+  const failurePersistence = deferred<void>();
+  let markFailureStarted!: () => void;
+  const failureStarted = new Promise<void>((resolve) => { markFailureStarted = resolve; });
+  const oldRuntime = {
+    bridge: {
+      onClientEvent: undefined as ((next: (event: any) => void) => void) | undefined,
+      emitClientEvent: undefined as ((event: any) => void) | undefined,
+    },
+  } as any;
+  const replacement = {
+    bridge: {
+      turnState: () => "running",
+      onClientEvent: () => {},
+      emitClientEvent: () => {},
+    },
+  };
+  let oldListener: ((event: any) => void) | undefined;
+  oldRuntime.bridge.onClientEvent = (next: (event: any) => void) => { oldListener = next; };
+  oldRuntime.bridge.emitClientEvent = (event: any) => oldListener?.(event);
+  const runtimes = [oldRuntime, replacement];
+  let current: any;
+  let configureCalls = 0;
+  let failCalls = 0;
+  let resumeSettled = false;
+  const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
+    mgr: {
+      get: () => current,
+      createFor: () => { current = runtimes.shift(); return current; },
+      configure: async () => {
+        configureCalls += 1;
+        if (configureCalls === 1) await oldConfigure.promise;
+      },
+      start: () => {},
+      teardownIfCurrent: (_id: string, expected: any) => {
+        if (current !== expected) return false;
+        current = undefined;
+        return true;
+      },
+    } as any,
+    hub: { publish: () => 1, clear: () => {} } as any,
+    thtRunner: {
+      sessionNew: async () => ({ id: "s1" }),
+      searchPack: async () => {},
+      sessionShow: async () => ({ status: "open", archived: false }),
+      reopenSession: async () => {},
+      failSession: async () => {
+        failCalls += 1;
+        if (failCalls === 1) {
+          markFailureStarted();
+          await failurePersistence.promise;
+        }
+      },
+    } as any,
+    readiness: { ensure: async () => ({ ok: true }) } as any,
+    getSettings: () => ({ workspace: "local" }) as any,
+  });
+
+  await app.inject({ method: "POST", url: "/sessions", payload: { question: "q" } });
+  oldConfigure.reject(new Error("configure failed"));
+  await failureStarted;
+  const resumeResponse = app.inject({ method: "POST", url: "/sessions/s1/resume" })
+    .then((response) => { resumeSettled = true; return response; });
+  await new Promise((resolve) => setImmediate(resolve));
+  expect(resumeSettled).toBe(false);
+
+  failurePersistence.resolve();
+  expect((await resumeResponse).json()).toEqual({ id: "s1", alreadyActive: false });
+  await new Promise((resolve) => setImmediate(resolve));
+
+  expect(failCalls).toBe(1);
+  expect(current).toBe(replacement);
+});
+
+test("Delete followed by Resume cannot resurrect the deleted session", async () => {
+  const deletion = deferred<void>();
+  let markDeleteStarted!: () => void;
+  const deleteStarted = new Promise<void>((resolve) => { markDeleteStarted = resolve; });
+  const oldRuntime = { bridge: { turnState: () => "running" } };
+  let current: any = oldRuntime;
+  let deleted = false;
+  let resumeSettled = false;
+  let createCalls = 0;
+  const teardown = (expected?: any) => {
+    if (expected && current !== expected) return false;
+    current = undefined;
+    return true;
+  };
+  const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
+    mgr: {
+      get: () => current,
+      createFor: () => {
+        createCalls += 1;
+        current = { bridge: { onClientEvent: () => {}, emitClientEvent: () => {} } };
+        return current;
+      },
+      configure: async () => {},
+      start: () => {},
+      teardown: () => { teardown(); },
+      teardownIfCurrent: (_id: string, expected: any) => teardown(expected),
+    } as any,
+    hub: { publish: () => 1, clear: () => {}, forget: () => {} } as any,
+    thtRunner: {
+      deleteSession: async () => {
+        markDeleteStarted();
+        await deletion.promise;
+        deleted = true;
+      },
+      sessionShow: async () => {
+        if (deleted) throw new Error("session deleted");
+        return { status: "open", archived: false };
+      },
+      reopenSession: async () => {},
+    } as any,
+    readiness: { ensure: async () => ({ ok: true }) } as any,
+    getSettings: () => ({ workspace: "local" }) as any,
+  });
+
+  const deleteResponse = app.inject({ method: "DELETE", url: "/sessions/s1" });
+  await deleteStarted;
+  const resumeResponse = app.inject({ method: "POST", url: "/sessions/s1/resume" })
+    .then((response) => { resumeSettled = true; return response; });
+  await new Promise((resolve) => setImmediate(resolve));
+  expect(resumeSettled).toBe(false);
+
+  deletion.resolve();
+  await deleteResponse;
+  const resumed = await resumeResponse;
+
+  expect(resumed.statusCode).toBe(500);
+  expect(current).toBeUndefined();
+  expect(createCalls).toBe(0);
+});
+
 test("a replaced runtime cannot publish or fail the newly resumed session", async () => {
   const controlledBridge = (initialState: string) => {
     let state = initialState;
@@ -448,7 +971,11 @@ test("a replaced runtime cannot publish or fail the newly resumed session", asyn
       createFor: () => { current = runtimes.shift(); return current; },
       configure: async () => {},
       start: () => {},
-      teardown: () => { current = undefined; },
+      teardownIfCurrent: (_id: string, expected: any) => {
+        if (current !== expected) return false;
+        current = undefined;
+        return true;
+      },
     } as any,
     hub: {
       clear: () => {},
@@ -505,7 +1032,11 @@ test("a deleted runtime cannot repopulate or fail the forgotten session", async
       createFor: () => { current = runtime; return runtime; },
       configure: async () => {},
       start: () => {},
-      teardown: () => { current = undefined; },
+      teardownIfCurrent: (_id: string, expected: any) => {
+        if (current !== expected) return false;
+        current = undefined;
+        return true;
+      },
     } as any,
     hub: {
       publish: (_id: string, event: string, data: object) => {
@@ -604,7 +1135,12 @@ test("POST resume tears down a created runtime when bridge binding fails", async
   const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
     mgr: {
       get: () => current,
-      teardown: () => { teardownCalls += 1; current = undefined; },
+      teardownIfCurrent: (_id: string, expected: any) => {
+        if (current !== expected) return false;
+        teardownCalls += 1;
+        current = undefined;
+        return true;
+      },
       createFor: () => {
         current = {
           bridge: {
@@ -688,9 +1224,17 @@ test("DELETE /sessions/:id calls deleteSession", async () => {
 
 test("DELETE /sessions/:id tears down the runtime before deleting on disk", async () => {
   const order: string[] = [];
+  const runtime = {} as any;
   const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
     thtRunner: { deleteSession: async (id: string) => { order.push(`del:${id}`); } } as any,
-    mgr: { teardown: (id: string) => { order.push(`teardown:${id}`); } } as any,
+    mgr: {
+      get: () => runtime,
+      teardownIfCurrent: (id: string, expected: any) => {
+        expect(expected).toBe(runtime);
+        order.push(`teardown:${id}`);
+        return true;
+      },
+    } as any,
     hub: { forget: (id: string) => { order.push(`forget:${id}`); } } as any,
     getSettings: () => ({ workspace: "w" }) as any,
     spawnFn: () => nodeSpawn("node", [FAKE, SCRIPT]) as any,
@@ -704,7 +1248,7 @@ test("POST /sessions/:id/close clears transient hub state without forgetting its
   const calls: string[] = [];
   const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
     thtRunner: { closeSession: async () => {} } as any,
-    mgr: { teardown: () => {} } as any,
+    mgr: { get: () => undefined } as any,
     hub: {
       clear: (id: string) => { calls.push(`clear:${id}`); },
       forget: (id: string) => { calls.push(`forget:${id}`); },
@@ -833,14 +1377,20 @@ test("POST /sessions returns after bridge attachment but starts only after retri
     emitClientEvent: () => {},
   };
   const runtime = { bridge } as any;
+  let current: any;
   const mgr = {
-    createFor: () => runtime,
+    get: () => current,
+    createFor: () => { current = runtime; return runtime; },
     configure: async () => {},
     start: () => {
       expect(bridgeAttached).toBe(true);
       started = true;
     },
-    teardown: () => {},
+    teardownIfCurrent: (_id: string, expected: any) => {
+      if (current !== expected) return false;
+      current = undefined;
+      return true;
+    },
   } as any;
   const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
     mgr,
@@ -871,13 +1421,18 @@ test("POST /sessions bootstrap failure emits only a fixed recovery message", asy
   const runtime = { bridge } as any;
   const rawFailure =
     "connect https://secret.invalid/bootstrap?token=DO_NOT_LEAK using /srv/private/model-key";
+  let current: any;
   const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
     mgr: {
-      get: () => undefined,
-      createFor: () => runtime,
+      get: () => current,
+      createFor: () => { current = runtime; return runtime; },
       configure: async () => { throw new Error(rawFailure); },
       start: () => {},
-      teardown: () => {},
+      teardownIfCurrent: (_id: string, expected: any) => {
+        if (current !== expected) return false;
+        current = undefined;
+        return true;
+      },
     } as any,
     hub: {
       publish: (_id: string, event: string, data: any) => published.push({ event, data }),

backend/test/sse-hub.test.ts

@@ -21,14 +21,34 @@ test("publish assigns monotonic ids and a subscriber replays only ids newer than
   ]);
 });
 
-test("clear drops stale subscribers and buffers but preserves the per-session id sequence", () => {
+test("clear closes every stale subscriber once and replays post-clear events from the cursor", () => {
   const hub = new SseHub();
-  const stale: any[] = [];
-  hub.subscribe("s1", (event, data, id) => stale.push({ event, data, id }));
+  const firstStale: any[] = [];
+  const secondStale: any[] = [];
+  const closeCalls = [0, 0];
+  let offFirst: () => void = () => undefined;
+  let offSecond: () => void = () => undefined;
+  offFirst = hub.subscribe(
+    "s1",
+    (event, data, id) => firstStale.push({ event, data, id }),
+    { close: () => { closeCalls[0] += 1; offFirst(); } },
+  );
+  offSecond = hub.subscribe(
+    "s1",
+    (event, data, id) => secondStale.push({ event, data, id }),
+    { close: () => { closeCalls[1] += 1; offSecond(); } },
+  );
   expect(hub.publish("s1", "info", { text: "before" })).toBe(1);
 
   hub.clear("s1");
+  offFirst();
+  offSecond();
+  expect(closeCalls).toEqual([1, 1]);
   expect(hub.publish("s1", "info", { text: "after" })).toBe(2);
+  expect(hub.publish("s1", "ui_request", {
+    type: "ui_request",
+    ui_request: { id: "gate-1", widget: "select" },
+  })).toBe(3);
   const resumed: any[] = [];
   hub.subscribe(
     "s1",
@@ -36,22 +56,47 @@ test("clear drops stale subscribers and buffers but preserves the per-session id
     { afterId: 1 },
   );
 
-  expect(stale).toEqual([{ event: "info", data: { text: "before" }, id: 1 }]);
-  expect(resumed).toEqual([{ event: "info", data: { text: "after" }, id: 2 }]);
+  const before = [{ event: "info", data: { text: "before" }, id: 1 }];
+  expect(firstStale).toEqual(before);
+  expect(secondStale).toEqual(before);
+  expect(resumed).toEqual([
+    { event: "info", data: { text: "after" }, id: 2 },
+    {
+      event: "ui_request",
+      data: { type: "ui_request", ui_request: { id: "gate-1", widget: "select" } },
+      id: 3,
+    },
+  ]);
 });
 
-test("forget drops subscribers, buffers, and the per-session id sequence", () => {
+test("forget closes every subscriber once and resets the per-session id sequence", () => {
   const hub = new SseHub();
-  const stale: any[] = [];
-  hub.subscribe("s1", (event, data, id) => stale.push({ event, data, id }));
+  const firstStale: any[] = [];
+  const secondStale: any[] = [];
+  const closeCalls = [0, 0];
+  let offFirst: () => void = () => undefined;
+  let offSecond: () => void = () => undefined;
+  offFirst = hub.subscribe(
+    "s1",
+    (event, data, id) => firstStale.push({ event, data, id }),
+    { close: () => { closeCalls[0] += 1; offFirst(); } },
+  );
+  offSecond = hub.subscribe(
+    "s1",
+    (event, data, id) => secondStale.push({ event, data, id }),
+    { close: () => { closeCalls[1] += 1; offSecond(); } },
+  );
   expect(hub.publish("s1", "info", { text: "before" })).toBe(1);
 
   hub.forget("s1");
+  expect(closeCalls).toEqual([1, 1]);
   expect(hub.publish("s1", "info", { text: "after" })).toBe(1);
   const fresh: any[] = [];
   hub.subscribe("s1", (event, data, id) => fresh.push({ event, data, id }));
 
-  expect(stale).toEqual([{ event: "info", data: { text: "before" }, id: 1 }]);
+  const before = [{ event: "info", data: { text: "before" }, id: 1 }];
+  expect(firstStale).toEqual(before);
+  expect(secondStale).toEqual(before);
   expect(fresh).toEqual([{ event: "info", data: { text: "after" }, id: 1 }]);
 });
 

backend/test/sse-route.test.ts

@@ -55,6 +55,17 @@ async function captureReplay(options: { query?: string; lastEventId?: string }):
   }
 }
 
+async function expectStreamEnd(reader: ReadableStreamDefaultReader<Uint8Array>): Promise<void> {
+  const result = await Promise.race([
+    reader.read(),
+    new Promise<{ timeout: true }>((resolve) =>
+      setTimeout(() => resolve({ timeout: true }), 1_000),
+    ),
+  ]);
+  expect(result).not.toEqual({ timeout: true });
+  expect("done" in result && result.done).toBe(true);
+}
+
 test("SSE emits ids and honors the native Last-Event-ID replay cursor", async () => {
   const body = await captureReplay({ lastEventId: "1" });
 
@@ -80,3 +91,54 @@ test("SSE uses the newer valid cursor when header and query are both present", a
   expect(body).not.toContain('"two"');
   expect(body).toContain("id: 3\nevent: info\n");
 });
+
+test("clear ends every live SSE response and a cursor reconnect replays post-clear events", async () => {
+  const hub = new SseHub();
+  const app = buildApp(loadConfig({ THT_HARNESS_DIR: "/tmp/h" }), {
+    hub,
+    thtRunner: {} as any,
+  });
+  await app.listen({ port: 0, host: "127.0.0.1" });
+  const port = (app.server.address() as { port: number }).port;
+  const controllers = [new AbortController(), new AbortController(), new AbortController()];
+
+  try {
+    const responses = await Promise.all(controllers.slice(0, 2).map((controller) => fetch(
+      `http://127.0.0.1:${port}/sessions/s1/events`,
+      { signal: controller.signal },
+    )));
+    const readers = responses.map((response) => response.body!.getReader());
+
+    expect(hub.publish("s1", "info", { type: "info", text: "before" })).toBe(1);
+    const initial = await Promise.all(readers.map((reader) =>
+      readUntil(reader, (text) => text.includes('"before"'))));
+    expect(initial.every((body) => body.includes("id: 1\nevent: info\n"))).toBe(true);
+
+    hub.clear("s1");
+    await Promise.all(readers.map(expectStreamEnd));
+
+    expect(hub.publish("s1", "info", { type: "info", text: "after" })).toBe(2);
+    expect(hub.publish("s1", "ui_request", {
+      type: "ui_request",
+      ui_request: { id: "gate-1", widget: "select" },
+    })).toBe(3);
+
+    const reconnected = await fetch(
+      `http://127.0.0.1:${port}/sessions/s1/events`,
+      {
+        headers: { "Last-Event-ID": "1" },
+        signal: controllers[2].signal,
+      },
+    );
+    const reconnectReader = reconnected.body!.getReader();
+    const replay = await readUntil(reconnectReader, (text) => text.includes('"gate-1"'));
+    expect(replay).toContain("id: 2\nevent: info\n");
+    expect(replay).toContain('data: {"type":"info","text":"after"}');
+    expect(replay).toContain("id: 3\nevent: ui_request\n");
+    expect(replay).toContain('"ui_request":{"id":"gate-1","widget":"select"}');
+    await reconnectReader.cancel();
+  } finally {
+    for (const controller of controllers) controller.abort();
+    await app.close();
+  }
+});

frontend/src/shell/AppShell.session-mgmt.test.tsx

@@ -20,6 +20,12 @@ const LIST = [
 const resumeResult = (id: string, alreadyActive = false) =>
   HttpResponse.json({ id, alreadyActive });
 
+function deferred() {
+  let resolve!: () => void;
+  const promise = new Promise<void>((onResolve) => { resolve = onResolve; });
+  return { promise, resolve };
+}
+
 beforeEach(() => {
   FakeEventSource.instances = [];
   (globalThis as any).EventSource = FakeEventSource;
@@ -134,58 +140,104 @@ test("an already-active same-session Resume preserves its EventSource and store"
   expect(useSessionStore.getState().activityLog).toEqual(before);
 });
 
-test("a false then true pair of concurrent Resume completions preserves the cold stream", async () => {
+test("concurrent same-id Resume invocations share one cold request and replacement source", async () => {
   let resumeCalls = 0;
-  let releaseCold!: () => void;
-  let releaseAlreadyActive!: () => void;
-  let markColdStarted!: () => void;
-  let markAlreadyActiveStarted!: () => void;
-  const coldStarted = new Promise<void>((resolve) => { markColdStarted = resolve; });
-  const alreadyActiveStarted = new Promise<void>((resolve) => { markAlreadyActiveStarted = resolve; });
-  const coldReleased = new Promise<void>((resolve) => { releaseCold = resolve; });
-  const alreadyActiveReleased = new Promise<void>((resolve) => { releaseAlreadyActive = resolve; });
+  const coldGate = deferred();
+  const coldStarted = deferred();
   server.use(
     http.post("http://localhost:8787/sessions/:id/resume", async () => {
       resumeCalls += 1;
-      if (resumeCalls === 1) {
-        markColdStarted();
-        await coldReleased;
-        return resumeResult("s1", false);
-      }
-      markAlreadyActiveStarted();
-      await alreadyActiveReleased;
-      return resumeResult("s1", true);
+      if (resumeCalls === 1) return resumeResult("s1", false);
+      if (resumeCalls === 2) coldStarted.resolve();
+      await coldGate.promise;
+      return resumeResult("s1", false);
     }),
     http.get("http://localhost:8787/sessions/:id", () =>
       HttpResponse.json({ id: "s1", status: "open", phase: 1 })),
   );
   wrap();
   await userEvent.click(await screen.findByText("Attiva uno"));
-  const resumeButton = await screen.findByRole("button", { name: /resume/i });
+  await userEvent.click(await screen.findByRole("button", { name: /resume/i }));
+  await waitFor(() => expect(FakeEventSource.instances).toHaveLength(1));
+  const oldSource = FakeEventSource.instances[0];
+  act(() => oldSource.emitNamed("info", { type: "info", text: "Old generation" }, "900"));
 
+  await userEvent.click(screen.getByText("Attiva uno"));
+  const resumeButton = await screen.findByRole("button", { name: /resume/i });
   await userEvent.click(resumeButton);
-  await coldStarted;
+  await coldStarted.promise;
   await userEvent.click(resumeButton);
-  await alreadyActiveStarted;
+  await new Promise((resolve) => setImmediate(resolve));
   expect(resumeCalls).toBe(2);
 
-  releaseCold();
-  await waitFor(() => expect(FakeEventSource.instances).toHaveLength(1));
-  const stream = FakeEventSource.instances[0];
-  act(() => stream.emitNamed(
+  coldGate.resolve();
+  await waitFor(() => expect(FakeEventSource.instances).toHaveLength(2));
+  const replacement = FakeEventSource.instances[1];
+  expect(oldSource.closed).toBe(true);
+  expect(replacement.url).toBe("http://localhost:8787/sessions/s1/events");
+  expect(useSessionStore.getState().activityLog).toEqual([
+    { kind: "lifecycle", phase: null, text: "Resuming session" },
+  ]);
+  act(() => replacement.emitNamed(
     "ui_request",
     { type: "ui_request", ui_request: { id: "cold-gate", widget: "select" } },
     "1",
   ));
-  const before = useSessionStore.getState().activityLog.map((entry) => ({ ...entry }));
 
-  releaseAlreadyActive();
-  await waitFor(() => expect(screen.queryByText("Domanda originale")).not.toBeInTheDocument());
-
-  expect(FakeEventSource.instances).toHaveLength(1);
-  expect(stream.closed).toBe(false);
   expect(useSessionStore.getState().pendingWidget?.id).toBe("cold-gate");
-  expect(useSessionStore.getState().activityLog).toEqual(before);
+});
+
+test("a committed Resume releases same-id single-flight before its manifest settles", async () => {
+  const other = {
+    ...LIST[0], id: "s3", question: "Attiva tre", group: null,
+    created_at: "2026-01-03T00:00:00Z",
+  };
+  let s1ResumeCalls = 0;
+  let s1ManifestCalls = 0;
+  const firstS1ManifestGate = deferred();
+  const firstS1ManifestStarted = deferred();
+  server.use(
+    http.get("http://localhost:8787/sessions", () => HttpResponse.json([LIST[0], other])),
+    http.post("http://localhost:8787/sessions/:id/resume", ({ params }) => {
+      const id = params.id as string;
+      if (id === "s1") s1ResumeCalls += 1;
+      return resumeResult(id);
+    }),
+    http.get("http://localhost:8787/sessions/:id", async ({ params }) => {
+      if (params.id === "s1") {
+        s1ManifestCalls += 1;
+        if (s1ManifestCalls === 1) {
+          firstS1ManifestStarted.resolve();
+          await firstS1ManifestGate.promise;
+          return HttpResponse.json({ id: "s1", status: "open", phase: 7 });
+        }
+        return HttpResponse.json({ id: "s1", status: "open", phase: 4 });
+      }
+      return HttpResponse.json({ id: "s3", status: "open", phase: 3 });
+    }),
+  );
+  wrap();
+
+  await userEvent.click(await screen.findByText("Attiva uno"));
+  await userEvent.click(await screen.findByRole("button", { name: /resume/i }));
+  await firstS1ManifestStarted.promise;
+  expect(s1ResumeCalls).toBe(1);
+
+  await userEvent.click(screen.getByText("Attiva tre"));
+  await userEvent.click(await screen.findByRole("button", { name: /resume/i }));
+  await waitFor(() => expect(useSessionStore.getState().currentPhase).toBe("F3"));
+
+  await userEvent.click(screen.getByText("Attiva uno"));
+  await userEvent.click(await screen.findByRole("button", { name: /resume/i }));
+  await waitFor(() => expect(s1ResumeCalls).toBe(2));
+  await waitFor(() => expect(useSessionStore.getState().currentPhase).toBe("F4"));
+  expect(FakeEventSource.instances.at(-1)?.url).toContain("/sessions/s1/events");
+
+  firstS1ManifestGate.resolve();
+  await new Promise((resolve) => setImmediate(resolve));
+
+  expect(useSessionStore.getState().currentPhase).toBe("F4");
+  expect(FakeEventSource.instances.at(-1)?.url).toContain("/sessions/s1/events");
 });
 
 test("cold same-session Resume keeps the old stream until success then receives post-clear events once", async () => {
@@ -321,6 +373,122 @@ test("resuming a different already-active session binds it only after success",
   }));
 });
 
+test("competing Resume requests for different ids commit only the latest intent", async () => {
+  const other = {
+    ...LIST[0], id: "s3", question: "Attiva tre", group: null,
+    created_at: "2026-01-03T00:00:00Z",
+  };
+  const s1Gate = deferred();
+  const s3Gate = deferred();
+  const s1Started = deferred();
+  const s3Started = deferred();
+  server.use(
+    http.get("http://localhost:8787/sessions", () => HttpResponse.json([LIST[0], other])),
+    http.post("http://localhost:8787/sessions/:id/resume", async ({ params }) => {
+      const id = params.id as string;
+      if (id === "s1") {
+        s1Started.resolve();
+        await s1Gate.promise;
+      } else {
+        s3Started.resolve();
+        await s3Gate.promise;
+      }
+      return resumeResult(id);
+    }),
+    http.get("http://localhost:8787/sessions/:id", ({ params }) =>
+      HttpResponse.json({ id: params.id, status: "open", phase: params.id === "s3" ? 3 : 1 })),
+  );
+  wrap();
+
+  await userEvent.click(await screen.findByText("Attiva uno"));
+  await userEvent.click(await screen.findByRole("button", { name: /resume/i }));
+  await s1Started.promise;
+  await userEvent.click(screen.getByText("Attiva tre"));
+  await userEvent.click(await screen.findByRole("button", { name: /resume/i }));
+  await s3Started.promise;
+
+  s3Gate.resolve();
+  await waitFor(() => expect(useSessionStore.getState().currentPhase).toBe("F3"));
+  expect(FakeEventSource.instances).toHaveLength(1);
+  const latestSource = FakeEventSource.instances[0];
+  expect(latestSource.url).toContain("/sessions/s3/events");
+  act(() => latestSource.emitNamed("info", { type: "info", text: "Latest target" }, "1"));
+
+  s1Gate.resolve();
+  await new Promise((resolve) => setImmediate(resolve));
+
+  expect(FakeEventSource.instances).toHaveLength(1);
+  expect(latestSource.closed).toBe(false);
+  expect(useSessionStore.getState().currentPhase).toBe("F3");
+  expect(useSessionStore.getState().activityLog).toContainEqual(expect.objectContaining({
+    text: "Latest target",
+  }));
+});
+
+test("a stale Resume manifest cannot repaint the latest session phase", async () => {
+  const other = {
+    ...LIST[0], id: "s3", question: "Attiva tre", group: null,
+    created_at: "2026-01-03T00:00:00Z",
+  };
+  const s1ManifestGate = deferred();
+  const s1ManifestStarted = deferred();
+  server.use(
+    http.get("http://localhost:8787/sessions", () => HttpResponse.json([LIST[0], other])),
+    http.post("http://localhost:8787/sessions/:id/resume", ({ params }) =>
+      resumeResult(params.id as string)),
+    http.get("http://localhost:8787/sessions/:id", async ({ params }) => {
+      if (params.id === "s1") {
+        s1ManifestStarted.resolve();
+        await s1ManifestGate.promise;
+        return HttpResponse.json({ id: "s1", status: "open", phase: 7 });
+      }
+      return HttpResponse.json({ id: "s3", status: "open", phase: 3 });
+    }),
+  );
+  wrap();
+
+  await userEvent.click(await screen.findByText("Attiva uno"));
+  await userEvent.click(await screen.findByRole("button", { name: /resume/i }));
+  await s1ManifestStarted.promise;
+  await userEvent.click(screen.getByText("Attiva tre"));
+  await userEvent.click(await screen.findByRole("button", { name: /resume/i }));
+  await waitFor(() => expect(useSessionStore.getState().currentPhase).toBe("F3"));
+  expect(FakeEventSource.instances.at(-1)?.url).toContain("/sessions/s3/events");
+
+  s1ManifestGate.resolve();
+  await new Promise((resolve) => setImmediate(resolve));
+
+  expect(useSessionStore.getState().currentPhase).toBe("F3");
+  expect(FakeEventSource.instances.at(-1)?.url).toContain("/sessions/s3/events");
+});
+
+test("starting a new question invalidates a pending Resume intent", async () => {
+  const resumeGate = deferred();
+  const resumeStarted = deferred();
+  server.use(
+    http.post("http://localhost:8787/sessions/:id/resume", async () => {
+      resumeStarted.resolve();
+      await resumeGate.promise;
+      return resumeResult("s1");
+    }),
+    http.post("http://localhost:8787/runtime/prewarm", () =>
+      HttpResponse.json({ status: "warming" }, { status: 202 })),
+  );
+  wrap();
+
+  await userEvent.click(await screen.findByText("Attiva uno"));
+  await userEvent.click(await screen.findByRole("button", { name: /resume/i }));
+  await resumeStarted.promise;
+  await userEvent.click(screen.getByRole("button", { name: /^new session$/i }));
+  expect(await screen.findByText(/type your question/i)).toBeInTheDocument();
+
+  resumeGate.resolve();
+  await new Promise((resolve) => setImmediate(resolve));
+
+  expect(FakeEventSource.instances).toHaveLength(0);
+  expect(screen.getByText(/type your question/i)).toBeInTheDocument();
+});
+
 test("a failed Resume with no active session preserves the panel and existing activity", async () => {
   server.use(http.post("http://localhost:8787/sessions/:id/resume", () => new HttpResponse(null, { status: 409 })));
   useSessionStore.setState({

frontend/src/shell/AppShell.tsx

@@ -33,6 +33,12 @@ import { useEffect, useMemo, useRef, useState } from "react";
 export function AppShell() {
   const [activeSessionId, setActiveSessionId] = useState<string | null>(null);
   const activeSessionIdRef = useRef<string | null>(null);
+  const resumeInvocationRef = useRef(0);
+  const latestResumeIntentRef = useRef<{ token: number; id: string } | null>(null);
+  const resumeInFlightRef = useRef(new Map<string, {
+    latestToken: number;
+    promise: Promise<void>;
+  }>());
   const [streamCursorResetEpoch, setStreamCursorResetEpoch] = useState(0);
   const [creatingSession, setCreatingSession] = useState(false);
   const [awaitingQuestion, setAwaitingQuestion] = useState(false);
@@ -72,6 +78,11 @@ export function AppShell() {
     setActiveSessionId(id);
   }
 
+  function invalidateResumeIntent() {
+    resumeInvocationRef.current += 1;
+    latestResumeIntentRef.current = null;
+  }
+
   // A background refresh can remove a session (for example from another browser).
   // Keep the local selection aligned with the authoritative list.
   useEffect(() => {
@@ -107,8 +118,37 @@ export function AppShell() {
     });
   }
   async function doResume(id: string) {
+    const token = ++resumeInvocationRef.current;
+    latestResumeIntentRef.current = { token, id };
+    const inFlight = resumeInFlightRef.current.get(id);
+    if (inFlight) {
+      // Repeated intent for the same target shares one backend lifecycle operation and one
+      // commit path. Updating its token still lets s1→s2→s1 make the final s1 intent authoritative.
+      inFlight.latestToken = token;
+      return inFlight.promise;
+    }
+
+    const operation = {
+      latestToken: token,
+      promise: Promise.resolve(),
+    };
+    operation.promise = runResume(id, operation).finally(() => {
+      if (resumeInFlightRef.current.get(id) === operation) {
+        resumeInFlightRef.current.delete(id);
+      }
+    });
+    resumeInFlightRef.current.set(id, operation);
+    return operation.promise;
+  }
+
+  async function runResume(
+    id: string,
+    operation: { latestToken: number; promise: Promise<void> },
+  ) {
     try {
       const result = await resumeSession(id);
+      const latest = latestResumeIntentRef.current;
+      if (latest?.token !== operation.latestToken || latest.id !== id) return;
       const reconnectSameSession = activeSessionIdRef.current === id;
       setPanelSession(null);
       setAwaitingQuestion(false);
@@ -129,10 +169,22 @@ export function AppShell() {
         setStreamCursorResetEpoch((value) => value + 1);
       }
 
+      // Single-flight covers only the backend Resume and its local binding commit. A slow
+      // manifest read must not prevent a later explicit intent from starting a new Resume.
+      if (resumeInFlightRef.current.get(id) === operation) {
+        resumeInFlightRef.current.delete(id);
+      }
+
       // Paint the persisted re-entry phase while the replacement stream starts replaying.
       // The manifest's `phase` is the 1-based current phase (1..8).
       try {
         const m = (await getSession(id)) as { phase?: number };
+        const latestAfterManifest = latestResumeIntentRef.current;
+        if (
+          latestAfterManifest?.token !== operation.latestToken
+          || latestAfterManifest.id !== id
+          || activeSessionIdRef.current !== id
+        ) return;
         if (typeof m.phase === "number" && m.phase >= 1 && m.phase <= 8) {
           setPhase(`F${m.phase}`);
         }
@@ -140,7 +192,12 @@ export function AppShell() {
         /* non-fatal: the first gate will set the phase */
       }
     } catch {
-      toast.error("Failed to resume session.");
+      if (
+        latestResumeIntentRef.current?.token === operation.latestToken
+        && latestResumeIntentRef.current.id === id
+      ) {
+        toast.error("Failed to resume session.");
+      }
     }
   }
   async function move(s: SessionSummary, group: string) {
@@ -183,6 +240,9 @@ export function AppShell() {
   }
 
   async function deleteSessions(targets: SessionSummary[]) {
+    if (targets.some((session) => session.id === activeSessionIdRef.current)) {
+      invalidateResumeIntent();
+    }
     try {
       const results = await Promise.allSettled(targets.map((session) => deleteSession(session.id)));
       const deletedIds = new Set(
@@ -246,6 +306,7 @@ export function AppShell() {
     if (ev === "session_exit") {
       // Never let a streamed event terminate the managed Pi child. Only the
       // explicit “Stop & save” action is allowed to call /close.
+      invalidateResumeIntent();
       resetSession();
       selectActiveSession(null);
       setAwaitingQuestion(false);
@@ -258,6 +319,7 @@ export function AppShell() {
   }, [lastSystemEvent]);
 
   function startNewSession() {
+    invalidateResumeIntent();
     resetSession();
     setAwaitingQuestion(true);
     setCreatingSession(false);
@@ -290,6 +352,7 @@ export function AppShell() {
 
   async function stopSession() {
     if (!activeSessionId) return;
+    invalidateResumeIntent();
     try {
       await closeSession(activeSessionId);
     } finally {