fix(desktop): keep queued drains out of the foreground session on session switch

A queue drain pairs two identifiers from different clocks: the queue key
(flips with the route) and the explicit runtime id (lags a resume behind).
Mid-switch the composer can fire a drain with storedSessionId=B but
sessionId=A-runtime, and prompt.submit then lands B's queued prompt — and
its whole answer turn — inside session A.

Make the central runtime binding authoritative for queued sends: when the
explicit runtime id no longer matches the binding recorded for the target
stored session, adopt the binding (or drop to the stored-id resume path
when none exists yet). The identity pair (storedSessionId === sessionId)
is the fresh-chat fallback and stays untouched.

Tests: re-home-via-resume and rebind-to-central-runtime; existing
background-drain and sleep/wake cases declare central bindings explicitly.
Null-fallback guard already on main; this PR is the remaining half.

apps/desktop/src/app/session/hooks/use-prompt-actions/index.test.tsx

@@ -1625,6 +1625,7 @@ describe('usePromptActions submit / queue drain semantics', () => {
     let handle: HarnessHandle | null = null
     render(
       <Harness
+        getRuntimeIdForStoredSession={storedId => (storedId === 'stored-session-a' ? 'rt-session-a' : null)}
         onReady={h => (handle = h)}
         onUpdateState={(sessionId, storedSessionId, state) => updates.push({ sessionId, state, storedSessionId })}
         refreshSessions={async () => undefined}
@@ -1656,6 +1657,102 @@ describe('usePromptActions submit / queue drain semantics', () => {
     expect($busy.get()).toBe(false)
   })
 
+  it('a fromQueue drain carrying a stale runtime id re-homes via session.resume instead of landing in the foreground session', async () => {
+    // The session-switch window this guards: the composer's queue key has
+    // already flipped to session B (route-driven) while the foreground runtime
+    // id prop still reads session A (resume-driven, one settle behind). Without
+    // the central-binding check, prompt.submit fires with session_id=A and B's
+    // queued prompt — plus its whole answer turn — lands inside A. With no
+    // binding recorded for B yet, the stale id must be dropped and the drain
+    // re-homed through the stored-session resume path.
+    const updates: { sessionId: string; state: Record<string, unknown>; storedSessionId: null | string | undefined }[] =
+      []
+
+    const requestGateway = vi.fn(
+      async (method: string) => (method === 'session.resume' ? { session_id: 'rt-session-b' } : {}) as never
+    )
+
+    let handle: HarnessHandle | null = null
+    render(
+      <Harness
+        onReady={h => (handle = h)}
+        onUpdateState={(sessionId, storedSessionId, state) => updates.push({ sessionId, state, storedSessionId })}
+        refreshSessions={async () => undefined}
+        requestGateway={requestGateway}
+      />
+    )
+
+    const accepted = await handle!.submitText('queued for B mid-switch', {
+      fromQueue: true,
+      sessionId: 'rt-session-a',
+      storedSessionId: 'stored-session-b'
+    })
+
+    expect(accepted).toBe(true)
+    expect(requestGateway).toHaveBeenCalledWith('session.resume', {
+      session_id: 'stored-session-b',
+      source: 'desktop'
+    })
+    expect(requestGateway).toHaveBeenCalledWith(
+      'prompt.submit',
+      {
+        session_id: 'rt-session-b',
+        text: 'queued for B mid-switch'
+      },
+      1_800_000
+    )
+    // The invariant: the stale foreground runtime never receives the prompt.
+    expect(
+      requestGateway.mock.calls.every(
+        ([method, params]) =>
+          method !== 'prompt.submit' || (params as { session_id?: string }).session_id !== 'rt-session-a'
+      )
+    ).toBe(true)
+    expect(
+      updates.some(update => update.sessionId === 'rt-session-b' && update.storedSessionId === 'stored-session-b')
+    ).toBe(true)
+  })
+
+  it('a fromQueue drain rebinds to the centrally recorded runtime when its explicit id is stale', async () => {
+    // Same window, but B's runtime binding is already known centrally — the
+    // drain should adopt the authoritative binding directly (no resume
+    // round-trip) rather than trusting the leftover foreground id.
+    const requestGateway = vi.fn(async () => ({}) as never)
+
+    let handle: HarnessHandle | null = null
+    render(
+      <Harness
+        getRuntimeIdForStoredSession={storedId => (storedId === 'stored-session-b' ? 'rt-session-b-live' : null)}
+        onReady={h => (handle = h)}
+        refreshSessions={async () => undefined}
+        requestGateway={requestGateway}
+      />
+    )
+
+    const accepted = await handle!.submitText('queued for B, B already re-bound', {
+      fromQueue: true,
+      sessionId: 'rt-session-a',
+      storedSessionId: 'stored-session-b'
+    })
+
+    expect(accepted).toBe(true)
+    expect(requestGateway).toHaveBeenCalledWith(
+      'prompt.submit',
+      {
+        session_id: 'rt-session-b-live',
+        text: 'queued for B, B already re-bound'
+      },
+      1_800_000
+    )
+    expect(requestGateway).not.toHaveBeenCalledWith('session.resume', expect.anything())
+    expect(
+      requestGateway.mock.calls.every(
+        ([method, params]) =>
+          method !== 'prompt.submit' || (params as { session_id?: string }).session_id !== 'rt-session-a'
+      )
+    ).toBe(true)
+  })
+
   it('a fromQueue drain with null runtime id does NOT land in the foreground session (cross-session leak guard)', async () => {
     // The cross-session leak: a background drain fires with sessionId=null
     // (the stored session's runtime was reaped by the gateway). Without the
@@ -2619,6 +2716,13 @@ describe('usePromptActions sleep/wake session recovery', () => {
     let handle: HarnessHandle | null = null
     render(
       <Harness
+        // The central binding is stale in lockstep with the caller here: the
+        // sleep/wake reaper only clears the GATEWAY's in-memory session, so
+        // client-side state still swears by the old runtime id. That is what
+        // routes this case to the reactive 404→resume→retry path instead of
+        // the proactive binding check (covered by the cross-session drain
+        // tests above).
+        getRuntimeIdForStoredSession={storedId => (storedId === STORED_SESSION_ID ? 'rt-background-stale' : null)}
         onReady={h => (handle = h)}
         refreshSessions={async () => undefined}
         requestGateway={requestGateway}

apps/desktop/src/app/session/hooks/use-prompt-actions/submit.ts

@@ -198,6 +198,30 @@ export function useSubmitPrompt(deps: SubmitPromptDeps) {
 
       let sessionId: null | string = options?.sessionId ?? (isBackgroundQueueDrain ? null : activeSessionIdRef.current)
 
+      // An explicit queued runtime id is authoritative ONLY while it still
+      // belongs to its stored session. On a session switch the composer's
+      // queue key flips with the route while the foreground runtime id lags a
+      // resume behind, so a drain can fire with storedSessionId=B but
+      // sessionId=A-runtime — and the prompt.submit below would land B's
+      // queued prompt (and its whole answer turn) inside A. Verify the pair
+      // against the central binding and drop a stale explicit id: the
+      // targetStoredSessionId resume path below then rebinds the right
+      // runtime, exactly as a background drain with an unknown binding does.
+      // The identity pair (storedSessionId === sessionId) is the fresh-chat
+      // fallback — an unpersisted conversation's queue key IS its runtime id,
+      // so it has no central binding to check against and is left untouched.
+      if (
+        options?.sessionId &&
+        options?.storedSessionId &&
+        options.storedSessionId !== options.sessionId
+      ) {
+        const boundRuntimeId = getRuntimeIdForStoredSession(options.storedSessionId)
+
+        if (boundRuntimeId !== options.sessionId) {
+          sessionId = boundRuntimeId
+        }
+      }
+
       // Pin the foreground session context for the whole async submit pipeline.
       // Without this, a fast session switch during session.resume / file.attach
       // can redirect the user's text into a different chat (#54527). Mutable —