diff --git a/frontend/src/lib/components/copilot/chat/AIChatManager.test.ts b/frontend/src/lib/components/copilot/chat/AIChatManager.test.ts
index f845f596a3..4560c9c655 100644
--- a/frontend/src/lib/components/copilot/chat/AIChatManager.test.ts
+++ b/frontend/src/lib/components/copilot/chat/AIChatManager.test.ts
@@ -13,7 +13,8 @@ import { runChatLoop } from './chatLoop'
 import { clearWorkspaceRoleCache } from '$lib/user'
 import {
 	noteDriverAlive,
-	noteRemoteTurnEnded
+	noteRemoteTurnEnded,
+	onDriverLost
 } from '$lib/components/sessions/sessionRunOwner.svelte'
 
 // This suite forces esm-env BROWSER=true (below). That makes @sveltejs/kit's
@@ -332,6 +333,9 @@ describe('AIChatManager cross-tab run guard', () => {
 		const manager = new AIChatManager()
 		manager.isSessionChat = true
 		manager.sessionId = 'session-catching-up'
+		// A tab showing a session has sessionRuntime loaded, which is what makes
+		// `catchingUp` a position it can actually leave.
+		onDriverLost(() => {})
 		noteDriverAlive('session-catching-up', false)
 		noteRemoteTurnEnded('session-catching-up')
 		const restoreInstructions = vi.fn(() => true)
diff --git a/frontend/src/lib/components/sessions/sessionRunOwner.svelte.ts b/frontend/src/lib/components/sessions/sessionRunOwner.svelte.ts
index 74bafdbf55..9e996622fa 100644
--- a/frontend/src/lib/components/sessions/sessionRunOwner.svelte.ts
+++ b/frontend/src/lib/components/sessions/sessionRunOwner.svelte.ts
@@ -84,10 +84,21 @@ export function noteDriverAlive(sessionId: string, planMode: boolean): void {
 }
 
 /** The driver says its turn is over. The re-read that follows is what actually
- *  frees this tab, so the position moves to `catchingUp` rather than to idle. */
+ *  frees this tab, so the position moves to `catchingUp` rather than to idle —
+ *  but only where something can perform that re-read.
+ *
+ *  The channel calls this from module scope in every tab that has it, including
+ *  ones that never load sessionRuntime (a page carrying the session sidebar with
+ *  no session open). Only sessionRuntime can call {@link noteCaughtUp}, so
+ *  entering `catchingUp` there would be entering a state with nothing able to
+ *  leave it — and `mirroringRemoteRun` reads true for as long as it lasts, which
+ *  locks the composer against a run that has already ended and drops the edits
+ *  mask and background-job writes that expect the re-read to reseed them. With
+ *  no runtime there is no mirrored transcript to be out of step with, so idle is
+ *  the truthful position rather than merely the convenient one. */
 export function noteRemoteTurnEnded(sessionId: string): void {
 	if (runPosition(sessionId).state !== 'watching') return
-	positions.set(sessionId, { state: 'catchingUp' })
+	positions.set(sessionId, canCompleteCatchUp() ? { state: 'catchingUp' } : { state: 'idle' })
 }
 
 /** The re-read finished: this tab's transcript and history are one conversation
@@ -214,6 +225,13 @@ export function onDriverLost(fn: (sessionId: string) => void): void {
 	driverLost = fn
 }
 
+/** Whether a module that can finish a catch-up is loaded in this tab. It is the
+ *  same registration either way: sessionRuntime owns both the re-read and the
+ *  handler, so having one means having the other. */
+function canCompleteCatchUp(): boolean {
+	return driverLost !== undefined
+}
+
 // Runs only while some session is being driven elsewhere, and stops itself once
 // none is — a browser with a single tab open never arms it at all.
 let reaperTimer: ReturnType<typeof setInterval> | undefined
diff --git a/frontend/src/lib/components/sessions/sessionRunOwner.test.ts b/frontend/src/lib/components/sessions/sessionRunOwner.test.ts
index 19bf5b9100..236cf4ec27 100644
--- a/frontend/src/lib/components/sessions/sessionRunOwner.test.ts
+++ b/frontend/src/lib/components/sessions/sessionRunOwner.test.ts
@@ -3,6 +3,7 @@ import {
 	clearRunPosition,
 	noteDriverAlive,
 	noteRemoteTurnEnded,
+	onDriverLost,
 	withSessionRunLock
 } from './sessionRunOwner.svelte'
 
@@ -28,6 +29,7 @@ describe('withSessionRunLock with no lock to take', () => {
 	// go — but the transcript on screen is still paired with the history from
 	// before that turn, and sending would put that pair to the model.
 	it('refuses while still catching up on a finished turn', async () => {
+		onDriverLost(() => {})
 		noteDriverAlive('session-catching-up', false)
 		noteRemoteTurnEnded('session-catching-up')
 		const body = vi.fn(async () => 'ran')
@@ -43,3 +45,25 @@ describe('withSessionRunLock with no lock to take', () => {
 		expect(body).toHaveBeenCalledTimes(1)
 	})
 })
+
+// The channel runs these transitions in every tab that loads it, including ones
+// that never load sessionRuntime — a page carrying the session sidebar with no
+// session open. Only sessionRuntime performs the re-read that leaves
+// `catchingUp`, so parking there would strand the session: `mirroringRemoteRun`
+// stays true, which locks the composer against a run that already ended and
+// drops the edits-mask and background-job writes that expect the re-read to
+// reseed them. A fresh module instance is the only honest way to test it — the
+// registration is process-wide and one-way, exactly as it is in a real tab.
+describe('a turn ending in a tab with no session runtime', () => {
+	it('settles to idle rather than waiting for a re-read nobody can do', async () => {
+		vi.resetModules()
+		const owner = await import('./sessionRunOwner.svelte')
+
+		owner.noteDriverAlive('session-no-runtime', false)
+		owner.noteRemoteTurnEnded('session-no-runtime')
+
+		expect(owner.isCatchingUp('session-no-runtime')).toBe(false)
+		expect(owner.isMirroring('session-no-runtime')).toBe(false)
+		owner.clearRunPosition('session-no-runtime')
+	})
+})