diff --git a/apps/desktop/src/app/contrib/hooks/live-status-spinner.test.ts b/apps/desktop/src/app/contrib/hooks/live-status-spinner.test.ts
new file mode 100644
index 000000000000..4fcad3b1a2ed
--- /dev/null
+++ b/apps/desktop/src/app/contrib/hooks/live-status-spinner.test.ts
@@ -0,0 +1,77 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+
+import { $selectedStoredSessionId, $unreadFinishedSessionIds } from '@/store/session'
+import { $workingSessionIds, clearAllSessionStates } from '@/store/session-states'
+
+import { rehydrateLiveSessionStatuses, resetLiveRuntimeTracking } from './use-background-sync'
+
+/**
+ * (C) The sidebar spinner is driven by `$workingSessionIds`, which is keyed by
+ * STORED session id. A turn that STARTS while Desktop isn't receiving stream
+ * events — a background profile, a degraded remote socket, a session opened on
+ * another surface — is only ever learned about through the `session.active_list`
+ * poll. If that poll can't seed a row the renderer has never seen, the thread
+ * name never gets its arc even though the backend is plainly working.
+ */
+describe('rehydrateLiveSessionStatuses — seeding a turn the renderer never saw start', () => {
+  beforeEach(() => {
+    vi.useFakeTimers()
+    $selectedStoredSessionId.set(null)
+    $unreadFinishedSessionIds.set([])
+    resetLiveRuntimeTracking()
+  })
+
+  afterEach(() => {
+    vi.clearAllTimers()
+    vi.useRealTimers()
+    clearAllSessionStates()
+    resetLiveRuntimeTracking()
+    $unreadFinishedSessionIds.set([])
+  })
+
+  it('shows the spinner for a turn that started with no stream events', () => {
+    rehydrateLiveSessionStatuses({
+      sessions: [{ id: 'runtime-cold', session_key: 'stored-cold', status: 'working' }]
+    })
+
+    expect($workingSessionIds.get()).toContain('stored-cold')
+  })
+
+  it('keeps the spinner across polls while the turn is still running', () => {
+    rehydrateLiveSessionStatuses({
+      sessions: [{ id: 'runtime-cold', session_key: 'stored-cold', status: 'working' }]
+    })
+    rehydrateLiveSessionStatuses({
+      sessions: [{ id: 'runtime-cold', session_key: 'stored-cold', status: 'working' }]
+    })
+
+    expect($workingSessionIds.get()).toContain('stored-cold')
+  })
+
+  it('shows the spinner when a runtime id is recycled onto a new stored session', () => {
+    // A respawned backend can mint the same runtime id for a different stored
+    // session. The row for the NEW stored id must light up, not the stale one.
+    rehydrateLiveSessionStatuses({
+      sessions: [{ id: 'runtime-1', session_key: 'stored-old', status: 'working' }]
+    })
+
+    rehydrateLiveSessionStatuses({
+      sessions: [{ id: 'runtime-1', session_key: 'stored-new', status: 'working' }]
+    })
+
+    expect($workingSessionIds.get()).toContain('stored-new')
+    expect($workingSessionIds.get()).not.toContain('stored-old')
+  })
+
+  it('leaves a starting session idle — the agent build is not proof of a turn', () => {
+    // `starting` = `agent_build_started` without `agent_ready`. _start_agent_build
+    // runs on the first prompt OR any incidental RPC that needs the agent, so it
+    // is not proof of a turn — lighting the spinner here would fire on merely
+    // opening a session. A real turn arrives as `working`.
+    rehydrateLiveSessionStatuses({
+      sessions: [{ id: 'runtime-boot', session_key: 'stored-boot', status: 'starting' }]
+    })
+
+    expect($workingSessionIds.get()).not.toContain('stored-boot')
+  })
+})
diff --git a/apps/desktop/src/store/working-ids-stored-id.test.ts b/apps/desktop/src/store/working-ids-stored-id.test.ts
new file mode 100644
index 000000000000..f302385a4442
--- /dev/null
+++ b/apps/desktop/src/store/working-ids-stored-id.test.ts
@@ -0,0 +1,32 @@
+import { afterEach, describe, expect, it } from 'vitest'
+
+import { createClientSessionState } from '@/lib/chat-runtime'
+import { $workingSessionIds, clearAllSessionStates, publishSessionState } from '@/store/session-states'
+
+/**
+ * (C) The sidebar spinner reads `$workingSessionIds`, which projects
+ * `$sessionStates` down to STORED session ids and drops any entry whose
+ * `storedSessionId` is null. `message.start` flips `busy` without carrying a
+ * stored id, so a runtime that was never seeded with one goes busy invisibly:
+ * the backend works, the thread name stays bare.
+ */
+describe('$workingSessionIds — a busy runtime with no stored id', () => {
+  afterEach(() => {
+    clearAllSessionStates()
+  })
+
+  it('cannot show a spinner for a busy runtime that has no stored id', () => {
+    publishSessionState('runtime-unmapped', { ...createClientSessionState(null), busy: true })
+
+    // Documents the constraint rather than asserting the bug is fine: the
+    // projection is keyed by stored id, so an unmapped runtime is unreachable
+    // from the sidebar no matter how busy it is.
+    expect($workingSessionIds.get()).toEqual([])
+  })
+
+  it('shows the spinner as soon as the stored id is known', () => {
+    publishSessionState('runtime-mapped', { ...createClientSessionState('stored-x'), busy: true })
+
+    expect($workingSessionIds.get()).toEqual(['stored-x'])
+  })
+})