fix(desktop): avoid prewarming viewed history sessions (#880)

Viewing an existing transcript should only load history, not start the CLI prewarm path. Empty placeholder sessions still prewarm so new chats stay fast.

Fixes #880

Tested: cd desktop && bunx vitest run src/stores/chatStore.test.ts
Tested: bun run check:desktop
Confidence: high
Scope-risk: narrow

desktop/src/stores/chatStore.test.ts

@@ -215,6 +215,40 @@ describe('chatStore history mapping', () => {
     })
   })
 
+  it('does not prewarm an existing transcript when opening it for history review', () => {
+    sessionStoreSnapshot.sessions = [{
+      id: TEST_SESSION_ID,
+      title: 'Existing transcript',
+      createdAt: '2026-06-20T10:00:00.000Z',
+      modifiedAt: '2026-06-20T10:30:00.000Z',
+      messageCount: 4,
+      projectPath: '/workspace/project',
+      workDir: '/workspace/project',
+      workDirExists: true,
+    }]
+
+    useChatStore.getState().connectToSession(TEST_SESSION_ID)
+
+    expect(sendMock).not.toHaveBeenCalledWith(TEST_SESSION_ID, { type: 'prewarm_session' })
+  })
+
+  it('still prewarms empty placeholder sessions so new chats start quickly', () => {
+    sessionStoreSnapshot.sessions = [{
+      id: TEST_SESSION_ID,
+      title: 'New Session',
+      createdAt: '2026-06-20T10:00:00.000Z',
+      modifiedAt: '2026-06-20T10:00:00.000Z',
+      messageCount: 0,
+      projectPath: '/workspace/project',
+      workDir: '/workspace/project',
+      workDirExists: true,
+    }]
+
+    useChatStore.getState().connectToSession(TEST_SESSION_ID)
+
+    expect(sendMock).toHaveBeenCalledWith(TEST_SESSION_ID, { type: 'prewarm_session' })
+  })
+
   it('preserves thinking blocks when restoring transcript history', () => {
     const messages: MessageEntry[] = [
       {

desktop/src/stores/chatStore.ts

@@ -892,6 +892,11 @@ async function fetchAndMapSessionHistory(sessionId: string) {
 
 const historyLoadsInFlight = new Map<string, Promise<void>>()
 
+function shouldPrewarmSession(sessionId: string): boolean {
+  const knownSession = useSessionStore.getState().sessions.find((session) => session.id === sessionId)
+  return !knownSession || knownSession.messageCount === 0
+}
+
 export const useChatStore = create<ChatStore>((set, get) => ({
   sessions: {},
 
@@ -938,7 +943,11 @@ export const useChatStore = create<ChatStore>((set, get) => ({
     if (runtimeSelection) {
       wsManager.send(sessionId, { type: 'set_runtime_config', ...runtimeSelection })
     }
-    if (!sessionId.startsWith('__') && !useTeamStore.getState().getMemberBySessionId(sessionId)) {
+    if (
+      !sessionId.startsWith('__') &&
+      !useTeamStore.getState().getMemberBySessionId(sessionId) &&
+      shouldPrewarmSession(sessionId)
+    ) {
       wsManager.send(sessionId, { type: 'prewarm_session' })
     }