fix(chat): preserve state across stale snapshots

Author: jaromil

src/client/features/chat/chat-projection.test.ts

@@ -8,6 +8,7 @@ import {
   acceptSnapshot,
   applyProjectionEvent,
   beginInteraction,
+  beginSnapshot,
   createChatProjection,
   deriveStatus,
   failInteraction,
@@ -90,6 +91,50 @@ describe('chat projection', () => {
       }).cursor,
     ).toBe(2);
   });
+  it('does not let a stale snapshot cut roll back sequenced timeline state', () => {
+    const base = acceptSnapshot(createChatProjection('s'), {
+      ...snapshot(2),
+      activeTurnId: 'turn-1',
+      items: [],
+      interactions: [],
+    });
+    const final = applyProjectionEvent(base, {
+      sequence: 3,
+      type: 'agentMessageDelta',
+      payload: { text: 'recovered final', phase: 'final_answer' },
+    });
+    const interaction = applyProjectionEvent(final, {
+      sequence: 4,
+      type: 'interaction.requested',
+      payload: { requestId: 'request-1', kind: 'commandApproval', turnId: 'turn-1', payload: {} },
+    });
+    const activity = applyProjectionEvent(interaction, {
+      sequence: 5,
+      type: 'activity.updated',
+      payload: { id: 'activity-1', label: 'Tool', detail: 'Finished' },
+    });
+
+    const retained = acceptSnapshot(beginSnapshot(activity), {
+      ...snapshot(2),
+      activeTurnId: null,
+      items: [],
+      interactions: [],
+    });
+
+    expect(retained).toMatchObject({
+      cursor: 5,
+      snapshotting: false,
+      activeTurnId: 'turn-1',
+      lifecycle: 'working',
+    });
+    expect(retained.messages).toEqual([
+      expect.objectContaining({ text: 'recovered final', phase: 'final_answer', complete: true }),
+    ]);
+    expect(retained.interactions).toEqual([
+      expect.objectContaining({ requestId: 'request-1', state: 'pending' }),
+    ]);
+    expect(retained.activities).toEqual([{ id: 'activity-1', label: 'Tool', detail: 'Finished' }]);
+  });
   it('does not promote duplicate identical optimistic prompts by ambiguous text', () => {
     const projection = queuePrompt(
       queuePrompt(createChatProjection('s'), 'one', 'same'),

src/client/features/chat/chat-projection.ts

@@ -262,6 +262,11 @@ export function hydrateCache(sessionId: string, cached: unknown): ChatProjection
 /** Accepts authoritative history without discarding a still-pending local operation. */
 export function acceptSnapshot(current: ChatProjection, snapshot: ChatSnapshot): ChatProjection {
   if (!Number.isInteger(snapshot.baseSequence) || snapshot.baseSequence < 0) return current;
+  // The projection has already applied state newer than this snapshot cut. Rebuilding
+  // from the stale snapshot would roll those sequenced events back, so retain the
+  // current projection and only drain events buffered while the request was in flight.
+  if (snapshot.baseSequence < current.cursor)
+    return replayBuffered({ ...current, snapshotting: false });
   const canonicalUsers = snapshot.items.filter((item) => item.kind === 'user');
   const prompts = current.prompts.map((prompt) => {
     const correlated = canonicalUsers.some(