Show persistent timestamps for operation and teacher messages

tests/unit/teacher-conversation.test.ts

@@ -37,6 +37,21 @@ function turn(index: number): TeacherRequest {
     omittedMessages: 0, status: 'completed', response: '回答 ' + index };
 }
 describe('continuous conversation persistence', () => {
+  it('preserves individual completion times and reads older replies without backfilling or rewriting them', async () => {
+    const dir = await root();
+    const store = new TeacherConversationStore(dir, 'account', 'agent-a');
+    const topic = await store.ensure(definition, 1);
+    const old = turn(1), recent = { ...turn(2), finishedAt: '2026-09-29T08:01:42.000Z' };
+    topic.requests = [old, recent];
+    await store.save(topic);
+    const first = await new TeacherConversationStore(dir, 'account', 'agent-a').read();
+    expect(first.requests[0]).not.toHaveProperty('finishedAt');
+    expect(first.requests[1].finishedAt).toBe(recent.finishedAt);
+    const second = await new TeacherConversationStore(dir, 'account', 'agent-a').read();
+    expect(second.requests).toEqual(first.requests);
+    expect(second.requests[0].createdAt).toBe(old.createdAt);
+  });
+
   it('repairs retained replies on every page without rewriting their files during restart recovery', async () => {
     const dir = await root();
     const store = new TeacherConversationStore(dir, 'account', 'agent-a');