fix(ui): reset transcript when replay starts

ui/src/lib/chat-transcript-session.test.ts

@@ -33,6 +33,21 @@ describe('chat-transcript-session', () => {
     expect(session.transcript.messages[0].streaming).toBe(false);
   });
 
+  it('replaces stale local transcript state when canonical replay starts', () => {
+    const session = createChatTranscriptSession('');
+    applyChatTranscriptUpdate(session, {
+      sessionUpdate: 'agent_message_chunk',
+      content: 'Based on the German residence law documents...',
+    });
+    finalizePromptStreaming(session);
+
+    noteReplayState(session, true);
+
+    expect(session.transcript.messages).toEqual([]);
+    expect(session.cacheHydrated).toBe(false);
+    expect(session.replaySawTranscriptUpdate).toBe(false);
+  });
+
   it('drops stale cached state when replay had no transcript-bearing updates', () => {
     const session = createChatTranscriptSession('');
     session.cacheHydrated = true;

ui/src/lib/chat-transcript-session.ts

@@ -44,10 +44,30 @@ export function replaceChatTranscriptSession(conversationId: string): ChatTransc
 /**
  * Resets replay bookkeeping when ACP begins replaying historical updates.
  */
-export function noteReplayState(session: ChatTranscriptSession, replaying: boolean): void {
-  if (replaying) {
-    session.replaySawTranscriptUpdate = false;
+export function noteReplayState(session: ChatTranscriptSession, replaying: boolean): ACPTranscript | null {
+  if (!replaying) {
+    return null;
   }
+
+  session.replaySawTranscriptUpdate = false;
+
+  const shouldReplaceTranscript =
+    session.cacheHydrated ||
+    session.transcript.messages.length > 0 ||
+    session.transcript.thinkingChunks.length > 0 ||
+    session.transcript.availableCommands.length > 0 ||
+    session.transcript.currentMode !== '' ||
+    session.transcript.usage !== null;
+
+  if (!shouldReplaceTranscript) {
+    return null;
+  }
+
+  // Backend replay is the source of truth, so drop any cached/live transcript
+  // state before historical updates start arriving.
+  session.transcript = createTranscript();
+  session.cacheHydrated = false;
+  return session.transcript;
 }
 
 /**

ui/src/lib/use-chat-connection.ts

@@ -249,7 +249,10 @@ export function useChatConnection({
           },
           onReplayStateChange: (replaying) => {
             if (cancelled) return;
-            noteReplayState(transcriptSessionRef.current, replaying);
+            const nextTranscript = noteReplayState(transcriptSessionRef.current, replaying);
+            if (replaying && nextTranscript) {
+              syncTranscript(nextTranscript);
+            }
           },
           onUpdate: (update, updateOptions) => {
             if (cancelled) return;

ui/src/pages/chat.test.tsx

@@ -759,6 +759,51 @@ describe('ChatPage draft persistence', () => {
     });
   });
 
+  it('replaces stale live assistant turns with canonical replay order on reconnect', async () => {
+    await renderChat('/c/covo/conv-1');
+
+    act(() => {
+      emitUpdate({
+        sessionUpdate: 'agent_message_chunk',
+        content: { type: 'text', text: 'Based on the German residence law documents...' },
+      });
+    });
+
+    await waitFor(() => {
+      const messages = screen.getAllByTestId('chat-message').map((element) => element.textContent);
+      expect(messages).toEqual(['assistant:Based on the German residence law documents...']);
+    });
+
+    act(() => {
+      emitReplayState(true);
+      emitUpdate({
+        sessionUpdate: 'agent_message_chunk',
+        historyMessageId: 'assistant-1',
+        content: { type: 'text', text: 'I need to clarify: tc is the TextCortex CLI.' },
+      }, { historical: true });
+      emitUpdate({
+        sessionUpdate: 'agent_message_chunk',
+        historyMessageId: 'assistant-2',
+        content: { type: 'text', text: "You're right — `tc kb search` lets you search your own knowledge bases." },
+      }, { historical: true });
+      emitUpdate({
+        sessionUpdate: 'agent_message_chunk',
+        historyMessageId: 'assistant-3',
+        content: { type: 'text', text: 'Based on the German residence law documents...' },
+      }, { historical: true });
+      emitReplayState(false);
+    });
+
+    await waitFor(() => {
+      const messages = screen.getAllByTestId('chat-message').map((element) => element.textContent);
+      expect(messages).toEqual([
+        'assistant:I need to clarify: tc is the TextCortex CLI.',
+        "assistant:You're right — `tc kb search` lets you search your own knowledge bases.",
+        'assistant:Based on the German residence law documents...',
+      ]);
+    });
+  });
+
   it('restores the original conversation draft when send fails after switching chats', async () => {
     const user = userEvent.setup();
     const deferred = createDeferred<unknown>();