v1.12.3: stale-stream banner with Retry/Discard

When an assistant message sits status='streaming' with no token activity
for 60+ seconds, the chat shows a banner above the input offering Retry
or Discard. Both clear the stale row via a new backend endpoint
POST /api/chats/:id/discard_stale that updates status='failed' and
publishes chat_status='idle'.

Closes the UX gap that caused the 2026-05-21 debugging spiral —
slow streams and dead streams now look different to the user.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/web/src/components/panes/ChatPane.tsx

@@ -5,6 +5,7 @@ import { api } from '@/api/client';
 import { useSessionStream } from '@/hooks/useSessionStream';
 import { MessageList } from '@/components/MessageList';
 import { ChatInput } from '@/components/ChatInput';
+import { StaleStreamBanner } from '@/components/StaleStreamBanner';
 import {
   DropdownMenu,
   DropdownMenuContent,
@@ -44,6 +45,38 @@ export function ChatPane({ sessionId, chatId, projectId, agentId, onAgentChange,
 
   const chatMessages = stream.messages.filter((m) => m.chat_id === chatId);
   const streaming = chatMessages.some((m) => m.status === 'streaming');
+
+  // v1.12.3: stale-stream detection. Watches the (at most one) streaming
+  // assistant row. If its content length doesn't grow for STALE_THRESHOLD_MS,
+  // assume the upstream call is dead and surface the recovery banner. We use
+  // content length as the activity signal because every token delta extends
+  // it; last_seq isn't currently bumped per delta.
+  const STALE_THRESHOLD_MS = 60_000;
+  const streamingMsg = chatMessages.find((m) => m.status === 'streaming' && m.role === 'assistant');
+  const streamingId = streamingMsg?.id ?? null;
+  const streamingLen = streamingMsg?.content.length ?? 0;
+  const lastActivityRef = useRef<{ id: string; len: number; at: number } | null>(null);
+  const [stale, setStale] = useState(false);
+  useEffect(() => {
+    if (!streamingId) {
+      lastActivityRef.current = null;
+      setStale(false);
+      return;
+    }
+    const prev = lastActivityRef.current;
+    if (!prev || prev.id !== streamingId || prev.len !== streamingLen) {
+      lastActivityRef.current = { id: streamingId, len: streamingLen, at: Date.now() };
+      setStale(false);
+    }
+    const interval = setInterval(() => {
+      const a = lastActivityRef.current;
+      if (!a) return;
+      if (Date.now() - a.at >= STALE_THRESHOLD_MS) {
+        setStale(true);
+      }
+    }, 5_000);
+    return () => clearInterval(interval);
+  }, [streamingId, streamingLen]);
   // v1.11.5: per-chat model context limit comes from chat.model_context_limit
   // populated by GET /api/sessions/:id/chats. Threaded into ChatInput so
   // ContextBar can render a zero-state before the first assistant message.
@@ -87,6 +120,45 @@ export function ChatPane({ sessionId, chatId, projectId, agentId, onAgentChange,
     }
   }
 
+  const handleDiscardStale = useCallback(async () => {
+    if (!streamingId) return;
+    try {
+      await api.chats.discardStale(chatId, streamingId);
+      setStale(false);
+      lastActivityRef.current = null;
+    } catch (err) {
+      // 409 (race) is benign — the row already terminated some other way.
+      const msg = err instanceof Error ? err.message : 'discard failed';
+      if (!msg.includes('409')) toast.error(msg);
+      setStale(false);
+    }
+  }, [chatId, streamingId]);
+
+  const handleRetryStale = useCallback(async () => {
+    if (!streamingId) return;
+    const lastUser = [...chatMessages].reverse().find((m) => m.role === 'user' && m.kind === 'message');
+    if (!lastUser) {
+      toast.error('no prior user message to retry');
+      return;
+    }
+    try {
+      await api.chats.discardStale(chatId, streamingId);
+    } catch (err) {
+      const msg = err instanceof Error ? err.message : 'discard failed';
+      if (!msg.includes('409')) {
+        toast.error(msg);
+        return;
+      }
+    }
+    setStale(false);
+    lastActivityRef.current = null;
+    try {
+      await api.messages.send(chatId, lastUser.content);
+    } catch (err) {
+      toast.error(err instanceof Error ? err.message : 'retry send failed');
+    }
+  }, [chatId, streamingId, chatMessages]);
+
   const handleForceSend = useCallback(async (content: string) => {
     const trimmed = content.trim();
     if (!trimmed) return;
@@ -187,6 +259,13 @@ export function ChatPane({ sessionId, chatId, projectId, agentId, onAgentChange,
         </div>
       )}
 
+      {stale && streamingId && (
+        <StaleStreamBanner
+          onRetry={() => void handleRetryStale()}
+          onDiscard={() => void handleDiscardStale()}
+        />
+      )}
+
       <ChatInput
         disabled={false}
         projectId={projectId}