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/server/src/routes/chats.ts

@@ -18,6 +18,12 @@ const ForkBody = z.object({
   name: z.string().min(1).max(200).optional(),
 });
 
+const DiscardStaleBody = z.object({
+  message_id: z.string().uuid(),
+});
+
+const STALE_MIN_AGE_SECONDS = 60;
+
 export function registerChatRoutes(
   app: FastifyInstance,
   sql: Sql,
@@ -320,6 +326,73 @@ export function registerChatRoutes(
     }
   );
 
+  // v1.12.3: explicit recovery from a stuck-streaming assistant row. The
+  // frontend gates this behind a 60s no-token-activity timer; the server
+  // re-checks the age and current status for safety. Non-streaming rows
+  // return 409 (frontend race; idempotent retry is fine).
+  app.post<{ Params: { id: string } }>(
+    '/api/chats/:id/discard_stale',
+    async (req, reply) => {
+      const parsed = DiscardStaleBody.safeParse(req.body ?? {});
+      if (!parsed.success) {
+        reply.code(400);
+        return { error: 'invalid body', details: parsed.error.flatten() };
+      }
+      const rows = await sql<{
+        id: string;
+        session_id: string;
+        chat_id: string;
+        status: string;
+        age_seconds: number;
+      }[]>`
+        SELECT id, session_id, chat_id, status,
+               EXTRACT(EPOCH FROM (clock_timestamp() - created_at))::int AS age_seconds
+        FROM messages
+        WHERE id = ${parsed.data.message_id} AND chat_id = ${req.params.id}
+      `;
+      if (rows.length === 0) {
+        reply.code(404);
+        return { error: 'message not found in chat' };
+      }
+      const msg = rows[0]!;
+      if (msg.status !== 'streaming') {
+        reply.code(409);
+        return { error: 'message is no longer streaming', current_status: msg.status };
+      }
+      if (msg.age_seconds < STALE_MIN_AGE_SECONDS) {
+        reply.code(409);
+        return { error: 'message is not stale yet', age_seconds: msg.age_seconds };
+      }
+      const updated = await sql<Message[]>`
+        UPDATE messages
+        SET status = 'failed',
+            content = COALESCE(content, ''),
+            finished_at = clock_timestamp()
+        WHERE id = ${msg.id} AND status = 'streaming'
+        RETURNING id, session_id, chat_id, role, content, kind, tool_calls, tool_results,
+                  status, last_seq, tokens_used, ctx_used, ctx_max, started_at, finished_at,
+                  created_at, metadata, summary, tail_start_id, compacted_at
+      `;
+      if (updated.length === 0) {
+        // Race: the row flipped out of 'streaming' between our SELECT and UPDATE.
+        reply.code(409);
+        return { error: 'message status changed mid-request' };
+      }
+      broker.publishUser('default', {
+        type: 'chat_status',
+        chat_id: msg.chat_id,
+        status: 'idle',
+        at: new Date().toISOString(),
+      });
+      broker.publish(msg.session_id, {
+        type: 'message_complete',
+        message_id: msg.id,
+        chat_id: msg.chat_id,
+      });
+      return updated[0];
+    }
+  );
+
   app.get<{ Params: { id: string } }>(
     '/api/chats/:id/messages',
     async (req, reply) => {