diff --git a/src/agent_runs.py b/src/agent_runs.py
index 7fc661d..8adbab9 100644
--- a/src/agent_runs.py
+++ b/src/agent_runs.py
@@ -15,6 +15,7 @@ Durability scope: in-memory, survives as long as the server process runs (tab
 close / navigation / refresh). It does NOT survive a server restart.
 """
 import asyncio
+import json
 import logging
 from typing import AsyncGenerator, Dict, Optional
 
@@ -41,6 +42,17 @@ _RUNS: Dict[str, _Run] = {}
 _EVICT_GRACE_S = 180
 
 
+def _publish(run: _Run, ev: str) -> None:
+    """Append one SSE event and fan it out to every live subscriber."""
+    run.buffer.append(ev)
+    seq = len(run.buffer) - 1
+    for q in list(run.subscribers):
+        try:
+            q.put_nowait((seq, ev))
+        except Exception:
+            pass
+
+
 def _schedule_evict(session_id: str) -> None:
     """(Re)arm a grace-period eviction for a terminal run with no subscribers.
     Identity-checked so a run that gets replaced/reused is never evicted by a
@@ -93,13 +105,7 @@ async def _drain(session_id: str, agen: AsyncGenerator[str, None],
             pass
     try:
         async for ev in agen:
-            run.buffer.append(ev)
-            seq = len(run.buffer) - 1
-            for q in list(run.subscribers):
-                try:
-                    q.put_nowait((seq, ev))
-                except Exception:
-                    pass
+            _publish(run, ev)
         if run.status == "running":
             run.status = "done"
     except asyncio.CancelledError:
@@ -113,6 +119,12 @@ async def _drain(session_id: str, agen: AsyncGenerator[str, None],
     except Exception as e:
         logger.error("[agent-run] %s failed: %s", session_id, e, exc_info=True)
         run.status = "error"
+        _publish(
+            run,
+            "event: error\n"
+            f"data: {json.dumps({'error': 'Agent run failed before completion.', 'status': 500})}\n\n",
+        )
+        _publish(run, "data: [DONE]\n\n")
     finally:
         # Wake every subscriber with the end sentinel so their SSE closes.
         for q in list(run.subscribers):
diff --git a/static/js/chat.js b/static/js/chat.js
index 8c10bcf..118399c 100644
--- a/static/js/chat.js
+++ b/static/js/chat.js
@@ -1213,6 +1213,7 @@ import createResearchSynapse from './researchSynapse.js';
       }
 
       let _nextIsError = false;
+      let _streamSawDone = false;
 
       while (true) {
         const { done, value } = await reader.read();
@@ -1255,6 +1256,7 @@ import createResearchSynapse from './researchSynapse.js';
             }
 
             if (data === '[DONE]') {
+              _streamSawDone = true;
               // Always update background map if entry exists (even if user switched back)
               var bgDone = _backgroundStreams.get(streamSessionId);
               if (bgDone) {
@@ -2220,6 +2222,10 @@ import createResearchSynapse from './researchSynapse.js';
         }
       }
 
+      if (!_streamSawDone) {
+        throw new Error('Stream closed before completion');
+      }
+
       _renderStream();
       _cancelThinkingTimer();
       _removeThinkingSpinner();