diff --git a/api/server/routes/agents/index.js b/api/server/routes/agents/index.js
index eb42046bed..bbb39f5d2c 100644
--- a/api/server/routes/agents/index.js
+++ b/api/server/routes/agents/index.js
@@ -143,7 +143,8 @@ router.get('/chat/stream/:streamId', async (req, res) => {
   }
 
   if (!result) {
-    return res.status(404).json({ error: 'Failed to subscribe to stream' });
+    onError('Failed to subscribe to stream');
+    return;
   }
 
   req.on('close', () => {
diff --git a/client/src/hooks/SSE/__tests__/useResumableSSE.spec.ts b/client/src/hooks/SSE/__tests__/useResumableSSE.spec.ts
index 1717d27c22..269f38c865 100644
--- a/client/src/hooks/SSE/__tests__/useResumableSSE.spec.ts
+++ b/client/src/hooks/SSE/__tests__/useResumableSSE.spec.ts
@@ -281,4 +281,51 @@ describe('useResumableSSE - 404 error path', () => {
       unmount();
     },
   );
+
+  it('treats responseCode === 0 with raw SSE buffer data as transport failure (reconnect path)', async () => {
+    const submission = buildSubmission();
+    const chatHelpers = buildChatHelpers();
+
+    const { unmount } = renderHook(() => useResumableSSE(submission, chatHelpers));
+
+    await act(async () => {
+      await Promise.resolve();
+    });
+
+    const sse = getLastSSE();
+
+    await act(async () => {
+      sse._emit('error', {
+        responseCode: 0,
+        data: 'event: message\ndata: {"created":true,"message":{}}\n\n',
+      });
+    });
+
+    expect(mockErrorHandler).not.toHaveBeenCalled();
+    unmount();
+  });
+
+  it('parses and surfaces server-sent error events (no responseCode, JSON data)', async () => {
+    const submission = buildSubmission();
+    const chatHelpers = buildChatHelpers();
+
+    const { unmount } = renderHook(() => useResumableSSE(submission, chatHelpers));
+
+    await act(async () => {
+      await Promise.resolve();
+    });
+
+    const sse = getLastSSE();
+
+    const errorPayload = JSON.stringify({
+      error: JSON.stringify({ type: 'token_limit' }),
+    });
+
+    await act(async () => {
+      sse._emit('error', { data: errorPayload });
+    });
+
+    expect(mockErrorHandler).toHaveBeenCalledTimes(1);
+    unmount();
+  });
 });
diff --git a/client/src/hooks/SSE/useResumableSSE.ts b/client/src/hooks/SSE/useResumableSSE.ts
index 39dc610dae..f0ee12ecc1 100644
--- a/client/src/hooks/SSE/useResumableSSE.ts
+++ b/client/src/hooks/SSE/useResumableSSE.ts
@@ -391,8 +391,10 @@ export default function useResumableSSE(
          * Server-sent error event (event: error with data) - no responseCode.
          * These are known errors (ErrorTypes, ViolationTypes) that should be displayed to user.
          * Only check e.data if there's no HTTP responseCode, since HTTP errors may also have body data.
+         * Note: responseCode === 0 means transport failure (connection dropped) - treat as network error,
+         * not a server-sent error payload. Use `== null` to only match undefined/null (no HTTP status).
          */
-        if (!responseCode && e.data) {
+        if (responseCode == null && e.data) {
           console.log('[ResumableSSE] Server-sent error event received:', e.data);
           sse.close();
           removeActiveJob(currentStreamId);