fix: Stream errors as simple JSON objects in ADK web server SSE

The ADK web server's /run_sse endpoint now yields a JSON object like {"error": "..."} when an exception occurs during event generation. The adk_web_server_client is updated to detect this error payload and raise a RuntimeError.

Close #4291

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 863475838

src/google/adk/cli/adk_web_server.py

@@ -1474,17 +1474,7 @@ class AdkWebServer:
                 yield f"data: {sse_event}\n\n"
         except Exception as e:
           logger.exception("Error in event_generator: %s", e)
-          # Yield a proper Event object for the error
-          error_event = Event(
-              author="system",
-              content=types.Content(
-                  role="model", parts=[types.Part(text=f"Error: {e}")]
-              ),
-          )
-          yield (
-              "data:"
-              f" {error_event.model_dump_json(by_alias=True, exclude_none=True)}\n\n"
-          )
+          yield f"data: {json.dumps({'error': str(e)})}\n\n"
 
       # Returns a streaming response with the proper media type for SSE
       return StreamingResponse(

src/google/adk/cli/conformance/adk_web_server_client.py

@@ -228,6 +228,7 @@ class AdkWebServerClient:
       ValueError: If mode is provided but test_case_dir or user_message_index is None
       httpx.HTTPStatusError: If the request fails
       json.JSONDecodeError: If event data cannot be parsed
+      RuntimeError: If the server streams an error payload
     """
     # Add recording parameters to state_delta for conformance tests
     if mode:
@@ -262,6 +263,8 @@ class AdkWebServerClient:
         async for line in response.aiter_lines():
           if line.startswith("data:") and (data := line[5:].strip()):
             event_data = json.loads(data)
+            if isinstance(event_data, dict) and "error" in event_data:
+              raise RuntimeError(event_data["error"])
             yield Event.model_validate(event_data)
           else:
             logger.debug("Non data line received: %s", line)

tests/unittests/cli/conformance/test_adk_web_server_client.py

@@ -224,6 +224,44 @@ async def test_run_agent():
     assert events[1].invocation_id == "test_invocation_2"
 
 
+@pytest.mark.asyncio
+async def test_run_agent_raises_on_streamed_error():
+  client = AdkWebServerClient()
+
+  class MockStreamResponse:
+
+    def raise_for_status(self):
+      pass
+
+    async def aiter_lines(self):
+      yield 'data: {"error": "boom"}'
+
+    async def __aenter__(self):
+      return self
+
+    async def __aexit__(self, exc_type, exc_val, exc_tb):
+      pass
+
+  def mock_stream(*_args, **_kwargs):
+    return MockStreamResponse()
+
+  with patch("httpx.AsyncClient") as mock_client_class:
+    mock_client = AsyncMock()
+    mock_client.stream = mock_stream
+    mock_client_class.return_value = mock_client
+
+    request = RunAgentRequest(
+        app_name="test_app",
+        user_id="test_user",
+        session_id="test_session",
+        new_message=types.Content(role="user", parts=[types.Part(text="Hi")]),
+    )
+
+    with pytest.raises(RuntimeError, match="boom"):
+      async for _ in client.run_agent(request):
+        pass
+
+
 @pytest.mark.asyncio
 async def test_close():
   client = AdkWebServerClient()

tests/unittests/cli/test_fast_api.py

@@ -1019,6 +1019,48 @@ def test_agent_run_sse_splits_artifact_delta(
   assert sse_events[1]["actions"]["artifactDelta"] == {"artifact.txt": 0}
 
 
+def test_agent_run_sse_yields_error_object_on_exception(
+    test_app, create_test_session, monkeypatch
+):
+  """Test /run_sse streams an error object if streaming raises."""
+  info = create_test_session
+
+  async def run_async_raises(
+      self,
+      *,
+      user_id: str,
+      session_id: str,
+      invocation_id: Optional[str] = None,
+      new_message: Optional[types.Content] = None,
+      state_delta: Optional[dict[str, Any]] = None,
+      run_config: Optional[RunConfig] = None,
+  ):
+    del user_id, session_id, invocation_id, new_message, state_delta, run_config
+    raise ValueError("boom")
+    if False:  # pylint: disable=using-constant-test
+      yield _event_1()
+
+  monkeypatch.setattr(Runner, "run_async", run_async_raises)
+
+  payload = {
+      "app_name": info["app_name"],
+      "user_id": info["user_id"],
+      "session_id": info["session_id"],
+      "new_message": {"role": "user", "parts": [{"text": "Hello agent"}]},
+      "streaming": True,
+  }
+
+  response = test_app.post("/run_sse", json=payload)
+  assert response.status_code == 200
+
+  sse_events = [
+      json.loads(line.removeprefix("data: "))
+      for line in response.text.splitlines()
+      if line.startswith("data: ")
+  ]
+  assert sse_events == [{"error": "boom"}]
+
+
 def test_list_artifact_names(test_app, create_test_session):
   """Test listing artifact names for a session."""
   info = create_test_session