fix: Prevent splitting of SSE events with artifactDelta for function resume requests

When handling a /run_sse request that includes a functionCallEventId, do not split events that contain both content and artifactDelta

Close #4487

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 877435561

src/google/adk/cli/adk_web_server.py

@@ -208,6 +208,8 @@ class RunAgentRequest(common.BaseModel):
   new_message: Optional[types.Content] = None
   streaming: bool = False
   state_delta: Optional[dict[str, Any]] = None
+  # for long-running function resume requests (e.g., OAuth callback)
+  function_call_event_id: Optional[str] = None
   # for resume long-running functions
   invocation_id: Optional[str] = None
 
@@ -1707,7 +1709,8 @@ class AdkWebServer:
               # 2) a content-less "action-only" event carrying `artifactDelta`
               events_to_stream = [event]
               if (
-                  event.actions.artifact_delta
+                  not req.function_call_event_id
+                  and event.actions.artifact_delta
                   and event.content
                   and event.content.parts
               ):

tests/unittests/cli/test_fast_api.py

@@ -1116,6 +1116,57 @@ def test_agent_run_sse_splits_artifact_delta(
   assert sse_events[1]["actions"]["artifactDelta"] == {"artifact.txt": 0}
 
 
+def test_agent_run_sse_does_not_split_artifact_delta_for_function_resume(
+    test_app, create_test_session, monkeypatch
+):
+  """Test /run_sse keeps artifactDelta with content for function resume flow."""
+  info = create_test_session
+
+  async def run_async_with_artifact_delta(
+      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
+    yield Event(
+        author="dummy agent",
+        invocation_id="invocation_id",
+        content=types.Content(
+            role="model", parts=[types.Part(text="LLM reply")]
+        ),
+        actions=EventActions(artifact_delta={"artifact.txt": 0}),
+    )
+
+  monkeypatch.setattr(Runner, "run_async", run_async_with_artifact_delta)
+
+  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,
+      "functionCallEventId": "function-call-event-id",
+  }
+
+  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 len(sse_events) == 1
+  assert sse_events[0]["content"]["parts"][0]["text"] == "LLM reply"
+  assert sse_events[0]["actions"]["artifactDelta"] == {"artifact.txt": 0}
+
+
 def test_agent_run_sse_yields_error_object_on_exception(
     test_app, create_test_session, monkeypatch
 ):