fix: Split SSE events with both content and artifactDelta in ADK Web Server

This change modifies the /run_sse endpoint to split events that contain both content and an artifactDelta. The original event is split into two separate SSE events: one containing only the content (with artifactDelta cleared) and another containing only the artifactDelta (with content cleared)

Close #4036

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 852945249

src/google/adk/cli/adk_web_server.py

@@ -1531,14 +1531,31 @@ class AdkWebServer:
               )
           ) as agen:
             async for event in agen:
-              # Format as SSE data
-              sse_event = event.model_dump_json(
-                  exclude_none=True, by_alias=True
-              )
-              logger.debug(
-                  "Generated event in agent run streaming: %s", sse_event
-              )
-              yield f"data: {sse_event}\n\n"
+              # ADK Web renders artifacts from `actions.artifactDelta`
+              # during part processing *and* during action processing
+              # 1) the original event with `artifactDelta` cleared (content)
+              # 2) a content-less "action-only" event carrying `artifactDelta`
+              events_to_stream = [event]
+              if (
+                  event.actions.artifact_delta
+                  and event.content
+                  and event.content.parts
+              ):
+                content_event = event.model_copy(deep=True)
+                content_event.actions.artifact_delta = {}
+                artifact_event = event.model_copy(deep=True)
+                artifact_event.content = None
+                events_to_stream = [content_event, artifact_event]
+
+              for event_to_stream in events_to_stream:
+                sse_event = event_to_stream.model_dump_json(
+                    exclude_none=True,
+                    by_alias=True,
+                )
+                logger.debug(
+                    "Generated event in agent run streaming: %s", sse_event
+                )
+                yield f"data: {sse_event}\n\n"
         except Exception as e:
           logger.exception("Error in event_generator: %s", e)
           # You might want to yield an error event here

tests/unittests/cli/test_fast_api.py

@@ -130,6 +130,7 @@ async def dummy_run_async(
     new_message,
     state_delta=None,
     run_config: Optional[RunConfig] = None,
+    invocation_id: Optional[str] = None,
 ):
   run_config = run_config or RunConfig()
   yield _event_1()
@@ -959,6 +960,62 @@ def test_agent_run_passes_state_delta(test_app, create_test_session):
   assert data[3]["actions"]["stateDelta"] == payload["state_delta"]
 
 
+def test_agent_run_sse_splits_artifact_delta(
+    test_app, create_test_session, monkeypatch
+):
+  """Test /run_sse splits artifact deltas to avoid double-rendering in web."""
+  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,
+  }
+
+  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) == 2
+
+  # First event: content but artifactDelta cleared.
+  assert sse_events[0]["content"]["parts"][0]["text"] == "LLM reply"
+  assert sse_events[0]["actions"]["artifactDelta"] == {}
+
+  # Second event: artifactDelta but no content.
+  assert "content" not in sse_events[1]
+  assert sse_events[1]["actions"]["artifactDelta"] == {"artifact.txt": 0}
+
+
 def test_list_artifact_names(test_app, create_test_session):
   """Test listing artifact names for a session."""
   info = create_test_session