diff --git a/python/packages/ag-ui/agent_framework_ag_ui/_endpoint.py b/python/packages/ag-ui/agent_framework_ag_ui/_endpoint.py
index d80ecea7a1..f4116b221e 100644
--- a/python/packages/ag-ui/agent_framework_ag_ui/_endpoint.py
+++ b/python/packages/ag-ui/agent_framework_ag_ui/_endpoint.py
@@ -9,7 +9,7 @@
 from collections.abc import AsyncGenerator, Sequence
 from typing import Any
 
-from ag_ui.core import RunErrorEvent
+from ag_ui.core import BaseEvent, RunErrorEvent
 from ag_ui.encoder import EventEncoder
 from agent_framework import SupportsAgentRun, Workflow
 from fastapi import FastAPI, HTTPException
@@ -93,6 +93,19 @@ async def event_generator() -> AsyncGenerator[str]:
                 event_count = 0
                 try:
                     async for event in protocol_runner.run(input_data):
+                        # Guard: only BaseEvent instances can be SSE-encoded.
+                        # Non-BaseEvent objects (e.g. AgentResponseUpdate) lack
+                        # model_dump_json() and would cause an AttributeError
+                        # in EventEncoder.encode(). Skip them with a warning.
+                        if not isinstance(event, BaseEvent):
+                            logger.warning(
+                                "[%s] Skipping non-BaseEvent object of type %s; "
+                                "only ag_ui.core.BaseEvent instances can be SSE-encoded.",
+                                path,
+                                type(event).__name__,
+                            )
+                            continue
+
                         event_count += 1
                         event_type_name = getattr(event, "type", type(event).__name__)
                         # Log important events at INFO level
diff --git a/python/packages/ag-ui/tests/ag_ui/test_endpoint.py b/python/packages/ag-ui/tests/ag_ui/test_endpoint.py
index 51ab468b84..80e7f1abaf 100644
--- a/python/packages/ag-ui/tests/ag_ui/test_endpoint.py
+++ b/python/packages/ag-ui/tests/ag_ui/test_endpoint.py
@@ -9,6 +9,7 @@
 from ag_ui.core import RunStartedEvent
 from agent_framework import (
     Agent,
+    AgentResponseUpdate,
     ChatResponseUpdate,
     Content,
     WorkflowBuilder,
@@ -603,3 +604,36 @@ async def run(self, input_data: dict[str, Any]):
 
     # Should still get 200 (SSE stream), just with no events
     assert response.status_code == 200
+
+
+async def test_endpoint_skips_non_base_event_objects():
+    """Non-BaseEvent objects (e.g. AgentResponseUpdate) are skipped with a warning.
+
+    Regression test for https://github.com/microsoft/agent-framework/issues/4929
+    """
+
+    class MixedEventWorkflow(AgentFrameworkWorkflow):
+        async def run(self, input_data: dict[str, Any]):
+            del input_data
+            yield RunStartedEvent(run_id="run-1", thread_id="thread-1")
+            # Yield a non-BaseEvent object — this should be skipped with a warning, not crash
+            yield AgentResponseUpdate(  # type: ignore[misc]
+                contents=[Content.from_text(text="leaked update")],
+                role="assistant",
+            )
+
+    app = FastAPI()
+    add_agent_framework_fastapi_endpoint(app, MixedEventWorkflow(), path="/mixed-events")
+    client = TestClient(app)
+
+    response = client.post("/mixed-events", json={"messages": [{"role": "user", "content": "Hello"}]})
+
+    assert response.status_code == 200
+    content = response.content.decode("utf-8")
+    lines = [line for line in content.split("\n") if line.startswith("data: ")]
+    event_types = [json.loads(line[6:]).get("type") for line in lines]
+
+    # RUN_STARTED should be present; the AgentResponseUpdate should have been
+    # skipped with a warning — no RUN_ERROR or crash.
+    assert "RUN_STARTED" in event_types
+    assert "RUN_ERROR" not in event_types