diff --git a/python/packages/core/agent_framework/_workflows/_runner_context.py b/python/packages/core/agent_framework/_workflows/_runner_context.py
index edbbfea50c9..205d3bd9538 100644
--- a/python/packages/core/agent_framework/_workflows/_runner_context.py
+++ b/python/packages/core/agent_framework/_workflows/_runner_context.py
@@ -585,9 +585,17 @@ async def send_request_info_response(self, request_id: str, response: Any) -> No
 
         # Validate response type if specified
         if event.response_type and not is_instance_of(response, event.response_type):
+            # response_type may be a union/generic (e.g. int | str), which has no usable
+            # __name__ on Python 3.10/3.11 (AttributeError) and only yields "Union" on 3.12+.
+            # Use the readable repr for anything that isn't a plain class.
+            expected = (
+                event.response_type.__name__
+                if isinstance(event.response_type, type)
+                else str(event.response_type)
+            )
             raise TypeError(
                 f"Response type mismatch for request_id {request_id}: "
-                f"expected {event.response_type.__name__}, got {type(response).__name__}"
+                f"expected {expected}, got {type(response).__name__}"
             )
 
         source_executor_id = event.source_executor_id
diff --git a/python/packages/core/tests/workflow/test_request_info_event_rehydrate.py b/python/packages/core/tests/workflow/test_request_info_event_rehydrate.py
index 1be0a75f58e..88cb3dd7049 100644
--- a/python/packages/core/tests/workflow/test_request_info_event_rehydrate.py
+++ b/python/packages/core/tests/workflow/test_request_info_event_rehydrate.py
@@ -532,3 +532,24 @@ async def test_apply_checkpoint_preserves_streaming_flag() -> None:
     await target.apply_checkpoint(checkpoint)
 
     assert target.is_streaming() is True
+
+
+async def test_send_request_info_response_union_type_mismatch_message() -> None:
+    """A union response_type must produce a readable TypeError, not crash on __name__.
+
+    ``types.UnionType`` (e.g. ``int | str``) has no ``__name__`` on Python 3.10/3.11
+    (AttributeError) and only yields the unhelpful ``"Union"`` on 3.12+. The mismatch
+    message should name the union itself.
+    """
+    event = WorkflowEvent.request_info(
+        request_id="union-req",
+        source_executor_id="gateway",
+        request_data=MockRequest(),
+        response_type=int | str,
+    )
+
+    runner_context = InProcRunnerContext(InMemoryCheckpointStorage())
+    await runner_context.add_request_info_event(event)
+
+    with pytest.raises(TypeError, match=r"expected int \| str, got float"):
+        await runner_context.send_request_info_response("union-req", 3.14)