diff --git a/temporalio/contrib/openai_agents/_invoke_model_activity.py b/temporalio/contrib/openai_agents/_invoke_model_activity.py
index a43f9aeaf..3f7a639dd 100644
--- a/temporalio/contrib/openai_agents/_invoke_model_activity.py
+++ b/temporalio/contrib/openai_agents/_invoke_model_activity.py
@@ -393,6 +393,9 @@ async def invoke_model_activity_streaming(
                     conversation_id=input.get("conversation_id"),
                     prompt=input.get("prompt"),
                 ):
+                    # OpenAI models set defer_build=True, so an event's pydantic
+                    # schema may still be an unbuilt placeholder.
+                    type(event).model_rebuild()
                     events.append(event)
                     events_topic.publish(event)
             except APIStatusError as e: