diff --git a/pydantic_ai_slim/pydantic_ai/models/openai.py b/pydantic_ai_slim/pydantic_ai/models/openai.py
index b968ac61fd..92d79c6340 100644
--- a/pydantic_ai_slim/pydantic_ai/models/openai.py
+++ b/pydantic_ai_slim/pydantic_ai/models/openai.py
@@ -1051,7 +1051,7 @@ async def _get_event_iterator(self) -> AsyncIterator[ModelResponseStreamEvent]:
                     vendor_part_id=chunk.item_id,
                     tool_name=None,
                     args=chunk.delta,
-                    tool_call_id=chunk.item_id,
+                    tool_call_id=None,
                 )
                 if maybe_event is not None:  # pragma: no branch
                     yield maybe_event