diff --git a/src/agents/models/chatcmpl_stream_handler.py b/src/agents/models/chatcmpl_stream_handler.py
index 0cf1e6a3e..83fa32abc 100644
--- a/src/agents/models/chatcmpl_stream_handler.py
+++ b/src/agents/models/chatcmpl_stream_handler.py
@@ -276,7 +276,7 @@ async def handle_stream(
                     state.function_calls[tc_delta.index].name += (
                         tc_function.name if tc_function else ""
                     ) or ""
-                    state.function_calls[tc_delta.index].call_id += tc_delta.id or ""
+                    state.function_calls[tc_delta.index].call_id = tc_delta.id or ""
 
         if state.reasoning_content_index_and_output:
             yield ResponseReasoningSummaryPartDoneEvent(