diff --git a/litellm/responses/streaming_iterator.py b/litellm/responses/streaming_iterator.py
index d6d3e2576f6..8d766cf1cd0 100644
--- a/litellm/responses/streaming_iterator.py
+++ b/litellm/responses/streaming_iterator.py
@@ -450,11 +450,7 @@ class BaseResponsesAPIStreamingIterator:
                     _terminal_chunk: Final = (
                         openai_responses_api_chunk
                         if _billed_response is None or _billed_response is _response_obj
-                        else (
-                            openai_responses_api_chunk.model_copy(update={"response": _billed_response})
-                            if issubclass(type(openai_responses_api_chunk), BaseModel)  # pyright: ignore[reportUnnecessaryIsInstance]  # test stubs use spec'd Mocks whose __class__ reports BaseModel but whose model_copy returns a Mock
-                            else _replace_response(openai_responses_api_chunk, _billed_response)
-                        )
+                        else openai_responses_api_chunk.model_copy(update={"response": _billed_response})
                     )
                     self.completed_response = _terminal_chunk
                     _stamp_responses_usage_cost(_billed_response, self.logging_obj)
@@ -1359,13 +1355,6 @@ def _billed_terminal_response(
     )
 
 
-def _replace_response(
-    event: ResponsesAPIStreamingResponse, response: ResponsesAPIResponse
-) -> ResponsesAPIStreamingResponse:
-    setattr(event, "response", response)
-    return event
-
-
 def _dump_json_safely(response: BaseModel) -> str | None:
     try:
         return response.model_dump_json()
diff --git a/tests/test_litellm/responses/test_streaming_iterator.py b/tests/test_litellm/responses/test_streaming_iterator.py
index b33cc4e93c2..dbf54ec3b9b 100644
--- a/tests/test_litellm/responses/test_streaming_iterator.py
+++ b/tests/test_litellm/responses/test_streaming_iterator.py
@@ -47,10 +47,10 @@ def _mock_config() -> Mock:
     def _transform(model, parsed_chunk, logging_obj):
         evt_type = parsed_chunk.get("type")
         if evt_type == "response.completed":
-            completed = Mock(spec=ResponseCompletedEvent)
-            completed.type = ResponsesAPIStreamEvents.RESPONSE_COMPLETED
-            completed.response = mock_responses_api_response
-            return completed
+            return ResponseCompletedEvent(
+                type=ResponsesAPIStreamEvents.RESPONSE_COMPLETED,
+                response=mock_responses_api_response,
+            )
         stub = Mock()
         stub.type = evt_type
         return stub
@@ -348,10 +348,10 @@ def _mock_config_with_completed_response(response: ResponsesAPIResponse) -> Mock
     def _transform(model, parsed_chunk, logging_obj):
         evt_type = parsed_chunk.get("type")
         if evt_type == "response.completed":
-            completed = Mock(spec=ResponseCompletedEvent)
-            completed.type = ResponsesAPIStreamEvents.RESPONSE_COMPLETED
-            completed.response = response
-            return completed
+            return ResponseCompletedEvent(
+                type=ResponsesAPIStreamEvents.RESPONSE_COMPLETED,
+                response=response,
+            )
         stub = Mock()
         stub.type = evt_type
         if "delta" in parsed_chunk: