test(logging): lock in streaming response_cost survival across sync success handler

tests/test_litellm/litellm_core_utils/test_litellm_logging.py

@@ -2796,6 +2796,39 @@ async def test_async_success_handler_preserves_response_cost_for_pass_through_en
     assert slo is not None
     assert slo["response_cost"] > 0
 
+@pytest.mark.asyncio
+async def test_sync_success_handler_preserves_streaming_cost_computed_by_async_handler():
+    """Regression for #34875: the sync success_handler used to reset response_cost to None
+    for streams, so custom callbacks reading kwargs["response_cost"] saw None whenever it ran
+    after the async handler had already computed the cost on the shared model_call_details."""
+    import litellm
+
+    litellm.callbacks = []
+    litellm.success_callback = []
+
+    response = await litellm.acompletion(
+        model="gpt-4o-mini",
+        messages=[{"role": "user", "content": "hi"}],
+        stream=True,
+        mock_response="hello there",
+        stream_options={"include_usage": True},
+    )
+    logging_obj = response.logging_obj
+    async for _chunk in response:
+        pass
+    await asyncio.sleep(0.5)
+
+    cost_from_async_handler = logging_obj.model_call_details.get("response_cost")
+    assert cost_from_async_handler is not None and cost_from_async_handler > 0
+
+    assembled = logging_obj.model_call_details.get(
+        "async_complete_streaming_response"
+    ) or logging_obj.model_call_details.get("complete_streaming_response")
+
+    for _ in range(2):
+        logging_obj.success_handler(assembled, None, None, False)
+        assert logging_obj.model_call_details["response_cost"] == cost_from_async_handler
+
 
 def test_process_hidden_params_recalculates_cost_after_failure_handler_zero():
     """