fix(router): include partial usage in streaming fallback totals

litellm/router.py

@@ -1899,11 +1899,7 @@ class Router:
 
         usage = cast(Optional[Usage], getattr(fallback_item, "usage", None))
         usage_objects = [usage] if usage is not None else []
-        if (
-            complete_response_object_usage is not None
-            and hasattr(complete_response_object_usage, "usage")
-            and complete_response_object_usage.usage is not None  # type: ignore
-        ):
+        if complete_response_object_usage is not None:
             usage_objects.append(complete_response_object_usage)
         combined_usage = BaseTokenUsageProcessor.combine_usage_objects(usage_objects=usage_objects)
         setattr(fallback_item, "usage", combined_usage)

tests/test_litellm/test_router.py

@@ -4025,12 +4025,10 @@ def test_update_kwargs_with_deployment_model_info_in_metadata():
     assert model_info["output_cost_per_token"] == 0.0015
 
 
-def test_combine_fallback_usage():
-    """Test that _combine_fallback_usage merges partial and fallback usage."""
+def test_combine_fallback_usage_without_partial_usage():
     from litellm.router import Router
     from litellm.types.utils import Usage
 
-    # Create a stream chunk with usage
     chunk = litellm.ModelResponseStream(
         id="test",
         model="gpt-4o",
@@ -4038,7 +4036,6 @@ def test_combine_fallback_usage():
         usage=Usage(prompt_tokens=10, completion_tokens=5, total_tokens=15),
     )
 
-    # Call _combine_fallback_usage with no extra usage
     Router._combine_fallback_usage(chunk, None)
     assert chunk.usage is not None
     assert chunk.usage.prompt_tokens == 10
@@ -4046,6 +4043,26 @@ def test_combine_fallback_usage():
     assert chunk.usage.total_tokens == 15
 
 
+def test_combine_fallback_usage():
+    from litellm.router import Router
+    from litellm.types.utils import Usage
+
+    chunk = litellm.ModelResponseStream(
+        id="test",
+        model="gpt-4o",
+        choices=[],
+        usage=Usage(prompt_tokens=10, completion_tokens=5, total_tokens=15),
+    )
+    partial_usage = Usage(prompt_tokens=7, completion_tokens=3, total_tokens=10)
+
+    Router._combine_fallback_usage(chunk, partial_usage)
+
+    assert chunk.usage is not None
+    assert chunk.usage.prompt_tokens == 17
+    assert chunk.usage.completion_tokens == 8
+    assert chunk.usage.total_tokens == 25
+
+
 @pytest.mark.asyncio
 async def test_acompletion_streaming_iterator_does_not_log_success_on_terminal_failure():
     """A mid-stream failure with no successful fallback raises and is logged as