fix(streaming): preserve upstream custom fields on final chunk

Ensure final finish_reason chunks retain non-OpenAI attributes from original provider chunks, including the holding_chunk flush path where delta is non-empty. Add regression tests for both final-chunk branches.

Made-with: Cursor

litellm/litellm_core_utils/streaming_handler.py

@@ -31,7 +31,7 @@ from litellm.litellm_core_utils.model_response_utils import (
 )
 from litellm.litellm_core_utils.redact_messages import LiteLLMLoggingObject
 from litellm.litellm_core_utils.thread_pool_executor import executor
-from litellm.types.llms.openai import ChatCompletionChunk
+from litellm.types.llms.openai import OpenAIChatCompletionChunk
 from litellm.types.router import GenericLiteLLMParams
 from litellm.types.utils import (
     Delta,
@@ -745,7 +745,7 @@ class CustomStreamWrapper:
 
     def copy_model_response_level_provider_specific_fields(
         self,
-        original_chunk: Union[ModelResponseStream, ChatCompletionChunk],
+        original_chunk: Union[ModelResponseStream, OpenAIChatCompletionChunk],
         model_response: ModelResponseStream,
     ) -> ModelResponseStream:
         """
@@ -1012,6 +1012,15 @@ class CustomStreamWrapper:
             # if delta is None
             _is_delta_empty = self.is_delta_empty(delta=model_response.choices[0].delta)
 
+            # Preserve custom attributes from original chunk (applies to both
+            # empty and non-empty delta final chunks).
+            _original_chunk = response_obj.get("original_chunk", None)
+            if _original_chunk is not None:
+                preserve_upstream_non_openai_attributes(
+                    model_response=model_response,
+                    original_chunk=_original_chunk,
+                )
+
             if _is_delta_empty:
                 model_response.choices[0].delta = Delta(
                     content=None

tests/test_litellm/litellm_core_utils/test_streaming_handler.py

@@ -615,6 +615,79 @@ def test_streaming_handler_with_stop_chunk(
     assert returned_chunk is None
 
 
+def test_finish_reason_chunk_preserves_non_openai_attributes(
+    initialized_custom_stream_wrapper: CustomStreamWrapper,
+):
+    """
+    Regression test for #23444:
+    Preserve upstream non-OpenAI attributes on final finish_reason chunk.
+    """
+    initialized_custom_stream_wrapper.received_finish_reason = "stop"
+
+    original_chunk = ModelResponseStream(
+        id="chatcmpl-test",
+        created=1742093326,
+        model=None,
+        object="chat.completion.chunk",
+        choices=[
+            StreamingChoices(
+                finish_reason="stop",
+                index=0,
+                delta=Delta(content=""),
+                logprobs=None,
+            )
+        ],
+    )
+    setattr(original_chunk, "custom_field", {"key": "value"})
+
+    returned_chunk = initialized_custom_stream_wrapper.return_processed_chunk_logic(
+        completion_obj={"content": ""},
+        response_obj={"original_chunk": original_chunk},
+        model_response=ModelResponseStream(),
+    )
+
+    assert returned_chunk is not None
+    assert getattr(returned_chunk, "custom_field", None) == {"key": "value"}
+
+
+def test_finish_reason_with_holding_chunk_preserves_non_openai_attributes(
+    initialized_custom_stream_wrapper: CustomStreamWrapper,
+):
+    """
+    Regression test for #23444 holding-chunk path:
+    preserve custom attributes when _is_delta_empty is False after flushing
+    holding_chunk.
+    """
+    initialized_custom_stream_wrapper.received_finish_reason = "stop"
+    initialized_custom_stream_wrapper.holding_chunk = "filtered text"
+
+    original_chunk = ModelResponseStream(
+        id="chatcmpl-test-2",
+        created=1742093327,
+        model=None,
+        object="chat.completion.chunk",
+        choices=[
+            StreamingChoices(
+                finish_reason="stop",
+                index=0,
+                delta=Delta(content=""),
+                logprobs=None,
+            )
+        ],
+    )
+    setattr(original_chunk, "custom_field", {"key": "value"})
+
+    returned_chunk = initialized_custom_stream_wrapper.return_processed_chunk_logic(
+        completion_obj={"content": ""},
+        response_obj={"original_chunk": original_chunk},
+        model_response=ModelResponseStream(),
+    )
+
+    assert returned_chunk is not None
+    assert returned_chunk.choices[0].delta.content == "filtered text"
+    assert getattr(returned_chunk, "custom_field", None) == {"key": "value"}
+
+
 def test_set_response_id_propagation_empty_to_valid(
     initialized_custom_stream_wrapper: CustomStreamWrapper,
 ):