fix: address PR review comments for model_dump_with_preserved_fields

- Restore preserve_fields param for backward compatibility (deprecated)
- Use zip() instead of index-based iteration to prevent IndexError
- Add backward compatibility test

litellm/proxy/utils.py

@@ -4565,9 +4565,6 @@ def validate_model_access(
             )
 
 
-# Fields within each choice that must be preserved as null (not stripped)
-# for OpenAI API compatibility. Each tuple is (sub_object, field_name).
-# To add a new preserved field, just append a tuple here.
 _PRESERVED_NONE_FIELDS: List[tuple] = [
     ("message", "content"),  # null when tool_calls present (issue #6677)
     ("message", "role"),  # always required by OpenAI spec
@@ -4577,6 +4574,7 @@ _PRESERVED_NONE_FIELDS: List[tuple] = [
 
 def model_dump_with_preserved_fields(
     obj: Any,
+    preserve_fields: Optional[List[str]] = None,
     exclude_unset: bool = True,
 ) -> Dict[str, Any]:
     """
@@ -4588,6 +4586,7 @@ def model_dump_with_preserved_fields(
 
     Args:
         obj: The Pydantic BaseModel instance to serialize
+        preserve_fields: Deprecated, kept for backward compatibility.
         exclude_unset: Whether to exclude fields that were not explicitly set
 
     Returns:
@@ -4600,9 +4599,7 @@ def model_dump_with_preserved_fields(
         return result
 
     obj_choices = obj.choices
-    for i, choice_dict in enumerate(choices):
-        choice_obj = obj_choices[i]
-
+    for choice_obj, choice_dict in zip(obj_choices, choices):
         for sub_object, field_name in _PRESERVED_NONE_FIELDS:
             sub_dict = choice_dict.get(sub_object)
             if sub_dict is None:

tests/test_litellm/proxy/test_model_dump_with_preserved_fields.py

@@ -375,3 +375,39 @@ def test_delta_dynamic_attributes_in_model_dump():
     delta_no_role = Delta(content=None, role=None)
     dump_no_role = delta_no_role.model_dump(exclude_none=True)
     assert "role" not in dump_no_role
+
+
+def test_preserve_fields_param_backward_compat():
+    """preserve_fields parameter is accepted (deprecated) without error."""
+    response = ModelResponse(
+        choices=[
+            Choices(
+                finish_reason="tool_calls",
+                index=0,
+                message=Message(
+                    content=None,
+                    role="assistant",
+                    tool_calls=[
+                        {
+                            "id": "call_1",
+                            "type": "function",
+                            "function": {"name": "f", "arguments": "{}"},
+                        }
+                    ],
+                ),
+            )
+        ],
+    )
+    result_default = model_dump_with_preserved_fields(response, exclude_unset=True)
+    result_explicit = model_dump_with_preserved_fields(
+        response,
+        preserve_fields=[
+            "choices.*.message.content",
+            "choices.*.message.role",
+            "choices.*.delta.content",
+        ],
+        exclude_unset=True,
+    )
+    assert result_default == result_explicit
+    assert result_default["choices"][0]["message"]["content"] is None
+    assert result_default["choices"][0]["message"]["role"] == "assistant"