fix(utils): avoid pydantic serializer warnings in validate_and_fix_openai_messages

Copy pydantic Messages (via model_dump) and dicts before rewriting
tool_calls, so the later model_dump() no longer warns and the caller's
messages are left untouched.

Fixes #25880

Signed-off-by: Christian-Sidak <61099993+Christian-Sidak@users.noreply.github.com>

litellm/utils.py

@@ -8427,6 +8427,13 @@ def validate_and_fix_openai_messages(messages: list):
     """
     new_messages: Final = []
     for message in messages:
+        # Copy before mutating: assigning dicts into a pydantic Message's
+        # tool_calls triggers PydanticSerializationUnexpectedValue warnings on
+        # the later model_dump(), and it rewrites the caller's objects.
+        if isinstance(message, BaseModel):
+            message = message.model_dump(exclude_none=True)
+        elif isinstance(message, dict):
+            message = message.copy()
         if not message.get("role"):
             message["role"] = "assistant"
         if message.get("tool_calls"):

tests/unit/test_validate_and_fix_openai_messages.py

@@ -0,0 +1,82 @@
+"""Tests for validate_and_fix_openai_messages (issue #25880)."""
+
+import warnings
+
+from litellm.types.utils import ChatCompletionMessageToolCall, Function, Message
+from litellm.utils import validate_and_fix_openai_messages
+
+
+def _make_tool_call_message() -> Message:
+    tc = ChatCompletionMessageToolCall(
+        id="toolu_1",
+        type="function",
+        function=Function(name="add", arguments='{"a":1,"b":2}'),
+    )
+    return Message(content="", role="assistant", tool_calls=[tc])
+
+
+def test_no_pydantic_serialization_warning_with_tool_calls():
+    msg = _make_tool_call_message()
+
+    with warnings.catch_warnings():
+        warnings.filterwarnings(
+            "error",
+            message=".*serialized value may not be as expected.*",
+            category=UserWarning,
+        )
+        result = validate_and_fix_openai_messages(
+            [
+                {"role": "user", "content": "hi"},
+                msg,
+                {"role": "tool", "tool_call_id": "toolu_1", "content": "3"},
+            ]
+        )
+
+    assert len(result) == 3
+    assert all(isinstance(m, dict) for m in result)
+    assistant_msg = result[1]
+    assert assistant_msg["role"] == "assistant"
+    assert isinstance(assistant_msg["tool_calls"][0], dict)
+    assert assistant_msg["tool_calls"][0]["function"]["name"] == "add"
+
+
+def test_original_message_not_mutated():
+    msg = _make_tool_call_message()
+    original_tc = msg.tool_calls[0]
+
+    validate_and_fix_openai_messages([{"role": "user", "content": "hi"}, msg])
+
+    assert isinstance(msg.tool_calls[0], ChatCompletionMessageToolCall)
+    assert msg.tool_calls[0] is original_tc
+
+
+def test_dict_messages_with_tool_calls_still_work():
+    messages = [
+        {"role": "user", "content": "hello"},
+        {
+            "role": "assistant",
+            "content": None,
+            "tool_calls": [
+                {
+                    "id": "call_1",
+                    "type": "function",
+                    "function": {"name": "greet", "arguments": "{}"},
+                }
+            ],
+        },
+        {"role": "tool", "tool_call_id": "call_1", "content": "hi"},
+    ]
+
+    result = validate_and_fix_openai_messages(messages)
+
+    assert len(result) == 3
+    assert result[1]["tool_calls"][0]["function"]["name"] == "greet"
+
+
+def test_missing_role_defaults_to_assistant_without_mutating_input():
+    original = {"content": "some content"}
+
+    result = validate_and_fix_openai_messages([original])
+
+    assert result[0]["role"] == "assistant"
+    assert original == {"content": "some content"}