diff --git a/litellm/utils.py b/litellm/utils.py
index 2125875ee1d..6b7d835ba2a 100644
--- a/litellm/utils.py
+++ b/litellm/utils.py
@@ -7853,6 +7853,13 @@ def validate_and_fix_openai_messages(messages: List):
     """
     new_messages = []
     for message in messages:
+        # Convert pydantic models to dicts first to avoid mutating model
+        # fields in-place, which triggers PydanticSerializationUnexpectedValue
+        # warnings when model_dump() is called later.
+        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"):
diff --git a/tests/local_testing/test_validate_openai_messages.py b/tests/local_testing/test_validate_openai_messages.py
new file mode 100644
index 00000000000..3a96521ba02
--- /dev/null
+++ b/tests/local_testing/test_validate_openai_messages.py
@@ -0,0 +1,114 @@
+"""
+Tests for validate_and_fix_openai_messages in litellm/utils.py
+
+Covers the fix for https://github.com/BerriAI/litellm/issues/25880:
+Pydantic Message objects with tool_calls should not trigger
+PydanticSerializationUnexpectedValue warnings.
+"""
+
+import warnings
+
+import pytest
+
+from litellm.types.utils import (
+    ChatCompletionMessageToolCall,
+    Function,
+    Message,
+)
+from litellm.utils import validate_and_fix_openai_messages
+
+
+def _make_tool_call_message():
+    """Helper: build an assistant Message with a single tool_call."""
+    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():
+    """Regression: pydantic Messages with tool_calls must not emit warnings."""
+    msg = _make_tool_call_message()
+
+    with warnings.catch_warnings():
+        warnings.filterwarnings("error", message="Pydantic serializer warnings")
+        # Should not raise
+        result = validate_and_fix_openai_messages(
+            [
+                {"role": "user", "content": "hi"},
+                msg,
+                {"role": "tool", "tool_call_id": "toolu_1", "content": "3"},
+            ]
+        )
+
+    # Basic sanity: all messages come back as dicts
+    assert isinstance(result, list)
+    assert len(result) == 3
+    for m in result:
+        assert isinstance(m, dict)
+
+    # The assistant message should have tool_calls serialized as dicts
+    assistant_msg = result[1]
+    assert assistant_msg["role"] == "assistant"
+    assert isinstance(assistant_msg["tool_calls"], list)
+    assert isinstance(assistant_msg["tool_calls"][0], dict)
+    assert assistant_msg["tool_calls"][0]["function"]["name"] == "add"
+
+
+def test_original_message_not_mutated():
+    """The original pydantic Message should not be mutated by validation."""
+    msg = _make_tool_call_message()
+    original_tc = msg.tool_calls[0]
+
+    validate_and_fix_openai_messages(
+        [
+            {"role": "user", "content": "hi"},
+            msg,
+            {"role": "tool", "tool_call_id": "toolu_1", "content": "3"},
+        ]
+    )
+
+    # tool_calls on the original message should still be the pydantic model
+    assert isinstance(msg.tool_calls[0], ChatCompletionMessageToolCall)
+    assert msg.tool_calls[0] is original_tc
+
+
+def test_dict_messages_still_work():
+    """Plain dict messages should still be processed correctly."""
+    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_gets_default():
+    """Messages without a role should default to 'assistant'."""
+    result = validate_and_fix_openai_messages(
+        [{"content": "some content"}]
+    )
+    assert result[0]["role"] == "assistant"
+
+
+def test_dict_messages_not_mutated_in_place():
+    """Original dict messages should not be mutated."""
+    original = {"role": "user", "content": "test"}
+    validate_and_fix_openai_messages([original])
+    # Should still be the same dict content
+    assert original == {"role": "user", "content": "test"}