diff --git a/litellm/utils.py b/litellm/utils.py
index 7de7ce6188d..d65ef631b69 100644
--- a/litellm/utils.py
+++ b/litellm/utils.py
@@ -8921,14 +8921,23 @@ def convert_list_message_to_dict(
     return [dict(cleanup_none_field_in_message(message=_as_message_value(message))) for message in messages]
 
 
-def validate_and_fix_openai_messages(messages: list):
+def validate_and_fix_openai_messages(
+    messages: list[Any],  # mutable-ok: caller passes arbitrary message dicts
+):
     """
     Ensures all messages are valid OpenAI chat completion messages.
 
     Handles missing role for assistant messages.
     """
-    new_messages: Final = []
+    new_messages: Final[list[AllMessageValues]] = []  # mutable-ok: list is populated by appending, name never rebound
     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"):
@@ -8941,7 +8950,9 @@ def validate_and_fix_openai_messages(messages: list):
             ),
         )
         cleaned_message = cleanup_none_field_in_message(message=convert_msg_to_dict)
-        new_messages.append(cleaned_message)
+        new_messages.append(
+            cast(AllMessageValues, cleaned_message)
+        )  # cast-ok: cleanup_none_field_in_message return satisfies AllMessageValues
     return validate_chat_completion_user_messages(messages=new_messages)
 
 
diff --git a/tests/unit/test_validate_and_fix_openai_messages.py b/tests/unit/test_validate_and_fix_openai_messages.py
new file mode 100644
index 00000000000..cd2d41bd8f2
--- /dev/null
+++ b/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"}