fix: strip extra fields from messages for Mistral chat

Mistral API enforces additionalProperties: false on its message
schema, rejecting extra fields like metadata, provider_specific_fields,
thinking_blocks, and cache_control. Strip these before sending.

Fixes #30882

litellm/llms/mistral/chat/transformation.py

@@ -265,6 +265,13 @@ class MistralConfig(OpenAIGPTConfig):
             if MistralConfig._is_empty_assistant_message(m):
                 continue
             m = strip_none_values_from_message(m)  # prevents 'extra_forbidden' error
+            # GH#30882: strip fields not permitted by Mistral
+            # (additionalProperties: false on their message schema)
+            if isinstance(m, dict):
+                m.pop("metadata", None)
+                m.pop("provider_specific_fields", None)
+                m.pop("thinking_blocks", None)
+                m.pop("cache_control", None)
             new_messages.append(m)
 
         if is_async:

tests/test_litellm/llms/mistral/test_mistral_chat_transformation.py

@@ -249,6 +249,36 @@ class TestMistralReasoningSupport:
         assert result == messages
         assert len(result) == 1
 
+    def test_transform_messages_strips_extra_fields(self):
+        """GH#30882: extra fields like metadata should be stripped
+        from messages before sending to Mistral API."""
+        mistral_config = MistralConfig()
+        messages: List[AllMessageValues] = [
+            cast(
+                AllMessageValues,
+                {
+                    "role": "assistant",
+                    "content": "hello",
+                    "metadata": {
+                        "tool_outputs_trimmed": True,
+                        "trimmed_by": "async_context_compression",
+                    },
+                    "provider_specific_fields": {"foo": "bar"},
+                    "thinking_blocks": [],
+                    "cache_control": {"type": "ephemeral"},
+                },
+            )
+        ]
+        result = mistral_config._transform_messages(messages, "mistral/mistral-large-latest")
+        assert len(result) == 1
+        msg = cast(dict, result[0])
+        assert msg["role"] == "assistant"
+        assert msg["content"] == "hello"
+        assert "metadata" not in msg
+        assert "provider_specific_fields" not in msg
+        assert "thinking_blocks" not in msg
+        assert "cache_control" not in msg
+
     def test_transform_request_magistral_with_reasoning(self):
         """Test transform_request method for magistral model with reasoning."""
         mistral_config = MistralConfig()