fix(mistral): forward reasoning_effort on native hybrid reasoning models

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

litellm/llms/mistral/chat/transformation.py

@@ -24,7 +24,7 @@ from litellm.secret_managers.main import get_secret_str
 from litellm.types.llms.mistral import MistralThinkingBlock, MistralToolCallMessage
 from litellm.types.llms.openai import AllMessageValues
 from litellm.types.utils import ModelResponse, ModelResponseStream
-from litellm.utils import convert_to_model_response_object
+from litellm.utils import convert_to_model_response_object, supports_reasoning
 
 
 class MistralConfig(OpenAIGPTConfig):
@@ -98,12 +98,26 @@ class MistralConfig(OpenAIGPTConfig):
             "parallel_tool_calls",
         ]
 
-        # Add reasoning support for magistral models
-        if "magistral" in model.lower():
+        if self._is_magistral_model(model):
             supported_params.extend(["thinking", "reasoning_effort"])
+        elif self._supports_native_reasoning_effort(model):
+            supported_params.append("reasoning_effort")
 
         return supported_params
 
+    @staticmethod
+    def _is_magistral_model(model: str) -> bool:
+        return "magistral" in model.lower()
+
+    @staticmethod
+    def _supports_native_reasoning_effort(model: str) -> bool:
+        """
+        Mistral's hybrid reasoning models (mistral-medium-3-5 and newer) take ``reasoning_effort``
+        directly on /chat/completions, so it is forwarded as-is instead of being emulated with the
+        magistral system prompt: https://docs.mistral.ai/capabilities/reasoning
+        """
+        return supports_reasoning(model=model, custom_llm_provider="mistral")
+
     def _map_tool_choice(self, tool_choice: str) -> str:
         if tool_choice == "auto" or tool_choice == "none":
             return tool_choice
@@ -168,10 +182,12 @@ class MistralConfig(OpenAIGPTConfig):
                 optional_params["extra_body"] = {"random_seed": value}
             if param == "response_format":
                 optional_params["response_format"] = value
-            if param == "reasoning_effort" and "magistral" in model.lower():
-                # Flag that we need to add reasoning system prompt
-                optional_params["_add_reasoning_prompt"] = True
-            if param == "thinking" and "magistral" in model.lower():
+            if param == "reasoning_effort" and value is not None:
+                if self._is_magistral_model(model):
+                    optional_params["_add_reasoning_prompt"] = True
+                elif self._supports_native_reasoning_effort(model):
+                    optional_params["reasoning_effort"] = value
+            if param == "thinking" and self._is_magistral_model(model):
                 # Flag that we need to add reasoning system prompt
                 optional_params["_add_reasoning_prompt"] = True
             if param == "parallel_tool_calls":

tests/test_litellm/llms/mistral/test_mistral_chat_transformation.py

@@ -311,6 +311,48 @@ class TestMistralReasoningSupport:
         assert len(result["messages"]) == 1
         assert result["messages"][0]["role"] == "user"
 
+    @pytest.mark.parametrize(
+        "model",
+        ["mistral/mistral-medium-3-5", "mistral-medium-3-5", "mistral/mistral-medium-latest"],
+    )
+    def test_native_reasoning_model_supports_reasoning_effort(self, model):
+        """Hybrid reasoning models take reasoning_effort natively, see https://github.com/BerriAI/litellm/issues/36407"""
+        supported_params = MistralConfig().get_supported_openai_params(model)
+
+        assert "reasoning_effort" in supported_params
+        assert "thinking" not in supported_params
+
+    def test_native_reasoning_effort_forwarded_to_request_body(self):
+        mistral_config = MistralConfig()
+
+        optional_params = mistral_config.map_openai_params(
+            non_default_params={"reasoning_effort": "high"},
+            optional_params={},
+            model="mistral/mistral-medium-3-5",
+            drop_params=False,
+        )
+        assert optional_params == {"reasoning_effort": "high"}
+
+        request = mistral_config.transform_request(
+            model="mistral/mistral-medium-3-5",
+            messages=[{"role": "user", "content": "What is 15 * 7?"}],
+            optional_params=optional_params,
+            litellm_params={},
+            headers={},
+        )
+        assert request["reasoning_effort"] == "high"
+        assert len(request["messages"]) == 1
+
+    def test_magistral_reasoning_effort_stays_prompt_based(self):
+        optional_params = MistralConfig().map_openai_params(
+            non_default_params={"reasoning_effort": "high"},
+            optional_params={},
+            model="mistral/magistral-medium-2506",
+            drop_params=False,
+        )
+
+        assert optional_params == {"_add_reasoning_prompt": True}
+
     def test_case_insensitive_magistral_detection(self):
         """Test that magistral model detection is case-insensitive."""
         mistral_config = MistralConfig()