fix(mistral): reject reasoning_effort strings that are not a level, and leave "default" to Mistral

A string such as "HIGH" or "auto" went to Mistral as sent and Mistral answered 422, even with drop_params. It now takes the same path as a number: dropped with a warning under drop_params, a 400 without it. "default", which litellm.completion lists as a value, sends no reasoning_effort so Mistral applies its own default.

litellm/llms/mistral/chat/transformation.py

@@ -27,6 +27,7 @@ from litellm.litellm_core_utils.prompt_templates.common_utils import (
     strip_none_values_from_message,
 )
 from litellm.litellm_core_utils.reasoning_effort_capability import (
+    REASONING_EFFORT_ADVERTISEMENT_ORDER,
     declared_reasoning_efforts_for_model,
     nearest_declared_reasoning_effort,
 )
@@ -92,6 +93,8 @@ _MISTRAL_REQUEST_ROUTING: Final = TypeAdapter(MistralRequestRouting)
 _MISTRAL_CACHE_KEY_PARAMS: Final = TypeAdapter(MistralCacheKeyParams)
 _MISTRAL_REASONING_EFFORT_PARAMS: Final = TypeAdapter(MistralReasoningEffortParams)
 _MISTRAL_FILE_PART: Final = TypeAdapter(MistralFilePart)
+_REASONING_LEVELS: Final = frozenset(REASONING_EFFORT_ADVERTISEMENT_ORDER)
+_PROVIDER_DEFAULT_EFFORT: Final = "default"
 _NON_LEVEL_EFFORT_REASON: Final = (
     "the Mistral API takes it only as a level such as 'high', so Mistral applies its default effort."
 )
@@ -294,13 +297,16 @@ class MistralConfig(OpenAIGPTConfig):
     ) -> str | None:
         requested_effort: Final = requested.get("reasoning_effort")
         level, summary = _reasoning_level_and_summary(requested_effort)
+        leaves_level_to_mistral: Final = level == _PROVIDER_DEFAULT_EFFORT
         sendable: Final = (
-            _accepted_reasoning_effort(model, level, self.custom_llm_provider) if isinstance(level, str) else None
+            _accepted_reasoning_effort(model, level, self.custom_llm_provider)
+            if isinstance(level, str) and level in _REASONING_LEVELS
+            else None
         )
-        if sendable is not None and summary is None:
+        if summary is None and (sendable is not None or leaves_level_to_mistral):
             return sendable
         if drop_params or litellm.drop_params is True:
-            if sendable is None and level is not None:
+            if sendable is None and level is not None and not leaves_level_to_mistral:
                 warn_dropped_reasoning(
                     "reasoning_effort", requested_effort, model, _NON_LEVEL_EFFORT_REASON, drop_params_gated=True
                 )

tests/unit/llms/mistral/test_mistral_chat_transformation.py

@@ -1556,7 +1556,9 @@ def test_non_magistral_reasoning_effort_is_sent_without_warning(caplog: pytest.L
     assert _warnings_naming(caplog, "reasoning_effort") == ()
 
 
-@pytest.mark.parametrize("reasoning_effort", [5, {"effort": 5}, {"effort": 5, "summary": "auto"}])
+@pytest.mark.parametrize(
+    "reasoning_effort", [5, {"effort": 5}, {"effort": 5, "summary": "auto"}, "HIGH", "auto", "", {"effort": "HIGH"}]
+)
 def test_mistral_warns_once_when_drop_params_drops_a_level_it_cannot_send(
     caplog: pytest.LogCaptureFixture, reasoning_effort: object
 ) -> None:
@@ -1575,7 +1577,10 @@ def test_mistral_warns_once_when_drop_params_drops_a_level_it_cannot_send(
     assert "(drop_params=True)" in warnings[0]
 
 
-@pytest.mark.parametrize("reasoning_effort", [{"summary": "auto"}, {"effort": "high", "summary": "auto"}])
+@pytest.mark.parametrize(
+    "reasoning_effort",
+    [{"summary": "auto"}, {"effort": "high", "summary": "auto"}, {"effort": "default", "summary": "auto"}],
+)
 def test_mistral_stays_quiet_under_drop_params_when_no_level_is_lost(
     caplog: pytest.LogCaptureFixture, reasoning_effort: object
 ) -> None:
@@ -1590,6 +1595,44 @@ def test_mistral_stays_quiet_under_drop_params_when_no_level_is_lost(
     assert _warnings_naming(caplog, "reasoning_effort") == ()
 
 
+@pytest.mark.parametrize("reasoning_effort", ["HIGH", "auto", "", {"effort": "HIGH"}])
+def test_magistral_rejects_a_reasoning_effort_that_is_not_a_level(
+    local_model_cost_map: None, reasoning_effort: object
+) -> None:
+    import litellm
+
+    with pytest.raises(litellm.UnsupportedParamsError, match=r"\['reasoning_effort'\]"):
+        litellm.get_optional_params(
+            model="magistral-small-latest",
+            custom_llm_provider="mistral",
+            reasoning_effort=reasoning_effort,
+        )
+
+
+@pytest.mark.parametrize("drop_params", [False, True])
+@pytest.mark.parametrize("reasoning_effort", ["default", {"effort": "default"}])
+@pytest.mark.parametrize("model", ["magistral-small-latest", "mistral-medium-latest"])
+def test_default_reasoning_effort_leaves_the_level_to_mistral(
+    local_model_cost_map: None,
+    caplog: pytest.LogCaptureFixture,
+    model: str,
+    reasoning_effort: object,
+    drop_params: bool,
+) -> None:
+    import litellm
+
+    with caplog.at_level(logging.WARNING, logger=verbose_logger.name):
+        optional_params: Final = litellm.get_optional_params(
+            model=model,
+            custom_llm_provider="mistral",
+            reasoning_effort=reasoning_effort,
+            drop_params=drop_params,
+        )
+
+    assert "reasoning_effort" not in optional_params
+    assert _warnings_naming(caplog, "reasoning_effort") == ()
+
+
 @pytest.mark.parametrize(
     ("model", "custom_llm_provider"),
     [