fix(gpt-5): enforce supports_none_reasoning_effort for reasoning_effort=none

Adds a missing capability gate so that reasoning_effort="none" is dropped or refused when the model map marks supports_none_reasoning_effort=false (e.g. gpt-5, gpt-5-mini) instead of being forwarded to the provider and surfacing as a 400.

litellm/llms/openai/chat/gpt_5_transformation.py

@@ -300,6 +300,19 @@ class OpenAIGPT5Config(OpenAIGPTConfig):
                         message=(f"reasoning_effort={effective_effort} is not supported for this model."),
                         status_code=400,
                     )
+        elif effective_effort == "none":
+            # "none" is a declared capability: if the model map says the model does not
+            # support it, drop or refuse rather than forwarding it to the provider.
+            # Example: gpt-5 and gpt-5-mini are marked supports_none_reasoning_effort=false.
+            if not self._supports_reasoning_effort_level(model, effective_effort):
+                if litellm.drop_params or drop_params:
+                    non_default_params.pop("reasoning_effort", None)
+                    optional_params.pop("reasoning_effort", None)
+                else:
+                    raise litellm.utils.UnsupportedParamsError(
+                        message=(f"reasoning_effort={effective_effort} is not supported for this model."),
+                        status_code=400,
+                    )
 
         ################################################################
         # max_tokens is not supported for gpt-5 models on OpenAI API

tests/test_litellm/llms/openai/test_gpt5_reasoning_effort_none.py

@@ -0,0 +1,61 @@
+import pytest
+
+import litellm
+from litellm.litellm_core_utils.get_model_cost_map import get_model_cost_map
+from litellm.llms.openai.openai import OpenAIConfig
+
+
+@pytest.fixture()
+def config() -> OpenAIConfig:
+    return OpenAIConfig()
+
+
+@pytest.fixture(autouse=True)
+def use_local_model_cost_map(monkeypatch: pytest.MonkeyPatch):
+    monkeypatch.setenv("LITELLM_LOCAL_MODEL_COST_MAP", "True")
+    monkeypatch.setattr(litellm, "model_cost", get_model_cost_map(url=litellm.model_cost_map_url))
+    litellm.add_known_models(model_cost_map=litellm.model_cost)
+
+
+def test_gpt5_rejects_reasoning_effort_none_for_unsupported_models(config: OpenAIConfig):
+    """Models marked supports_none_reasoning_effort=false must not forward reasoning_effort=none."""
+    for model in ("gpt-5", "gpt-5-mini"):
+        with pytest.raises(litellm.utils.UnsupportedParamsError):
+            config.map_openai_params(
+                non_default_params={"reasoning_effort": "none"},
+                optional_params={},
+                model=model,
+                drop_params=False,
+            )
+
+        # Dict form {"effort": "none"} should be gated the same way.
+        with pytest.raises(litellm.utils.UnsupportedParamsError):
+            config.map_openai_params(
+                non_default_params={"reasoning_effort": {"effort": "none", "summary": "detailed"}},
+                optional_params={},
+                model=model,
+                drop_params=False,
+            )
+
+
+def test_gpt5_drops_reasoning_effort_none_when_requested(config: OpenAIConfig):
+    """drop_params=True should strip unsupported reasoning_effort=none instead of raising."""
+    for model in ("gpt-5", "gpt-5-mini"):
+        params = config.map_openai_params(
+            non_default_params={"reasoning_effort": "none"},
+            optional_params={},
+            model=model,
+            drop_params=True,
+        )
+        assert "reasoning_effort" not in params
+
+
+def test_gpt5_1_passes_through_reasoning_effort_none(config: OpenAIConfig):
+    """Models marked supports_none_reasoning_effort=true still forward reasoning_effort=none."""
+    params = config.map_openai_params(
+        non_default_params={"reasoning_effort": "none"},
+        optional_params={},
+        model="gpt-5.1",
+        drop_params=False,
+    )
+    assert params["reasoning_effort"] == "none"