fix(responses): apply the chat o-series temperature rule to openai responses

litellm/llms/openai/responses/transformation.py

@@ -519,6 +519,22 @@ class OpenAIResponsesAPIConfig(BaseResponsesAPIConfig):
                     status_code=400,
                 )
 
+        o_series_temperature: Final = params.get("temperature")
+        if (
+            self.custom_llm_provider == LlmProviders.OPENAI
+            and o_series_temperature not in (None, 1)
+            and litellm.openaiOSeriesConfig.is_model_o_series_model(model=model)
+        ):
+            if not (drop_params or litellm.drop_params):
+                raise litellm.UnsupportedParamsError(
+                    message=(
+                        f"O-series models don't support temperature={o_series_temperature}. Only temperature=1 is "
+                        "supported. To drop unsupported params set `litellm.drop_params = True`"
+                    ),
+                    status_code=400,
+                )
+            params.pop("temperature", None)
+
         lookup_name: Final = self._model_map_lookup_name(model)
         if self._is_gpt_5_model(model=lookup_name):
             reasoning: Final = params.get("reasoning") or {}

tests/unit/llms/openai/responses/test_openai_responses_transformation.py

@@ -2383,6 +2383,31 @@ class TestPromptCacheOptionsOnResponsesPath:
         }
 
 
+@pytest.mark.parametrize("model", ["o1", "o3", "o3-mini", "o4-mini"])
+@pytest.mark.parametrize("temperature", [0, 0.5, 1])
+def test_o_series_temperature_is_dropped_like_chat_completions(model, temperature):
+    chat = litellm.get_optional_params(
+        model=model, custom_llm_provider="openai", temperature=temperature, drop_params=True
+    )
+    mapped = OpenAIResponsesAPIConfig().map_openai_params(
+        response_api_optional_params={"temperature": temperature},
+        model=model,
+        drop_params=True,
+    )
+    assert mapped.get("temperature") == chat.get("temperature")
+
+
+def test_o_series_temperature_raises_like_chat_completions_without_drop_params():
+    with pytest.raises(litellm.UnsupportedParamsError):
+        litellm.get_optional_params(model="o3-mini", custom_llm_provider="openai", temperature=0.5)
+    with pytest.raises(litellm.UnsupportedParamsError):
+        OpenAIResponsesAPIConfig().map_openai_params(
+            response_api_optional_params={"temperature": 0.5},
+            model="o3-mini",
+            drop_params=False,
+        )
+
+
 class TestResponsesSurfaceSharesTheEffortRule:
     """The Responses API reaches the same gpt-5 models over a different wire, and the default
     /v1/messages bridge for openai models routes through it. It carried its own copy of the