fix(openai): Allow temperature when reasoning is set to the string 'none' (#34298)

Co-authored-by: Chester Curme <chester.curme@gmail.com>

libs/partners/openai/langchain_openai/chat_models/base.py

@@ -846,9 +846,15 @@ class BaseChatOpenAI(BaseChatModel):
         if model_lower.startswith("o1") and "temperature" not in values:
             values["temperature"] = 1
 
-        # For gpt-5 models, handle temperature restrictions
-        # Note that gpt-5-chat models do support temperature
-        if model_lower.startswith("gpt-5") and "chat" not in model_lower:
+        # For gpt-5 models, handle temperature restrictions. Temperature is supported
+        # by gpt-5-chat and gpt-5 models with reasoning_effort='none' or
+        # reasoning={'effort': 'none'}.
+        if (
+            model_lower.startswith("gpt-5")
+            and ("chat" not in model_lower)
+            and values.get("reasoning_effort") != "none"
+            and (values.get("reasoning") or {}).get("effort") != "none"
+        ):
             temperature = values.get("temperature")
             if temperature is not None and temperature != 1:
                 # For gpt-5 (non-chat), only temperature=1 is supported
@@ -3745,8 +3751,14 @@ def _construct_responses_api_payload(
         payload["reasoning"] = {"effort": payload.pop("reasoning_effort")}
 
     # Remove temperature parameter for models that don't support it in responses API
+    # gpt-5-chat supports temperature, and gpt-5 models with reasoning.effort='none'
+    # also support temperature
     model = payload.get("model") or ""
-    if model.startswith("gpt-5") and "chat" not in model:  # gpt-5-chat supports
+    if (
+        model.startswith("gpt-5")
+        and ("chat" not in model)  # gpt-5-chat supports
+        and (payload.get("reasoning") or {}).get("effort") != "none"
+    ):
         payload.pop("temperature", None)
 
     payload["input"] = _construct_responses_api_input(messages)

libs/partners/openai/tests/unit_tests/chat_models/test_base.py

@@ -3056,3 +3056,63 @@ def test_gpt_5_temperature_case_insensitive(
         messages = [HumanMessage(content="Hello")]
         payload = llm._get_request_payload(messages)
         assert payload["temperature"] == 0.7
+
+
+@pytest.mark.parametrize("use_responses_api", [False, True])
+def test_gpt_5_1_temperature_with_reasoning_effort_none(
+    use_responses_api: bool,
+) -> None:
+    """Test that temperature is preserved when reasoning_effort is explicitly 'none'."""
+    # Test with reasoning_effort='none' explicitly set
+    llm = ChatOpenAI(
+        model="gpt-5.1",
+        temperature=0.5,
+        reasoning_effort="none",
+        use_responses_api=use_responses_api,
+    )
+    messages = [HumanMessage(content="Hello")]
+    payload = llm._get_request_payload(messages)
+    assert payload["temperature"] == 0.5
+
+    # Test with reasoning={'effort': 'none'}
+    llm = ChatOpenAI(
+        model="gpt-5.1",
+        temperature=0.5,
+        reasoning={"effort": "none"},
+        use_responses_api=use_responses_api,
+    )
+    messages = [HumanMessage(content="Hello")]
+    payload = llm._get_request_payload(messages)
+    assert payload["temperature"] == 0.5
+
+    # Test that temperature is restricted by default (no reasoning_effort)
+    llm = ChatOpenAI(
+        model="gpt-5.1",
+        temperature=0.5,
+        use_responses_api=use_responses_api,
+    )
+    messages = [HumanMessage(content="Hello")]
+    payload = llm._get_request_payload(messages)
+    assert "temperature" not in payload
+
+    # Test that temperature is still restricted when reasoning_effort is something else
+    llm = ChatOpenAI(
+        model="gpt-5.1",
+        temperature=0.5,
+        reasoning_effort="low",
+        use_responses_api=use_responses_api,
+    )
+    messages = [HumanMessage(content="Hello")]
+    payload = llm._get_request_payload(messages)
+    assert "temperature" not in payload
+
+    # Test with reasoning={'effort': 'low'}
+    llm = ChatOpenAI(
+        model="gpt-5.1",
+        temperature=0.5,
+        reasoning={"effort": "low"},
+        use_responses_api=use_responses_api,
+    )
+    messages = [HumanMessage(content="Hello")]
+    payload = llm._get_request_payload(messages)
+    assert "temperature" not in payload