fix(openai): allow temperature parameter for gpt-5-chat models (#32624)

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

@@ -716,7 +716,8 @@ class BaseChatOpenAI(BaseChatModel):
         """Validate temperature parameter for different models.
 
         - o1 models only allow temperature=1
-        - gpt-5 models only allow temperature=1 or unset (defaults to 1)
+        - gpt-5 models (excluding gpt-5-chat) only allow temperature=1 or unset
+          (defaults to 1)
         """
         model = values.get("model_name") or values.get("model") or ""
 
@@ -725,10 +726,12 @@ class BaseChatOpenAI(BaseChatModel):
             values["temperature"] = 1
 
         # For gpt-5 models, handle temperature restrictions
-        if model.startswith("gpt-5"):
+        # Note that gpt-5-chat models do support temperature
+        if model.startswith("gpt-5") and "chat" not in model:
             temperature = values.get("temperature")
             if temperature is not None and temperature != 1:
-                # For gpt-5, only temperature=1 is supported, so remove non-defaults
+                # For gpt-5 (non-chat), only temperature=1 is supported
+                # So we remove any non-defaults
                 values.pop("temperature", None)
 
         return values
@@ -3520,7 +3523,7 @@ def _construct_responses_api_payload(
 
     # Remove temperature parameter for models that don't support it in responses API
     model = payload.get("model", "")
-    if model.startswith("gpt-5"):
+    if model.startswith("gpt-5") and "chat" not in model:  # gpt-5-chat supports
         payload.pop("temperature", None)
 
     payload["input"] = _construct_responses_api_input(messages)

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

@@ -2678,3 +2678,21 @@ def test_extra_body_with_model_kwargs() -> None:
     assert payload["extra_body"]["ttl"] == 600
     assert payload["custom_non_openai_param"] == "test_value"
     assert payload["temperature"] == 0.5
+
+
+@pytest.mark.parametrize("use_responses_api", [False, True])
+def test_gpt_5_temperature(use_responses_api: bool) -> None:
+    llm = ChatOpenAI(
+        model="gpt-5-nano", temperature=0.5, use_responses_api=use_responses_api
+    )
+
+    messages = [HumanMessage(content="Hello")]
+    payload = llm._get_request_payload(messages)
+    assert "temperature" not in payload  # not supported for gpt-5 family models
+
+    llm = ChatOpenAI(
+        model="gpt-5-chat", temperature=0.5, use_responses_api=use_responses_api
+    )
+    messages = [HumanMessage(content="Hello")]
+    payload = llm._get_request_payload(messages)
+    assert payload["temperature"] == 0.5  # gpt-5-chat is exception