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)