openai[patch]: use max_completion_tokens in place of max_tokens (#26917)

`max_tokens` is deprecated:
https://platform.openai.com/docs/api-reference/chat/create#chat-create-max_tokens

---------

Co-authored-by: Bagatur <baskaryan@gmail.com>

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

@@ -435,7 +435,7 @@ class BaseChatOpenAI(BaseChatModel):
     """Number of chat completions to generate for each prompt."""
     top_p: Optional[float] = None
     """Total probability mass of tokens to consider at each step."""
-    max_tokens: Optional[int] = None
+    max_tokens: Optional[int] = Field(default=None)
     """Maximum number of tokens to generate."""
     tiktoken_model_name: Optional[str] = None
     """The model name to pass to tiktoken when using this class. 
@@ -699,6 +699,7 @@ class BaseChatOpenAI(BaseChatModel):
         messages = self._convert_input(input_).to_messages()
         if stop is not None:
             kwargs["stop"] = stop
+
         return {
             "messages": [_convert_message_to_dict(m) for m in messages],
             **self._default_params,
@@ -853,7 +854,9 @@ class BaseChatOpenAI(BaseChatModel):
             ls_model_type="chat",
             ls_temperature=params.get("temperature", self.temperature),
         )
-        if ls_max_tokens := params.get("max_tokens", self.max_tokens):
+        if ls_max_tokens := params.get("max_tokens", self.max_tokens) or params.get(
+            "max_completion_tokens", self.max_tokens
+        ):
             ls_params["ls_max_tokens"] = ls_max_tokens
         if ls_stop := stop or params.get("stop", None):
             ls_params["ls_stop"] = ls_stop
@@ -1501,7 +1504,7 @@ class BaseChatOpenAI(BaseChatModel):
         return filtered
 
 
-class ChatOpenAI(BaseChatOpenAI):
+class ChatOpenAI(BaseChatOpenAI):  # type: ignore[override]
     """OpenAI chat model integration.
 
     .. dropdown:: Setup
@@ -1963,6 +1966,9 @@ class ChatOpenAI(BaseChatOpenAI):
     message chunks will be generated during the stream including usage metadata.
     """
 
+    max_tokens: Optional[int] = Field(default=None, alias="max_completion_tokens")
+    """Maximum number of tokens to generate."""
+
     @property
     def lc_secrets(self) -> Dict[str, str]:
         return {"openai_api_key": "OPENAI_API_KEY"}
@@ -1992,6 +1998,29 @@ class ChatOpenAI(BaseChatOpenAI):
         """Return whether this model can be serialized by Langchain."""
         return True
 
+    @property
+    def _default_params(self) -> Dict[str, Any]:
+        """Get the default parameters for calling OpenAI API."""
+        params = super()._default_params
+        if "max_tokens" in params:
+            params["max_completion_tokens"] = params.pop("max_tokens")
+
+        return params
+
+    def _get_request_payload(
+        self,
+        input_: LanguageModelInput,
+        *,
+        stop: Optional[List[str]] = None,
+        **kwargs: Any,
+    ) -> dict:
+        payload = super()._get_request_payload(input_, stop=stop, **kwargs)
+        # max_tokens was deprecated in favor of max_completion_tokens
+        # in September 2024 release
+        if "max_tokens" in payload:
+            payload["max_completion_tokens"] = payload.pop("max_tokens")
+        return payload
+
     def _should_stream_usage(
         self, stream_usage: Optional[bool] = None, **kwargs: Any
     ) -> bool: