fix(xai): Fix 400 error for AIMessages with tool_calls but no content

libs/partners/xai/langchain_xai/chat_models.py

@@ -5,9 +5,15 @@
 from typing import TYPE_CHECKING, Any, Literal, TypeAlias, cast
 
 import openai
-from langchain_core.messages import AIMessageChunk
+from langchain_core.messages import AIMessage, AIMessageChunk, BaseMessage
 from langchain_core.utils import secret_from_env
-from langchain_openai.chat_models.base import BaseChatOpenAI
+from langchain_openai.chat_models.base import (
+    BaseChatOpenAI,
+    _convert_from_v1_to_chat_completions,
+)
+from langchain_openai.chat_models.base import (
+    _convert_message_to_dict as _openai_convert_message_to_dict,
+)
 from pydantic import BaseModel, ConfigDict, Field, SecretStr, model_validator
 from typing_extensions import Self
 
@@ -37,6 +43,36 @@ def _get_default_model_profile(model_name: str) -> ModelProfile:
     return default.copy()
 
 
+def _convert_message_to_dict_xai(
+    message: BaseMessage,
+    api: Literal["chat/completions", "responses"] = "chat/completions",
+) -> dict:
+    """Convert a LangChain message to dictionary format expected by xAI.
+
+    xAI's API requires that all messages have at least an empty content field,
+    unlike OpenAI which allows None for messages with tool_calls.
+
+    Args:
+        message: The LangChain message to convert.
+        api: The API format to use. Defaults to "chat/completions".
+
+    Returns:
+        Dictionary representation of the message compatible with xAI's API.
+    """
+    message_dict = _openai_convert_message_to_dict(message, api=api)
+
+    # xAI requires content to be at least an empty string, not None
+    # This is especially important for AIMessages with tool_calls but no content
+    if (
+        isinstance(message, AIMessage)
+        and message_dict.get("content") is None
+        and ("tool_calls" in message_dict or "function_call" in message_dict)
+    ):
+        message_dict["content"] = ""
+
+    return message_dict
+
+
 class ChatXAI(BaseChatOpenAI):  # type: ignore[override]
     r"""ChatXAI chat model.
 
@@ -547,6 +583,42 @@ def _default_params(self) -> dict[str, Any]:
 
         return params
 
+    def _get_request_payload(
+        self,
+        input_: LanguageModelInput,
+        *,
+        stop: list[str] | None = None,
+        **kwargs: Any,  # noqa: ANN401
+    ) -> dict:
+        """Prepare the request payload for xAI's API.
+
+        Overrides the base implementation to use xAI-specific message conversion
+        that ensures all messages have at least an empty content field.
+
+        Args:
+            input_: The input to convert into messages.
+            stop: List of stop sequences. Defaults to None.
+            **kwargs: Additional keyword arguments to pass to the API.
+
+        Returns:
+            Dictionary containing the request payload for xAI's API.
+        """
+        messages = self._convert_input(input_).to_messages()
+        if stop is not None:
+            kwargs["stop"] = stop
+
+        payload = {**self._default_params, **kwargs}
+
+        # Use xAI-specific message converter for all message conversion
+        # xAI requires all messages to have at least empty content field
+        payload["messages"] = [
+            _convert_message_to_dict_xai(_convert_from_v1_to_chat_completions(m))
+            if isinstance(m, AIMessage)
+            else _convert_message_to_dict_xai(m)
+            for m in messages
+        ]
+        return payload
+
     def _create_chat_result(
         self,
         response: dict | openai.BaseModel,