ChatXAI: 400 error when AIMessage has tool_calls but no content

## Description When using `ChatXAI` with LangGraph agents, xAI's API returns a 400 error if an `AIMessage` contains `tool_calls` but has empty/null `content`: ``` openai.BadRequestError: Error code: 400 - {'code': 'Client specified an invalid argument', 'error': 'Invalid request content: Each message must have at least one content element.'} ``` ## Environment - `langchain-xai`: 1.1.0 - `langchain-core`: 1.1.0 - Model: `grok-4-1-fast-reasoning` - Using LangGraph with tool-calling agents ## Reproduction This occurs when using ChatXAI in a LangGraph agent that makes tool calls. After several tool call rounds, the message history contains AIMessages with only `tool_calls` and no `content` field. xAI's API is stricter than OpenAI's and rejects these messages. ## Workaround Subclass ChatXAI and ensure content is always set: ```python from langchain_xai import ChatXAI from langchain_core.messages import AIMessage class ChatXAISanitized(ChatXAI): def _convert_messages_to_dicts(self, messages): for msg in messages: if isinstance(msg, AIMessage) and not msg.content and msg.tool_calls: msg.content = "" # xAI requires content, even if empty return super()._convert_messages_to_dicts(messages) ``` ## Suggested Fix `ChatXAI._convert_messages_to_dicts` (or the underlying message conversion) should ensure all messages have at least an empty content field to satisfy xAI's API requirements.