Some models' ToolMessage content fields do not support arrays.

### Checked other resources - [x] This is a bug, not a usage question. - [x] I added a clear and descriptive title that summarizes this issue. - [x] I used the GitHub search to find a similar question and didn't find it. - [x] I am sure that this is a bug in LangChain rather than my code. - [x] The bug is not resolved by updating to the latest stable version of LangChain (or the specific integration package). - [x] This is not related to the langchain-community package. - [x] I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS. ### Package (Required) - [x] langchain - [ ] langchain-openai - [ ] langchain-anthropic - [ ] langchain-classic - [ ] langchain-core - [ ] langchain-cli - [ ] langchain-model-profiles - [ ] langchain-tests - [ ] langchain-text-splitters - [ ] langchain-chroma - [ ] langchain-deepseek - [ ] langchain-exa - [ ] langchain-fireworks - [ ] langchain-groq - [ ] langchain-huggingface - [ ] langchain-mistralai - [ ] langchain-nomic - [ ] langchain-ollama - [ ] langchain-perplexity - [ ] langchain-prompty - [ ] langchain-qdrant - [ ] langchain-xai - [ ] Other / not sure / general ### Example Code (Python) ```python none ``` ### Error Message and Stack Trace (if applicable) ```shell BadRequestError: Error code: 400 - {'error': {'message': 'Failed to deserialize the JSON body into the target type: messages[7]: invalid type: sequence, expected a string at line 1 column 8935', 'type': 'invalid_request_error', 'param': '', 'code': 'invalid_request_error'}} ``` ### Description The issue occurs when the `ls` tool is invoked, returning an empty array, followed by an `llm` call to `chat/completion`. However, `llm` does not support `content` as an array in `AIMessage`. ### System Info "deepagents>=0.2.6", "fastapi[standard]>=0.121.1", "langchain-deepseek>=1.0.1", "langchain-mcp-adapters>=0.1.12", "langgraph-cli[inmem]>=0.4.7",