Checked other resources

• This is a bug, not a usage question.
• I added a clear and descriptive title that summarizes this issue.
• I used the GitHub search to find a similar question and didn't find it.
• I am sure that this is a bug in LangChain rather than my code.
• The bug is not resolved by updating to the latest stable version of LangChain (or the specific integration package).
• This is not related to the langchain-community package.
• I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS.

Package (Required)

• 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)

none

Error Message and Stack Trace (if applicable)

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",