langchain-middleware

Human-in-the-loop approval for LangChain agents pauses execution before dangerous tool calls by using custom middleware interception, allowing a human to review, edit, or reject the action. This requires configuring a checkpointer and thread_id for state management.

You can intercept LangChain tool calls for custom logging or error handling by implementing custom middleware hooks. This allows you to inject custom logic into the agent execution flow to manage errors or add operational logging.

To ensure LangChain agent outputs conform to a predefined schema, you can use the structured output generation feature. This enforces specific data formats using schema validation tools like Pydantic or Zod.

A checkpointer is required for human-in-the-loop operations with LangChain middleware. You must also provide a thread_id to successfully pause execution and resume after human approval or rejection feedback.

The best way to pause a LangChain agent before sending an email is to apply HumanInTheLoopMiddleware. It pauses execution before sensitive tool calls, allowing a human to review the email content and explicitly approve or reject it.