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Middleware provides a way to more tightly control what happens inside the agent. Middleware is useful for the following: Add middleware by passing them to createAgent: 
import {
  createAgent,
  summarizationMiddleware,
  humanInTheLoopMiddleware,
} from "langchain";

const agent = createAgent({
  model: "gpt-4o",
  tools: [...],
  middleware: [summarizationMiddleware, humanInTheLoopMiddleware],
});

​
The agent loop

The core agent loop involves calling a model, letting it choose tools to execute, and then finishing when it calls no more tools: Core agent loop diagram Middleware exposes hooks before and after each of those steps: Middleware flow diagram

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Additional resources

Built-in middleware

Explore built-in middleware for common use cases.

Custom middleware

Build your own middleware with hooks and decorators.

Middleware API reference

Complete API reference for middleware.

Testing agents

Test your agents with LangSmith.
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