Overview

Middleware provides a way to more tightly control what happens inside the agent. Middleware is useful for the following:

Tracking agent behavior with logging, analytics, and debugging.
Transforming prompts, tool selection, and output formatting.
Adding retries, fallbacks, and early termination logic.
Applying rate limits, guardrails, and PII detection.

Add middleware by passing them to createAgent:

import {
  createAgent,
  summarizationMiddleware,
  humanInTheLoopMiddleware,
} from "langchain";

const agent = createAgent({
  model: "gpt-5.5",
  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:

Middleware exposes hooks before and after each of those steps:

Use middleware inside a LangGraph workflow

Middleware is not a separate runtime: hooks run inside the compiled LangGraph that create_agent returns. You can drop the whole agent (middleware and all) into a larger StateGraph as a node or subgraph, and every middleware hook continues to run. Reach for this pattern when the surrounding topology is more than a standard “loop until done”: classifying input before routing to one of several agents, fanning out work in parallel, or stitching agent calls together with deterministic steps. HumanInTheLoopMiddleware matches against each tool’s .name. The key matches the name you pass to tool({...}, { name }).

import { AgentState, createAgent, humanInTheLoopMiddleware } from "langchain";
import { StateGraph, START } from "@langchain/langgraph";

// Assumes readEmail, sendEmail, classifyNode, and route are defined elsewhere.
// readEmail / sendEmail are registered with name: "read_email" / "send_email".
const emailAgent = createAgent({
  model: "claude-sonnet-4-6",
  tools: [readEmail, sendEmail],
  middleware: [humanInTheLoopMiddleware({ interruptOn: { send_email: true } })],
});

const graph = new StateGraph(AgentState)
  .addNode("classify", classifyNode)
  .addNode("emailAgent", emailAgent)
  .addEdge(START, "classify")
  .addConditionalEdges("classify", route)
  .compile();

The HITL interrupt, summarization, PII redaction, retries, and any custom hooks all travel with the agent node. See Use subgraphs for the full set of composition patterns, including subgraph checkpointer scoping (per-invocation versus per-thread).

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.

Middleware integrations

Provider-specific middleware for Anthropic, AWS, OpenAI, and more.

Testing agents

Test your agents with LangSmith.

Connect these docs to Claude, VSCode, and more via MCP for real-time answers.

Edit this page on GitHub or file an issue.

Get started

Core components

Middleware

Frontend

Advanced usage

Agent development

Production

The agent loop

Use middleware inside a LangGraph workflow

Additional resources

Built-in middleware

Custom middleware

Middleware API reference

Middleware integrations

Testing agents

​The agent loop

​Use middleware inside a LangGraph workflow

​Additional resources

Built-in middleware

Custom middleware

Middleware API reference

Middleware integrations

Testing agents

The agent loop

Use middleware inside a LangGraph workflow

Additional resources