How to add custom middleware

When deploying agents on the LangGraph platform, you can add custom middleware to your server to handle cross-cutting concerns like logging request metrics, injecting or checking headers, and enforcing security policies without modifying core server logic. This works the same way as adding custom routes - you just need to provide your own Hono app.

Adding middleware lets you intercept and modify requests globally across your deployment, whether they're hitting your custom endpoints or the built-in LangGraph Platform APIs.

:::warning[Requests only for built-in LangGraph Platform APIs]

Currently only intercepting and modifying requests are supported at the moment.

You can still add custom headers to responses, but modifying response headers or response body of a built-in LangGraph Platform endpoint is not yet supported.

Create app

Starting from an existing LangGraph Platform application, add the following middleware code to your app.ts file. If you are starting from scratch, you can create a new app from a template using the CLI.

npm create langgraph

Make sure to install hono as a dependency.

npm install hono

Once you have a LangGraph project, add the following app code:

import { Hono } from "hono";

export const app = new Hono();

app.use(async (c, next) => {
  c.header("X-Custom-Header", "Hello World");
  await next();
});

Configure langgraph.json

Add the following to your langgraph.json file. Make sure the path points to the app.py file you created above.

{
  "graphs": {
    "agent": "./src/agent/graph.ts:graph"
  },
  "env": ".env",
  "http": {
    "app": "./src/agent/app.ts:app"
  }
  // Other configuration options like auth, store, etc.
}

Start server

Test the server out locally:

npx langgraph-cli@latest dev --no-browser

Now any request to your server will include the custom header X-Custom-Header in its response.

Deploying

You can deploy this app as-is to the managed LangGraph Cloud or to your self-hosted platform.

Next steps

Now that you've added custom middleware to your deployment, you can use similar techniques to add custom routes.