What does Middleware do in Next.js? And what is Proxy?

It's logic that runs before a request is fully handled: it can redirect, rewrite the URL or change headers.

Watch the version: since Next.js 16, Middleware has been renamed to Proxy; you'll see proxy.ts instead of middleware.ts.

Example use: a redirect based on the address, or an early check that a session exists.

Pitfall: hiding a page or redirecting doesn't replace authorisation when data is read and written. Proxy isn't the place for slow data fetching or all of your session handling.