Pages Functions route by file path, and a _middleware file covers its whole directory

In a Pages project, files under the functions directory map to URL paths, so a file at functions/api/users.ts serves the /api/users path. Square brackets around a filename make a route parameter and doubled brackets make a catch-all. Each file exports onRequest, or a method-specific variant such as onRequestGet, receiving a context object with request, env, params, next and waitUntil.

A _middleware.ts file runs for every request handled under its directory, including nested ones, and must call the context next function to continue. Middleware placed at the functions root sees every request routed to the Functions worker, which by default includes static asset requests, and calling next falls through to the asset. Sibling middleware runs outermost first, following directory depth.