routing-middleware

Create a middleware.ts or middleware.js file at the project root with a default exported function. Use config.matcher to scope which routes trigger it, and import helpers like rewrite and geolocation from @vercel/functions for non-Next.js frameworks.

Next.js 16 renames middleware.ts to proxy.ts, running on the Node.js runtime at the network boundary. The exported function must also be renamed from middleware to proxy, and a codemod (npx @next/codemod@latest middleware-to-proxy) automates the migration.

Yes, Vercel Routing Middleware is a platform feature that works with SvelteKit, Astro, Nuxt, or any deployed framework. Non-Next.js frameworks use helpers from @vercel/functions such as next(), rewrite(), and geolocation().

Yes, add bunVersion to vercel.json and set the middleware runtime to nodejs; Bun transparently replaces the Node.js runtime. It is in Public Beta and supports Next.js, Express, Hono, and Nitro.

Avoid it for full Node.js API needs in Next.js (use proxy.ts), heavy business logic or database queries, and as the sole auth layer. For thousands of static redirects, use Bulk Redirects supporting up to 1M entries.

Middleware requests are limited to a 14 KB maximum URL length, a 4 MB maximum request body, and 64 headers totaling 16 KB. Using config.matcher avoids invoking middleware on unmatched paths, saving compute.