routing-middleware

name	routing-middleware
description	Vercel Routing Middleware guidance — request interception before cache, rewrites, redirects, personalization. Works with any framework. Supports Edge, Node.js, and Bun runtimes. Use when intercepting requests at the platform level.
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Vercel Routing Middleware

You are an expert in Vercel Routing Middleware — the platform-level request interception layer.

What It Is

Routing Middleware runs before the cache on every request matching its config. It is a Vercel platform feature (not framework-specific) that works with Next.js, SvelteKit, Astro, Nuxt, or any deployed framework. Built on Fluid Compute.

• File: middleware.ts or middleware.js at the project root
• Default export required (function name can be anything)
• Runtimes: Edge (default), Node.js (runtime: 'nodejs'), Bun (Node.js + bunVersion in vercel.json)

CRITICAL: Middleware Disambiguation

There are THREE "middleware" concepts in the Vercel ecosystem:

Concept	File	Runtime	Scope	When to Use
Vercel Routing Middleware	middleware.ts (root)	Edge/Node/Bun	Any framework, platform-level	Request interception before cache: rewrites, redirects, geo, A/B
Next.js 16 Proxy	proxy.ts (root, or src/proxy.ts if using --src-dir)	Node.js only	Next.js 16+ only	Network-boundary proxy needing full Node APIs. NOT for auth.
Edge Functions	Any function file	V8 isolates	General-purpose	Standalone edge compute endpoints, not an interception layer

Why the rename in Next.js 16: middleware.ts → proxy.ts clarifies it sits at the network boundary (not general-purpose middleware). Partly motivated by CVE-2025-29927 (middleware auth bypass via x-middleware-subrequest header). The exported function must also be renamed from middleware to proxy. Migration codemod: npx @next/codemod@latest middleware-to-proxy

Deprecation: Next.js 16 still accepts middleware.ts but treats it as deprecated and logs a warning. It will be removed in a future version.