How to Use Next.js Middleware for Auth, A/B Testing, and Locale in 2026

Next.js middleware, defined in a middleware.ts file at the project root, intercepts every matched request before caching, rendering, or route resolution, making it ideal for auth redirects, A/B test bucketing, and locale detection. As of 2026, middleware on Vercel runs on Fluid Compute using standard Node.js, shifting the primary concern from API limitations to latency, with a recommended target of under 10 milliseconds per execution. For authentication, the recommended approach is verifying a self-contained JWT locally using a library like jose, without making any database calls, and redirecting unauthenticated users to a login page. A/B testing can be implemented by checking for an existing bucket cookie and, if absent, randomly assigning a variant, then using a URL rewrite so the original address remains visible in the browser. Developers are advised to use the matcher configuration to limit middleware execution to only relevant routes, since omitting it causes middleware to run on every request including static assets.