Middleware

Middleware runs before page rendering and can short-circuit requests with a custom Response — useful for auth, geolocation, A/B testing, redirects, and bot detection.

File convention

Create middleware.ts at the project root (or in src/):

// middleware.ts export function middleware(request: Request) { // Return a Response to short-circuit (redirect / rewrite / custom response). // Return undefined / null to continue to the page handler. const url = new URL(request.url); if (url.pathname.startsWith("/admin") && !isAuthed(request)) { return Response.redirect(new URL("/login", request.url)); } }

Return value	Behavior
Response	Short-circuits — the response is served instead of the page
undefined / null	Continues to the page handler

How it's wired

• middleware.ts is compiled to .krate/middleware.js during the build.
• It's executed via the Node.js sidecar before page rendering.
• Combined with redirects & rewrites, middleware gives you request-level control over routing.

Use cases

• Auth — redirect unauthenticated users.
• Bot detection — serve a custom response to known bots.
• A/B testing — rewrite requests to different variants.
• Header injection — set security headers per route.