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Next.js App Router route handlers: CORS in Route Handlers, Edge versus Node runtime, and middleware layering
Export `OPTIONS` alongside `GET` and `POST` in `route.ts` files—browsers preflight before hitting your handler logic.
1 min read
nextjsapp-routercors
Caching
`export const dynamic` settings affect whether CORS headers appear on cached shell responses.
ISR revalidation should not strip Vary: Origin from JSON APIs behind the same hostname as the SPA.
Streaming
ReadableStream responses must emit CORS headers before the first chunk for credentialed fetches.
After() API from Next can extend background work—do not mutate headers post-commit.