Migrate Next.js: Pages → App Router

The Pages → App migration looks daunting but Next.js supports both routers in the same project. Migrate incrementally, route by route. This skill is the order and the gotchas.

When to use

• Existing Pages-router app on Next.js 13+ (App router available since 13.4 stable).
• Want Server Components (RSC), nested layouts, streaming, parallel routes.
• Refactoring opportunity coincides with router migration.

When NOT to use

• Greenfield app — start with App router directly.
• App on Next < 13 — upgrade Next first.
• Heavy getStaticPaths+ISR coverage where Pages router is already optimal — App router's equivalent is fine but the migration win is small.

The principle: coexist, don't cut over

Both routers can run in the same app. pages/ and app/ directories are read together. Migrate route-by-route. Never do a big-bang cutover for a non-trivial app.