nextjslisted

# Next.js — Skills Reference > **Version-sensitive.** Next.js moves fast. Caching semantics, middleware conventions, and > rendering defaults differ materially between v14, v15, and v16. Every claim here is anchored > to the official docs at version 16.2.7 (May 2026). When working on a project, check the > installed version (`package.json`) and consult the matching docs. Study resources and version > landmarks live in [references/study-resources.md](references/study-resources.md). ## Overview A strong Next.js engineer reasons about **three orthogonal axes** simultaneously: (1) where code runs (server vs client), (2) when it runs (build/prerender vs request time), and (3) how long the result is cached. Most bugs and performance problems come from getting one of these wrong — shipping secrets to the client, blocking renders on slow fetches, or serving stale data because a cache was not invalidated after a mutation. This playbook covers the App Router model (the `app/` directory). The Pages Router (`pages/`) is a legacy path — do not build new features there. > **Load this skill when…** building or reviewing a Next.js App Router application; debugging caching, PPR, or streaming behaviour; auditing Server Actions for auth/authz or CSRF exposure; reviewing Proxy (middleware) configuration. > **Not this skill:** React fundamentals (hooks, state, RTL testing) → see `react`; Node.js runtime and event-loop concerns → see `nodejs`; TypeScript compiler configuration → see `typescri