Next.js Core (App Router) Server Components by default; minimal "use client". Mutations in Server Actions (validate/authz; revalidate tags/paths). Route handlers for APIs/webhooks; add loading/error boundaries.
Anti-patterns:
❌ Fetch initial data in useEffect. ❌ Cache or revalidate too broadly. ❌ Client-only authz.
References: see references/ (server actions, fetching, caching, routing, auth, testing).