nextjslisted

# Next.js House conventions for Next.js App Router (14+). Apply them to code you are writing or changing — don't migrate untouched routes unless asked. ## Conventions - **App Router only.** Never add to `pages/`. If a project still has one, new work goes in `app/` and the two coexist until someone schedules the migration. - **Server Components by default.** `'use client'` marks a leaf that needs interactivity, not a page. Everything above that marker ships to the browser too, so the boundary belongs as deep in the tree as it will go. - **Verify caching behaviour against the installed version — it changed.** Through Next 14, `fetch` was cached by default and you opted out; from Next 15, `fetch`, Route Handlers, and client navigation are uncached by default and you opt in. Assuming the wrong default gives you either stale pages or an origin taking every request. Check `package.json`, then set the intent explicitly (`cache`, `next.revalidate`, or route segment config) rather than relying on whatever the default happens to be. - **`params` and `searchParams` are Promises from Next 15 on** — `const { slug } = await params`. Code written against 14 destructures them synchronously and breaks on upgrade, so check the installed major before copying either form. - **A `layout.tsx` auth check is not a security boundary.** Layouts don't re-render on every navigation between their child routes, so the check can be skipped on client-side transitions. Enforce access in middleware and aga