Codemod covers (high-level): moves Turbopack config, migrates next lint → ESLint CLI, migrates middleware → proxy, removes some unstable_ prefixes, removes route-level experimental_ppr.

TypeScript: also upgrade @types/react and @types/react-dom.

What’s New (v16)

Cache Components: opt-in caching via the "use cache" directive; evolves/absorbs PPR.
Next.js DevTools MCP: Model Context Protocol integration for AI-assisted debugging.
proxy.ts: clearer network boundary; middleware.ts deprecated for most use.
Better logs/metrics: more detailed next dev and build timing output.

Performance / DX

Turbopack: stable; default bundler (opt out with next dev --webpack, next build --webpack).
If you have a custom webpack config, next build may fail (to prevent misconfiguration). Fix by migrating config, using next build --webpack, or using Turbopack and removing/ignoring the webpack config.
Turbopack config moved: experimental.turbopack → top-level turbopack in next.config.*.
Turbopack migration gotchas:
- Sass imports: remove the Webpack-only ~ prefix (e.g. @import 'bootstrap/...';).
- Browser bundles must not import Node built-ins (e.g. fs). If unavoidable, use turbopack.resolveAlias as a stopgap.
Turbopack filesystem cache (dev, beta): experimental.turbopackFileSystemCacheForDev: true.
React Compiler support: stable opt-in via reactCompiler: true (expect higher build/compile cost).
Build Adapters API: alpha (custom build adapters).
Routing/prefetching rewrite: layout deduplication + incremental prefetching.

Caching APIs (key signatures)

revalidateTag(tag, profile) now requires a cacheLife profile (or { expire }) for SWR behavior.
updateTag(tag) (Server Actions only): read-your-writes semantics.
refresh() (Server Actions only): refresh uncached data; does not mutate cache.
cacheLife and cacheTag are stable (no unstable_ prefix).

Requirements (v16)

Node.js: 20.9+ (Node 18 not supported)
TypeScript: 5.1+
Browsers: Chrome/Edge/Firefox 111+, Safari 16.4+

Breaking / Behavior Changes (high-impact)

Async Request APIs: sync access removed. Use await params, await searchParams, await cookies(), await headers(), await draftMode().
Tip (TypeScript): npx next typegen can generate helpers like PageProps, LayoutProps, RouteContext to migrate params/searchParams types safely.
Metadata images: opengraph-image, twitter-image, icon, apple-icon now receive params (and id) as Promises in the image function.
Sitemaps: sitemap({ id }) now receives id as a Promise when using generateSitemaps.
Parallel routes: slots require explicit default.js.
next/image defaults changed (cache TTL, sizes/qualities); local src with query strings requires images.localPatterns.

Other notable behavior changes:

next dev and next build use separate output dirs (next dev → .next/dev) and a lockfile prevents concurrent instances.
Scroll behavior: Next.js no longer overrides global scroll-behavior: smooth during navigations; add data-scroll-behavior="smooth" on <html> to restore the previous override behavior.
ESLint: @next/eslint-plugin-next defaults to ESLint Flat Config; legacy .eslintrc projects may need migration.

Removed / Deprecated (high-level)

Removed: AMP support; next lint (use ESLint/Biome directly); eslint option in next.config.*; serverRuntimeConfig/publicRuntimeConfig (use env vars); experimental.ppr + route-level experimental_ppr; unstable_rootParams.
Deprecated: middleware.ts filename (prefer proxy.ts); next/legacy/image; images.domains (prefer images.remotePatterns); revalidateTag(tag) single-arg form.
proxy.ts note: proxy runs on nodejs only; Edge runtime is not supported in proxy. Keep middleware.ts if you must stay on Edge.
Config rename example: skipMiddlewareUrlNormalize → skipProxyUrlNormalize.

nextjs16-skills

Links

Upgrade