Summary

Next.js 15.5 delivers production beta support for Turbopack builds, stabilizes Node.js middleware, and introduces major TypeScript route typing and tooling improvements. The release also starts deprecations (including next lint and several Next.js 16 removals) to prepare projects for the Next.js 16 migration. This note highlights what engineers should try, upgrade commands, and migration actions.

Key Points

• Turbopack builds (beta): run builds with next build --turbopack. Turbopack is faster on multi-core machines and is used in production at Vercel; watch CSS ordering and bundle-optimization edge cases.
• Node.js middleware (stable): middleware can now run in the Node.js runtime via export const config = { runtime: 'nodejs' }, enabling access to Node APIs and packages.
• TypeScript improvements: stable typed routes, route export validation that works with Turbopack, automatic PageProps/LayoutProps types, and a new next typegen command for standalone type generation and CI checks.
• Linting change: next lint is deprecated (removed in Next.js 16). New projects use explicit ESLint or Biome configs; use the codemod npx @next/codemod@latest next-lint-to-eslint-cli to migrate scripts.
• Next.js 16 warnings: dev/build logs now show deprecation notices (notably removal of legacyBehavior on next/link and AMP support). Update code to the modern Link usage and remove AMP pages.

Upgrade & Quick Commands

• Automated upgrade (recommended):
  • npx @next/codemod@canary upgrade latest
• Manual upgrade:
  • npm install next@latest react@latest react-dom@latest
• New project:
  • npx create-next-app@latest
• Try Turbopack build locally:
  • next build --turbopack
• Generate route types / CI checks:
  • next typegen && tsc --noEmit
• Migrate lint scripts:
  • npx @next/codemod@latest next-lint-to-eslint-cli

Practical migration notes

• If you rely on custom CSS ordering or certain bundle optimizations, test thoroughly under --turbopack and report issues; workarounds and documentation are available.
• For middleware needing Node APIs, set runtime: 'nodejs' in middleware.ts and validate behavior in edge and server environments.
• Turn on typedRoutes: true in next.config.ts to opt into stable typed routes and use next typegen in CI to ensure route correctness.
• Replace legacyBehavior usage and remove AMP pages before upgrading to Next.js 16 to avoid breaking changes.