Summary

Next.js 15.1 adds stable React 19 support, improved developer debugging (better source maps, collapsed stack frames, consistent Edge runtime errors), the stable after() API for post-response work, and two experimental authorization helpers (forbidden() and unauthorized()). Turbopack is now stable and recommended for more accurate source maps and profiling.

Key Points

• React 19

  • Pages Router: official React 19 stable support (React 18 still supported).
  • App Router: continues to ship with React canary builds to validate new framework integrations.

• Improved error debugging

  • Better source maps with ignoreList support to hide external dependency frames.
  • Collapsed stack frames in the browser overlay and terminal; "Show ignored frames" reveals hidden frames.
  • Consistent error output and profiling in the Edge runtime.

• after() (stable)

  • Run non-blocking secondary work after a response finishes streaming (logging, analytics, sync tasks).
  • Now supports self-hosted servers, runtime APIs (cookies(), headers()), and extensibility for platform waitUntil() implementations.
  • Usage: import { after } from 'next/server'; after(() => { /* secondary task */ });

• forbidden()/unauthorized() (experimental)

  • New helper APIs to trigger 403/401 flows and provide customizable pages at app/forbidden.tsx and app/unauthorized.tsx.
  • Enable via next.config.ts: { experimental: { authInterrupts: true } }.
  • Note: if triggered after response headers are sent, the status code will be 200 (same behavior as notFound()).

• Upgrade notes & quick commands

  • Automated: npx @next/codemod@canary upgrade latest
  • Manual: npm install next@latest react@latest react-dom@latest
  • New project: npx create-next-app@latest

• Miscellaneous

  • Turbopack is stable — recommended for better source-map/method-name mapping and profiling.
  • Several small fixes and features: ESLint 9 in create-next-app, increased cache tags, CSS inlining option, and multiple bug fixes affecting rewrites, server actions, and routing.

Actionable guidance

• Upgrade to Next.js 15.1 and React 19 if you use the Pages Router; test App Router behavior with the built-in canary where needed.
• Adopt Turbopack for improved source-mapping and profiling when possible.
• Populate ignoreList in library sourcemaps to improve consumer debugging.
• Enable experimental authInterrupts to evaluate forbidden()/unauthorized() and provide custom error pages during development.