Summary

Next.js 15 RC 2 focuses on developer experience, performance, and security ahead of the general release. Key updates include an enhanced upgrade codemod, opt-in Turbopack for development, a breaking change that makes request-specific server APIs asynchronous, improved Server Action security, a new Form component, TypeScript support for next.config.ts, stable server instrumentation, and multiple build/dev performance improvements.

Key Points

• Upgrade tooling

  • Use the automated codemod CLI to upgrade: npx @next/codemod@canary upgrade rc.
  • Manual upgrade: npm install next@rc react@rc react-dom@rc.

• Turbopack (development)

  • Opt-in with next dev --turbo.
  • Improves cold compile performance: ~25–35% memory reduction and ~30–50% faster compilation for large pages vs the previous RC.
  • Work continues on persistent caching and build support.

• Breaking: Async Request APIs

  • Request-bound APIs (cookies, headers, draftMode, params in layout/page/route/default/generateMetadata/generateViewport, and searchParams in page.js) are transitioning to async; use await (example: const cookieStore = await cookies()).
  • Temporary synchronous access remains with dev/production warnings; migrate with npx @next/codemod@canary next-async-request-api and consult the upgrade guide for manual fixes.

• Server Actions security

  • Dead code elimination removes unused actions from client bundles and prevents public IDs.
  • Action IDs are now unguessable and periodically recalculated between builds.
  • Treat actions as public endpoints; minimize exposed logic.

• Forms and routing

  • New <Form> component (import Form from 'next/form') adds prefetching, client-side navigation, and progressive enhancement without boilerplate.

• Configuration and observability

  • next.config.ts is supported with NextConfig typing for type-safe config.
  • instrumentation.js and register() are stable; onRequestError hook is available for server error observability integrations.

• Development & build improvements

  • Server Components HMR reuses cached fetch responses to reduce re-fetching during development.
  • Faster static generation for the App Router by reusing the first render and sharing fetch cache across worker pages.
  • Experimental advanced static generation options added (use with caution — may increase resource usage).
  • Additional improvements: self-hosting Cache-Control controls and ESLint 9 support.

Actionable Notes for Engineers

• Try RC2 locally: npx @next/codemod@canary upgrade rc then npm install next@rc react@rc react-dom@rc.
• Audit code for synchronous usages of cookies(), headers(), params, and searchParams; run the async-request codemod and test thoroughly.
• Opt into Turbopack for dev when testing large apps: next dev --turbo.
• Review Server Actions to ensure unused exports are removed and sensitive logic is not exposed as public endpoints.
• Add next.config.ts if you want typed config and wire up instrumentation.js for observability integrations.

References

See the Next.js 15 RC 2 announcement and linked upgrade guides and codemods for migration details and full docs.