Summary

The Next.js 15 Release Candidate is available (install with npm install next@rc react@rc react-dom@rc). It introduces React 19 RC support, the experimental React Compiler, improved hydration error diagnostics, and several caching behavior changes. Notable new experiments include incremental Partial Prerendering (PPR) and the next/after API to run tasks after a response streams. create-next-app now prompts to enable Turbopack and adds a --empty starter. Stable improvements include unified options for bundling external packages.

Key Points

• React 19 RC: App Router now supports React 19 RC. Check third-party compatibility and follow the Next.js/React upgrade guides.
• Experimental React Compiler: opt-in via babel-plugin-react-compiler and experimental.reactCompiler = true in next.config.js to enable automatic optimizations (may slow builds during Babel usage).
• Hydration errors: improved error view shows source and remediation suggestions.
• Caching defaults changed to uncached: fetch, GET Route Handlers, and Client Router Page cache are uncached by default. To opt back in:
  • set fetch(..., { cache: 'force-cache' }) for individual calls
  • set route export const dynamic = 'force-static' for a route
  • set fetchCache route config to default-cache in a Layout/Page
  • restore client router caching with experimental.staleTimes (e.g. staleTimes.dynamic)
• Partial Prerendering (experimental): enable incremental adoption with experimental_ppr = true per Page/Layout and experimental.ppr = 'incremental' in next.config.js. Wrap dynamic UI in Suspense to stream dynamic parts.
• next/after (experimental): enable experimental.after = true and use unstable_after from next/server to schedule post-response work (logging, analytics) without blocking the response.
• create-next-app: new UX prompts for Turbopack (use --turbo) and --empty for minimal starter.
• Bundling external packages (stable): bundlePagesRouterDependencies: true and serverExternalPackages let you control bundling behavior across App and Pages routers.
• Breaking changes: minimum React is 19 RC; multiple next/image, next/font, Middleware and caching behavior changes—review breaking notes before upgrading.

Refer to the Next.js 15 upgrade guide and the RC changelog for full details and migration steps.