Summary

Next.js 9.5 (2020-07-27) adds several production-ready features that improve routing, caching, and developer experience. Highlights include Stable Incremental Static Regeneration (ISR) to update static pages in the background, a configurable basePath for serving Next.js from subpaths, first-class rewrites, redirects, and headers in next.config.js, optional trailing-slash behavior, content-hashed page bundles for long-term caching, Fast Refresh reliability improvements, a production React profiling flag, optional catch-all routes, and a beta opt-in for Webpack 5.

These changes are practical for teams migrating existing sites, optimizing cache behavior across deploys, or adopting incremental static generation at scale.

Key Points

• Incremental Static Regeneration (stable): add revalidate in getStaticProps to re-render pages in the background (e.g. return { props, revalidate: 1 }). Pages remain served from static cache while the new version is generated.
• basePath: host Next.js on a subpath by adding basePath: '/docs' to next.config.js. next/link and router APIs will automatically include the prefix.
• Rewrites / Redirects / Headers: configure routing and HTTP headers in next.config.js via async rewrites(), redirects(), and headers() functions to proxy, redirect, or attach headers to static and server-rendered routes.
• Trailing slash option: control trailing-slash behavior with trailingSlash: true or false in next.config.js to avoid needless redirects and match legacy URL conventions.
• Persistent caching: page bundles now use deterministic content hashes instead of a global build ID, enabling long-term caching (Cache-Control: public, max-age=31536000, immutable) for unchanged pages across deploys.
• Fast Refresh improvements: clearer, more focused error messages and more reliable live-editing during development.
• Other: production React profiling flag for measuring render cost, optional catch-all routes for SEO-driven patterns, and an opt-in Webpack 5 beta for smaller/faster builds.

Practical upgrade notes

• Audit custom servers or existing rewrites/redirects when upgrading to rely on next.config.js options.
• Expect improved cache hit rates after deploys due to content-hash bundles; verify CDN/edge caching settings if you had custom cache invalidation logic.
• Try ISR on a subset of pages using revalidate to validate behavior under your backend load patterns; consider webhooks or upcoming RFCs for multi-page invalidation.