What is the .next folder and what does it contain?

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The .next folder is Next.js's build output and dev cache. It's created automatically when you run next dev or next build , and it should never be edited by hand or committed to git. What's inside (after a production build): cache/ — incremental cache for the Next.js compiler. Lets next build reuse work between runs and powers HMR in dev. Also stores the Data Cache for ISR and fetch results. server/ — server-side bundles. Compiled Server Components, route handlers, page modules, middleware, and the manifest files Next uses to dispatch requests. static/ — client-side bundles, hashed for cache busting. Includes JavaScript chunks, CSS, and any imported static assets. Served at /_next/static/... . standalone/ — only created when output: 'standalone' is set. A self-contained Node bundle with just the files needed to run the app, useful for Docker. BUILD_ID — a unique ID for this build, used in asset URLs and as part of cache keys. build-manifest.json, prerender-manifest.json, routes-manifest.json — metadata Next reads at runtime to map URLs to chunks, decide which routes are static vs dynamic, and identify ISR pages. trace — performance trace data from the build, useful for debugging slow builds with next build --profile . Practical points: Always gitignore it — the default .gitignore from create-next-app already does. Generated artifacts shouldn't be in source control. Delete it to force a clean build — rm -rf .next when caching weirdness is making your life difficult. next build will rebuild from scratch. Don't ship it as your only artifact — for self-hosted deploys, you also need node_modules , public/ , package.json , and next.config.js . Or use output: 'standalone' to get just what you need. Cache the right parts in CI — caching .next/cache between CI runs speeds up builds dramatically. Don't cache .next/static — it should always be regenerated to match the build. It can get big — for large apps, .next can be hundreds of megabytes. The standalone output strips this down to just the runtime essentials. Treat it like a Webpack output directory — useful to inspect when debugging build issues, but otherwise an implementation detail you can ignore. Mention that deleting .next/ forces a clean rebuild — a practical debugging tip interviewers appreciate. Note that it should be in .gitignore. Knowing what the cache/ subdirectory does shows build process understanding.