What is the difference between next dev, next build, and next start?

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These are the three commands Next.js ships for running your app, each for a different phase of development. next dev — the development server. Compiles routes on demand, watches files, enables Fast Refresh for near-instant updates, and surfaces errors with source maps. It's not optimized for performance — it's optimized for the feedback loop. Run it locally with npm run dev . next build — the production build. Compiles your app ahead of time, runs type checks and lint (by default), statically generates pages that can be, and produces the .next/ output directory ready for deployment. This is the command your CI/CD runs. next start — the production server. Reads the artifacts from next build and serves them. Requires the build step to have finished first; it will refuse to start if .next/ is missing or stale. Typical lifecycle: # development npm run dev # next dev on localhost:3000 # deployment npm run build # next build npm run start # next start on the production host Key differences: Compilation timing — dev compiles a route when you visit it. build compiles everything once. start compiles nothing — it serves the pre-built artifacts. Error handling — dev shows a helpful overlay with stack traces. start returns a generic 500 and logs to the console. Caching — dev disables most caching so changes show up immediately. start enforces the real ISR, Data Cache, and Router Cache behavior. Performance — only start represents real production performance. Don't benchmark dev and assume production is slow. Port — all default to 3000, overridable with -p 4000 or PORT=4000 . Useful flags: next dev --turbo — use Turbopack (now the default in Next 15+). next build --debug — extra logging for troubleshooting builds. next build --profile — writes a trace file for analyzing build performance. next start -H 0.0.0.0 — bind to all interfaces when running in Docker. One other related command is next lint (running ESLint with the Next config) and next export (deprecated — use output: 'export' in config instead for fully static sites). On Vercel and most PaaS, you don't call next start directly — the platform runs your build output through its own serving layer. next start is for self-hosting. The practical insight: you must run next build before next start. Mentioning that next dev compiles on-demand while next build pre-compiles everything shows you understand the performance difference.