Fixing Common Next.js Deployment Errors

Missing environment variables

Variables in your local .env file are not available on the server automatically. Add them in your hosting dashboard and redeploy so the new values are picked up.

Remember that only variables prefixed with NEXT_PUBLIC_ reach the browser. Anything else stays on the server, so a value that works in an API route may be undefined in a client component.

Server and client mix-ups

Using browser APIs such as window or React hooks like useState inside a server component causes build errors. Add "use client" only where interactivity is needed and keep data fetching on the server.

Stale cache and outdated data

If new content does not appear after a change, check your revalidation settings. Pages built at deploy time stay the same until they are revalidated, so trigger a revalidate whenever content is added or removed.

Case-sensitive file paths

Windows and macOS ignore letter case in file names, but most Linux servers do not. An import of "Header" for a file called "header" works locally and fails in production. Keep names consistent.

Read the build log first

Most deployment errors are explained in the first red line of the build log. Reproduce it locally with a production build before changing anything, and fix one issue at a time.