Why your app works in preview but breaks when you deploy it

It worked in Bolt, Lovable or Cursor. You hit deploy, and now it's a blank page or a build error. This is one of the most frustrating moments in vibe coding, but it usually comes down to a few causes.

1. Missing environment variables

Your API keys live in a .env file or the tool's secrets panel, and they don't copy over automatically. Add each one in your host's settings. Browser code also needs the right prefix (NEXT_PUBLIC_ or VITE_). Env variable undefined: fix.

2. The production build is stricter

Preview mode lets some type and lint errors slide. A production build doesn't. Run the build command yourself and fix the first error it shows. Vercel build failed: fix.

3. Server and browser render differently

Dates, random numbers or browser-only code can make the server's HTML differ from the browser's. Hydration error: fix.

4. Your live domain isn't allowed

APIs and login providers often only allow your preview URL. Add your real domain to their allowed list. CORS error: fix.

5. Wrong port or address on Replit

Deployed apps need to listen on the port the host gives them. Replit app not reachable: fix.