Production build succeeds, bundle size is reasonable, environment variables are set, error boundaries wrap the app, loading and error states are everywhere, auth works in production, the app is responsive and accessible, and SEO metadata is set.

Because the production build is what users get, not your dev bundle. Run npm run build and confirm it succeeds with no errors. A build that fails or has errors in production is worse than in development.

Test the full flow on the deployed app: signup, signin, signout, protected route access, and persistence across reloads. Confirm authorized domains are configured so auth redirects work in production, not just on localhost.