The checkboxes are the whole story. Each env var in Vercel can have a different value per environment:
- Production: real Supabase project
- Preview: a second Supabase project (free tier is fine) with fake data
- Development: local Supabase (
supabase start) or the same staging project
Edit each variable, uncheck Preview, then add it again with the staging value for Preview only. Do the same for anything that sends real emails or charges money: Resend key, Stripe key (test mode key for Preview).
Redeploy the preview afterwards, env vars are read at build time.