90% of the time it's one of: (1) Missing env var in Vercel dashboard (compare with `vercel env pull`), (2) Node version mismatch, pin in `engines` field of package.json, (3) Edge runtime can't run a Node API you used, (4) File system writes that work locally don't on serverless. Check Vercel function logs first, they tell you which one.
Prompt to paste
My app works locally but breaks on Vercel: [paste the error or behavior]. Walk the four usual causes in order, missing env var, Node version mismatch, Edge runtime using a Node API, and serverless filesystem writes. For each, tell me exactly what to check in my project and the Vercel logs, plus the fix.