Works locally, breaks on Vercel

The fix

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.