Clean local builds failing on Vercel: the checks one developer runs first

A developer describes repeatedly losing evenings to deployments that fail on Vercel even though the same commit builds cleanly locally. They list the issues that most often cause this, including case-sensitive environment variable imports, environment variables that exist only on the local machine, lint rules that Next.js treats as hard build failures, server-only code leaking into the client bundle, and files that are present locally but never committed to git. The post also promotes a paid preflight scanning tool that checks public GitHub repositories for these problems, offering one free scan per day and paid scans for private repos. The author closes by asking readers what their first check is in the same situation.