Runtime Cache

This is Vercel's ephemeral key-value cache that lives at the edge, shared across functions, middleware, and builds. It's what you reach for when Next.js's built-in caching isn't granular enough and you need tag-based invalidation without managing Redis infrastructure. The skill covers the `@vercel/functions` APIs, tag expiration patterns, and the critical distinction between `cache.expireTag()` (runtime only) versus `invalidateByTag()` (purges CDN, runtime, and data layers). One thing to know: it's regionally isolated and scoped per environment, so production and preview caches never mix. Useful validation rule catches direct Redis/ioredis imports and suggests Upstash instead for serverless compatibility.