Developer Warns: In-Memory Caching Breaks Silently on Vercel Serverless APIs

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A developer spent three days debugging inconsistent API responses before discovering the root cause had nothing to do with their code. They had deployed a Node.js-based JSON API on Vercel using Next.js API routes, relying on a module-level in-memory cache to reduce repeated database lookups. Because Vercel's serverless functions spin up fresh instances after periods of inactivity, any data stored in module-level variables is wiped on each cold start. This made the cache work reliably during bursts of traffic on a warm instance, but silently reset whenever the function went idle — producing unpredictable responses. The developer now cautions that in-memory state cannot be trusted across serverless invocations, and that understanding the cold-start lifecycle is essential before choosing Vercel for stateful backend workloads.

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