Next.js 15 Migration: Turbopack Gains Come With Caching and Async API Pitfalls

Next.js 15 introduces stable Turbopack support in development, React 19 compatibility, and Partial Prerendering, delivering build speeds reported to be 4–7x faster and hot module replacement latencies under 100ms. However, the upgrade brings two key breaking changes that developers must address during migration. Request-scoped APIs such as cookies(), headers(), params, and searchParams now return Promises, meaning any code that reads them synchronously will compile but fail silently at runtime. Additionally, fetch calls and GET Route Handlers are no longer cached by default, which can cause unexpected spikes in backend traffic for apps that relied on implicit caching in earlier versions. Developers are advised to audit async usage across middleware and server components, and to declare caching behavior explicitly per fetch call or route handler.