When to move API logic out of Next.js

Next.js Route Handlers are convenient for small, internal APIs but struggle as complexity grows. An alternative to extracting a separate backend service is embedding ElysiaJS within the same Next.js project via a catch-all route. ElysiaJS provides integrated TypeBox-based validation, automatic OpenAPI/Swagger docs, and Eden Treaty for end-to-end type safety — eliminating the schema drift common in Route Handler codebases. The setup deploys as a single Next.js app, with Elysia handling the backend boundary internally. The post compares Route Handlers vs. a dedicated Elysia layer across type safety, validation, execution model, and structure, and walks through a practical implementation.