api-first-design

Contract-first API design enforces a single source of truth using OpenAPI specs. Define endpoints, DTOs, and response formats upfront in Swagger, then auto-generate types across NestJS backend, Next.js frontend, and Expo mobile clients to keep contracts synchronized and eliminate misalignment.

Establish standardized response formats and error code mappings in your OpenAPI specification before implementation. Document these contracts centrally so backend, frontend, and mobile teams follow the same structure, reducing integration bugs and simplifying error handling across the monorepo.

Yes. Tools like Orval read your OpenAPI specification and generate TypeScript DTOs, types, and client code automatically. This ensures frontend and backend types stay in sync without manual duplication, eliminating type inconsistencies in NestJS and Next.js environments.

Version your OpenAPI specification explicitly and include version identifiers in endpoint paths or response headers. Document breaking changes in the spec upfront so all teams—backend, frontend, and mobile—prepare migrations before deployment, reducing surprise integrations.

Spec-first development catches design issues before coding, prevents API drift, and generates accurate documentation automatically. It forces alignment between teams on contracts, reduces rework, and enables faster type generation with tools like Orval compared to reverse-engineering types from implemented code.