Nest‑OpenAPI is a suite of modular NestJS libraries that let you drive validation, serialization, mocking and MCP tooling directly from a single OpenAPI specification. By plugging the packages (validator, serializer, mock, mcp) into any NestJS (v9+) app—whether on Express or Fastify—you get automatic request/response validation, high‑performance response formatting, mock servers and spec‑driven operation tools with fine‑grained per‑route control. It’s aimed at backend developers building TypeScript/NestJS services who want a single source of truth for their API contracts.