What is the purpose of an API spec file in API documentation?

Short answer: Contains all endpoints, request/response formats, parameters, headers, and security details. Acts as a contract between client and server. Used for auto-generating documentation, client SDKs, and mock servers.

Real-world example (ShopNest)

ShopNest exposes REST APIs for cart and checkout. Controllers stay thin; business rules live in application services.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.