When should you return a 400 Bad Request?

Short answer: Returned when the request is malformed or invalid, such as: Missing required parameters. Invalid JSON format. Wrong data type provided.

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.