An API is not only a technical contract. It is a promise made to every person and system that will depend on it later.
Good API design begins with the consumer: clear naming, predictable errors, useful documentation, sensible versioning, and security that is understandable rather than mysterious. Reusable code matters, but reusable understanding matters too.
Build for the handover
The engineer who inherits an API should not need to reverse-engineer its intentions. A good contract makes the next change safer, the next integration faster, and the business more confident.