Backward compatibility

Existing clients keep working without modification after something behind the scenes has changed.

Mental model: Think of it like a phone charger that still works with your older phone case — the shape didn't change.

Example: A new API version adds an optional 'nickname' field, but old clients that never send it keep working unmodified.

In practice: APIs typically maintain this by adding new optional fields instead of altering existing ones, and by versioning endpoints when a truly incompatible change is unavoidable.

• opposite of Breaking change