Most Spring Boot applications have a global exception handler.

Far fewer have a well-defined error contract .

There's an important difference.

A global exception handler decides how an exception becomes an HTTP response. An error contract defines what that response means to the application consuming it.

Once services begin communicating over HTTP, that distinction becomes critical.

A frontend needs stable identifiers to display meaningful messages. A backend service needs to distinguish permanent business failures from potentially recoverable dependency failures.…