As cliched as it may sound, it...depends on how is the API consumed? Is the error code going be processed further and passed onto logger or other sub-systems? Are there possible cases of collision with other existing enumerations or type restriction (magic handling relies on error code being of a certain value, for example). Most common way is to have an array of enumerated constants in code and that also adds a maintenance debt across front and back ends to sync up unless there is a separate micro-service to enquire about those enumerations which translates to extra development cost. It could get complicated fast and so on. Oh, hopefully the documentation (or code comment) stays up to date as well.