I see where you're coming from now.

How do we create a guideline then for what messages to use?

There actually nothing limiting having a message that's not specified in the schema: https://github.com/balanced/balanced-api/blob/master/fixtures/_models/error.json#L147

How we make sure that messages are even well documented? #597, https://github.com/balanced/balanced-api/commit/9a80bb80923d29eb06a912e289cd055ea69ef34b#diff-83f6993bcc8577e77fb6ae6516971183L146