422
reason_requiredplatformThis action needs an X-Reason header
Returned by the implementation; no specification rule declares it yet.
Some actions are recorded with the reason they were taken, and the reason is part of the audit row rather than a nicety. It is 422, not 400: the request is well-formed, it just violates a rule. errors[0].field is reason.
Why the API refuses
this action requires a reason
What to do
Send X-Reason: <why>. Write it for the person who will read the audit log during an investigation, not for the validator — that reader is the only reason the field exists.