Cause
Every write requires an Idempotency-Key of 8 to 128 characters A-Z a-z 0-9 . _ : -. The content preview of Communications does not require it.
What to do
Send a unique key per intent, for example a UUID, and reuse it only to retry the same request.
application/problem+json
{
"type": "https://masnet.com/developers/errors/idempotency_key_required",
"title": "Idempotency key required",
"status": 428,
"detail": "...",
"code": "idempotency_key_required",
"correlation_id": "..."
}