WEBHOOK_URL_NOT_ALLOWED

That webhook URL cannot be registered.

HTTP status	422
Group	Platform
Retryable	No — sending it again changes nothing

What happened

Webhook egress is deliberately narrow: private, loopback and link-local destinations are refused so the notifier cannot be pointed at internal hosts.

What to do

Register a public HTTPS endpoint.

The problem document

Every error is application/problem+json (RFC 9457) with the same shape:

{
  "type": "https://docs.co-relayer.com/errors/webhook-url-not-allowed",
  "title": "That webhook URL cannot be registered.",
  "status": 422,
  "detail": "A sentence about this specific occurrence.",
  "instance": "req_01JB…",
  "code": "WEBHOOK_URL_NOT_ALLOWED",
  "retryable": false,
  "hint": "Register a public HTTPS endpoint."
}

See the error model for what each member means and how to handle unknown codes.