GUARDIAN_REQUIRED

The sender is guarded but the transaction is not.

HTTP status	422
Group	Relay path
Retryable	No — sending it again changes nothing
Re-sign	NONE — do not ask the user to sign anything

What happened

A guarded account can only move with the guarded option set and a guardian signature present.

What to do

Rebuild as a guarded transaction, collect the guardian signature, then relay.

The problem document

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

{
  "type": "https://docs.co-relayer.com/errors/guardian-required",
  "title": "The sender is guarded but the transaction is not.",
  "status": 422,
  "detail": "A sentence about this specific occurrence.",
  "instance": "req_01JB…",
  "code": "GUARDIAN_REQUIRED",
  "retryable": false,
  "resign": "NONE",
  "hint": "Rebuild as a guarded transaction, collect the guardian signature, then relay."
}

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