How do I tell an invalid VAT number apart from an error?

A well-formed VAT number that simply isn't registered returns HTTP 200 with valid: false — that's a normal answer, not an error. Errors are separate: INVALID_FORMAT (400) means the string isn't a valid VAT ID shape and never reached VIES; VIES_UNAVAILABLE (503) means the EU system was down; RATE_LIMITED (429) means you hit your monthly quota; INVALID_REQUESTER (422) means the requester VAT you configured is itself invalid. Every error response includes a machine-readable code and a requestId.