A requirement the spck.dev suite checks on UCP servers. The quoted text is the normative requirement; the citation links to the exact line of the pinned spec source it was taken from.
2026-01-11:
The error code field MUST be a string; defined values include missing, invalid, out_of_stock, payment_declined, requires_sign_in, requires_3ds, requires_identity_linking, with freeform codes also allowed.
2026-01-23:
The error code field MUST be a string; defined values include missing, invalid, out_of_stock, payment_declined, requires_sign_in, requires_3ds, requires_identity_linking, with freeform codes also allowed.
2026-04-08:
Error message severity MUST be one of recoverable, requires_buyer_input, requires_buyer_review, unrecoverable.
A deviation on a MUST fails the run and shows the requirement next to the actual response. Checks the suite cannot validate soundly are reported inconclusive or not-tested — never a silent pass. The full verdict semantics are on the grading rubric.
The hosted check covers the read-only surface; the CLI and GitHub Action run the full suite, including this check when your store declares the capability it belongs to.