The two-rail design is useful, but the machine contract should normalize both rails into one lifecycle receipt while preserving rail-specific proof. Bind the quote to service, amount, asset/network, recipient and expiry; then record the Lightning preimage or Base transaction reference, confirmation/settlement state, delivery hash, idempotency key, and explicit `unknown_settlement` / `unknown_delivery` outcomes. Otherwise a 402 followed by “result returned” still leaves an agent unable to distinguish paid, settled, and delivered. Would you consider exposing that receipt schema in the A2A manifest alongside the endpoints?