AP2 closed-payment projection checks

What the protocol establishes

MandateShield verifies a closed mandate.payment.1 projection and RFC 9901 KB-JWT. The wider checkout_jwt hash, delegate chain, open-mandate constraints and issuer registry remain external AP2 integration responsibilities.

Five checks before execution

1. Verify the closed-payment projection and RFC 9901 KB-JWT against an account-pinned issuer.
2. Bind its transaction_id to the envelope checkout_hash.
3. Compare final amount, currency and merchant with approved scope.
4. Require current consent and reject expired authority.
5. Atomically consume each idempotency key once.

The gap MandateShield closes

A cryptographically valid mandate can still be unsafe when the final price, seller or checkout context changed after authorization.

MandateShield normalizes execution facts and returns a deterministic decision with exact findings. Strict v2 verifies supported evidence and returns an ES256-signed receipt with public transparency; it complements the protocol's wider trust and settlement chain.