402 conformance check for an x402 or MPP API: can buyer agents' SDKs pay it? Checks the x402 v2 requirements (whole-unit amounts, addresses, the token's EIP-712 domain, the same terms on every request), that a malformed payment is refused, any MPP challenge against the paymentauth.org draft, and the live price against the Bazaar and the host's openapi.json. Unpaid requests only; nothing is signed or paid.

Payment Options