Vauban Research The x402 PAYMENT-RESPONSE carries a facilitator-issued settlement reference but does not provide a self-contained, offline-verifiable payment-condition proof. A verifier must today contact the facilitator to confirm whether a specific amount, currency, and payer attestation were satisfied at the time of payment. This gap prevents compliance with EU AI Act Art. 12 (transparency and documentation) and MiCA Art. 76 (settlement record-keeping) in automated payment pipelines. This document defines the receipt-format extension for x402 V2. The extension introduces a negotiable receipt format selection mechanism that allows resource servers, facilitators, and clients to agree on which cryptographic receipt variant to produce and verify, without altering the core PAYMENT-RESPONSE wire structure. Three variants are defined: a Stwo Circle STARK proof of payment conditions (post- quantum sound, offline verifiable), a hybrid ES256K + ML-DSA-65 dual- signature receipt (receipt integrity under quantum adversary), and a classical ES256K fallback. A canonical preimage discipline using JCS ([RFC8785]) ensures cross-implementation digest consistency. This specification reflects three-implementation consensus established in [X402-2357].