Design a cryptographic non-repudiation audit log for agent-initiated purchases that satisfies dispute and compliance evidence requirements across card networks and stablecoin rails

Verified steps

1. At the moment of agent purchase authorization, capture and store the complete evidence chain: for AP2 flows, archive the signed Intent, Cart, and Payment Mandate VCs; for Stripe SPT flows, store the SPT token ID, the Link OAuth grant record, and the consumer approval timestamp; for Mastercard Agent Pay, store the Agentic Token ID, the program_id field value, and the agent identity binding
2. Write evidence records to an append-only log store (e.g., an immutable object store with Object Lock, or a ledger database); never allow update or delete operations on evidence records — compliance requirements for card network disputes require evidence to be producible on demand and unaltered
3. Include the following fields in every evidence record: agent session ID, agent platform and version, principal (buyer) identifier, merchant ID, transaction amount and currency, payment rail (card network, stablecoin, BNPL), idempotency key used, PSP transaction ID, and UTC timestamp with millisecond precision
4. For stablecoin (x402, Circle) transactions, additionally record the on-chain transaction hash, the block number at confirmation, and the chain ID; these serve as the immutable on-chain receipt that substitutes for a card network dispute record
5. Implement a retention policy aligned to card network requirements (Visa and Mastercard generally require dispute evidence availability for 18 months from transaction date — verify current requirements with your acquirer) and applicable data protection law for PII fields (apply pseudonymization to buyer PII in the evidence record)
6. Test dispute evidence retrieval: simulate a chargeback scenario, retrieve the evidence package for the disputed transaction, and verify it includes all required fields in a format your acquirer's dispute portal accepts; gaps in the evidence package at retrieval time, not at write time, cause dispute losses