Book 6 of the AdValorem Syndicate Crypto Series. Autonomous agents will transact a trillion dollars before the end of the decade. The payment infrastructure to support that volume does not exist yet. This book is for the operators who are going to build it. Agentic Commerce x402 starts with the problem: every payment standard we have was designed for human actors. Credit cards require cardholder authentication. ACH assumes a human-initiated origination event. Ethereum's EOA model assumes someone is holding a key and signing at human speed. None of this works for agents operating at machine speed and machine frequency. The x402 standard — Coinbase's revival of the long-dormant HTTP 402 Payment Required status code — is the beginning of an answer. The book covers x402 in full technical detail: the payment manifest schema, the facilitator attestation model, the request-response lifecycle. Then it covers the full surrounding stack: ERC-4337 account abstraction and session keys for programmable agent authorization; USDC and Circle's CCTP for stablecoin routing across chains; private orderflow agreements with block builders so agents retain their MEV instead of donating it; agent identity through ENS and Ethereum Attestation Service; custody patterns for hot smart accounts and multi-agent treasuries; payment channels for high-frequency micropayment flows; Across Protocol and LayerZero v2 for cross-domain settlement; and a compliance framework that is honest about what is unresolved in the regulatory treatment of autonomous agents. It closes with a reference architecture and vendor decision tree: Safe vs. ZeroDev vs. Biconomy, CCTP vs. Across, Flashbots MEV-Share vs. dedicated builder relationships. Everything an operator needs to build the stack, not just understand the concepts.