Circle published a formal USDC method specification for the Machine Payments Protocol on Monday, standardizing how AI agents and automated services settle payments in USDC across EVM-compatible blockchains and Solana.
The specification, posted at paymentauth.org/draft-usdc-charge-00.html, outlines how USDC payments can be initiated through the MPP payment challenge-response flow. It introduces three capabilities: a standardized payment interface for agents transacting in USDC across supported EVM chains and Solana, the first crosschain payment profile in MPP via Circle Gateway, and support for USDC-backed stablecoins beginning with USDCx on Stacks. Circle announced the spec Monday afternoon via its official account.
MPP, the Machine Payments Protocol, is an HTTP-native payment standard that revives the long-dormant HTTP 402 "Payment Required" status code. When an agent calls an MPP-enabled endpoint, the server returns a 402 challenge. The agent signs a USDC payment authorization using EIP-3009, an Ethereum standard for off-chain token transfer authorizations, and retries the request. Circle Gateway verifies the authorization and queues it for batched on-chain settlement. No API key is required; every request is attributable by wallet address and transaction hash.
