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 and HTTP 402
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.
The protocol was developed by engineers from Tempo Labs and Stripe and is formally proposed to the IETF. Circle’s entry adds $USDC as a named method type alongside existing MPP charge specs covering card, EVM, Lightning, Solana, Stellar, and Stripe.
USDCx and Stacks
The spec’s third capability adds USDCx, a $USDC-backed stablecoin on Stacks, Bitcoin’s leading Layer 2 network, to the list of assets agents can spend within the MPP framework. USDCx is issued by Stacks and fully backed by $USDC held in Circle xReserve, which provides cryptographic attestations for deposits and minting. The inclusion extends MPP payment coverage to Bitcoin-layer settlement. No user figures, transaction volumes, or adoption metrics accompany the spec.
Agentic Payments Context
The Circle spec lands on the same day 0x Protocol enabled autonomous agent payments on its Swap API, letting AI agents pay $0.01 per swap request in $USDC via x402 or MPP without API credentials.
The release extends a wave of agentic-commerce infrastructure. AWS wired Coinbase’s x402 protocol into CloudFront in June, letting AI agents pay for content in $USDC at the CDN edge. Coinbase launched standalone agent accounts that week. Mastercard opened its card rails to AI agents through its Agent Pay for Machines (AP4M) network with more than 30 crypto partners. Circle itself launched gas-free nanopayments across 11 EVM chains in April. The $USDC charge spec formalizes $USDC‘s role as a named payment method within MPP’s open standard for machine-to-machine commerce.
