x402 v2
x402 extends HTTP 402 Payment Required with machine-readable payment requirements. A protected service advertises the network, asset, amount, recipient, scheme, and authorization window; the client then retries with a signed payment payload.
MPP
MPP standardizes machine-to-machine payments through the HTTP authentication model. The server sends a WWW-Authenticate: Payment challenge, and a successful response can include a Payment-Receipt for downstream reconciliation.
EIP-712
EIP-712 defines typed structured-data hashing and signing. Wallets can present the payment fields to the signer in a deterministic domain, reducing ambiguity compared with opaque message signing.
ERC-3009
ERC-3009 TransferWithAuthorization enables a token holder to authorize a transfer using an offchain signature. The authorization binds the source, destination, value, validity window, and nonce; the Facilitator submits it onchain and pays the network fee.
Network and settlement asset
Each XAPG service exposes exactly one isolated Base environment and its canonical USDC contract. Clients must read the active CAIP-2 network and asset address from GET /supported instead of assuming Testnet or Mainnet.