Instead of sending a transaction, the payer signs an EIP-712 message authorizing a specific transfer — amount, recipient, a one-time nonce and a validity window. A relayer (in x402, the facilitator) submits transferWithAuthorization on-chain and covers gas. The payer never needs ETH.
USDC implements EIP-3009 natively, which is why x402's "exact" payment scheme uses it on EVM networks like Base — an agent holding only USDC can pay for an API call with a single signature. On Solana, the equivalent role is played by SPL-token TransferChecked instructions.
Because it makes payment a signature, not a transaction: the agent authorizes exactly one transfer and the facilitator handles gas and submission. That removes the need for the payer to hold native gas tokens and fits a single request-response cycle.