Why agents need different rails
A payment processor assumes three things about its customer: it is a human, it buys occasionally at order-sized amounts, and it can complete an interactive checkout. AI agents violate all three. They are software, they buy thousands of times a day at cents per call, and the only interface they have is the API request itself.
x402 — the open protocol most of this site tracks — puts the payment inside that request: the server answers HTTP 402 with a price, the agent pays USDC from its wallet, and the response is served. The result is a genuine alternative to card rails for machine commerce, implemented by the open-source projects linked from each guide above. See the protocol categories for the full stack.