x402
A scheme for paying over HTTP: the server answers 402 with what it wants, the client pays, and retries the same request with the receipt attached.
a protocol · the vocabulary
402 paymentsagent payments
Proposed by Coinbase in 2025 and built on stablecoin settlement, though the design does not require any particular rail. It exists because agents cannot sign up, enter a card and wait for an invoice: they need to buy one call, now, without an account. Whether it becomes the way agents pay is unsettled; that the problem is real is not.