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Revives the dormant HTTP 402 “Payment Required” status so an API or agent can charge per call with stablecoins — pay-as-you-go settlement for machine-to-machine work. Scope: Agent ↔ API · pay-per-call. The key architectural concerns are the boundary being bridged, the trust model and interoperability with existing agents and services.
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Concrete cog in the system that works inside larger relationships.
x402 is an open protocol that lets HTTP services request payment and release a resource after payment succeeds.
x402 was introduced as an open protocol by Coinbase in 2025. It revives the unused HTTP 402 Payment Required pattern to enable machine-readable payments for APIs and agents without a conventional account or subscription barrier.
A service first responds with 402 and payment requirements. The client signs the payment and retries with proof, while a facilitator verifies the transaction before the service returns the resource.
A response describing amount, network, and payment address.
A service that verifies payment proof or coordinates settlement.
Mechanisms preventing a payment proof from being reused.
x402 can make individual API calls and agent services directly billable; volatility, fees, fraud, and regulatory questions must be assessed for each use.
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