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x402 is an open payment standard from Coinbase and the x402 Foundation. It revives the HTTP 402 status code so a client can pay for a resource in the same request it asks for it. No accounts, no API keys by default. A-Identity ships a real x402 rail: the server answers an unpaid request with 402 + machine-readable payment requirements, the client pays USDC on Arc, and the server verifies that payment on-chain (with replay protection) before serving the resource.

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How One Payment Works

1

Request

An agent requests a paid resource.
2

402 Payment Required

The server answers with status 402 and the payment terms.
3

Pay And Retry

The agent signs a stablecoin transfer, attaches the proof, and sends the request again. The server verifies and returns the resource.
Each 402 carries a single-use, resource-scoped nonce; the redemption must echo it. So a payment is bound to the request that quoted it — an unrelated or stockpiled USDC transfer can’t blind-unlock the resource, and every proof is spendable exactly once (replay protection persisted across restarts).

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Settlement

The x402 standard is asset- and network-agnostic, but A-Identity settles every x402 call in USDC on Circle Arc — the same dollar the rest of the platform uses, where gas is paid in USDC and finality is sub-second. Each call costs a real sub-cent amount (0.001 USDC in the shipped demo), verified as an on-chain USDC transfer before the resource is served.

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Two rails, same standard

A-Identity ships two x402 rails and labels which is which:
  1. On-chain, self-verifying (this page) — each call settles as a real USDC transfer on Arc, verified on-chain with replay protection and a request nonce. Provable, standalone, no hosted meter.
  2. Circle Nanopayments — the same 402 negotiation over Circle Gateway’s batched scheme: the buyer signs an EIP-3009 authorization off-chain (zero gas) and Gateway batches the on-chain settlement, making sub-cent streams economical.
We chose the open x402 standard because it is self-verifying — we can prove settlement on-chain rather than trust a hosted meter — and then added Circle Nanopayments as the gasless, batched rail alongside it. Both are live on Arc testnet.
The x402-mcp library puts an x402 paywall in front of a Model Context Protocol tool. An agent discovers the tool, pays, and calls it, all over HTTP.

Build A Paid Tool

See the SDK reference for a paid tool in a few lines.