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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 in a stablecoin on the chain the 402 names, 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, and so is our use of it. Paid calls settle in USDC on Arc Mainnet, Base, Arbitrum One, Stellar, Algorand and Celo, and in USDG on Robinhood Chain; the Trust Oracle’s 120 settlements on X Layer, all from our own buyer wallet, were in USD₮0. Arc Mainnet leads: gas there is paid in USDC itself and finality is sub-second. Every 402 challenge states its own price, network and asset. What makes a payment count differs by rail and is stated, not blurred: on our EIP-3009, Stellar and Algorand rails we read the transfer back from the chain ourselves; a Circle Gateway nanopayment counts once Circle’s transfers API returns it; on Celo the facilitator Celo operates answers settled, and we publish the transaction hash it returns rather than re-reading it. Each rail’s evidence, transaction by transaction, is at a-identity.xyz/proof.

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Two rails, same standard

On Arc, 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. Then we added Circle Nanopayments as the gasless, batched rail alongside it. Both are live on Arc Mainnet, and on Arc testnet for rehearsal.
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.