x402 Negotiation Standard

The x402 standard defines how MachPay Agents and Gateways negotiate payments over plain HTTP before invoking the Optimistic settlement layer.[file:///Users/abhishektomar/Desktop/git/machpay-docs/whitepaper/machpay_whitepaper.pdf]

Three-Step Handshake

1. Request → 402 Payment Required

   • Agent calls the vendor endpoint.

   • Gateway responds with status 402 and a JSON payload describing the offer plus a nonce challenge.

2. Agent Signs the Challenge

   • Agent serializes the payload into the PaymentIntent structure defined in the whitepaper (EIP-712 typed data).

   • Using its private key, the agent signs the challenge and caches the nonce to avoid double-use.

3. Request + Signature Header → 200 OK

   • Agent replays the request, attaching the signature in X-MachPay-Auth.

   • Gateway verifies the signature against the agent’s solvency bond and serves the resource (200 OK).

Challenge Payload

{
  "service_id": "weather.pro",
  "gateway_id": "0x1234abcd...",
  "price": "0.001",
  "currency": "USDC",
  "nonce": "ce4f3a40-07d1-4df2-9dc0-d2f62e87b6d3",
  "deadline": 1737168000,
  "chain_id": 84532,
  "token": "0xb53913...usdc",
  "metadata": {
    "city": "Reykjavik",
    "unit": "metric"
  }
}

This maps directly to the EIP-712 PaymentIntent fields (serviceId, amount, token, nonce, deadline).\n[file:///Users/abhishektomar/Desktop/git/machpay-docs/whitepaper/machpay_whitepaper.pdf]

Signed Header Example

X-MachPay-Auth: sig=0x9e1d...; agent=0xAaBbCc...; nonce=ce4f3a40-07d1-4df2-9dc0-d2f62e87b6d3

Gateways may optionally include a receipt identifier to help relayers construct Merkle leaves later.

Security: Nonce-Based Replay Protection

Every challenge includes a globally unique nonce. Once an agent signs and submits a nonce, the gateway marks it as consumed. If an attacker replays the same header, the gateway rejects it because the nonce no longer matches an open challenge. Combined with EIP-712 domain separation (chain ID, verifying contract), this prevents cross-service replay attacks and keeps fraudulent agents slashable.