x402 integration

Sigil meters paid tools with x402 v2. An unpaid call receives HTTP 402 Payment Required with a base64-encoded PAYMENT-REQUIRED header. The v1 JSON body remains available for older buyers. Both forms offer EIP‑3009 first and Permit2 as a fallback.

The challenge

Decoded from the PAYMENT-REQUIRED header:

{
  "x402Version": 2,
  "error": "PAYMENT-SIGNATURE header is required",
  "resource": {
    "url": "https://sigil-asp.archonaudit.xyz/mcp",
    "description": "Sigil verdict — sigil_screen_transaction",
    "mimeType": "application/json",
    "serviceName": "Sigil"
  },
  "accepts": [{
    "scheme": "exact",
    "network": "eip155:196",
    "amount": "20000",
    "asset": "0x779Ded0c9e1022225f8E0630b35a9b54bE713736",
    "payTo": "0xc9955733F8f16872a6c8DF375E9D13d79A43a4dc",
    "maxTimeoutSeconds": 300,
    "extra": {
      "assetTransferMethod": "eip3009",
      "name": "USD₮0",
      "version": "1",
      "decimals": 6,
      "assetSymbol": "USDT0"
    }
  }, {
    "scheme": "exact",
    "network": "eip155:196",
    "amount": "20000",
    "asset": "0x779Ded0c9e1022225f8E0630b35a9b54bE713736",
    "payTo": "0xc9955733F8f16872a6c8DF375E9D13d79A43a4dc",
    "maxTimeoutSeconds": 300,
    "extra": {
      "assetTransferMethod": "permit2",
      "decimals": 6,
      "assetSymbol": "USDT0"
    }
  }]
}

• network — CAIP-2 eip155:196 (X Layer mainnet; gas token OKB).
• amount — atomic token units. 20000 at 6 decimals is 0.02 USD₮0.
• asset — live X Layer USD₮0 0x779Ded…713736; the older bridged token is not used.
• scheme exact — the signed EIP‑3009 or Permit2 authorization must equal the advertised amount and recipient exactly.
• EIP‑3009 first — compatible with the OKX task-402-pay buyer and settled through USD₮0's native transferWithAuthorization.
• Permit2 fallback — retained for existing x402 clients that already use the OKX facilitator.

Paying and retrying

1. Decode PAYMENT-REQUIRED from the 402 response header.
2. Select the first supported requirement. The OKX marketplace buyer selects exact + EIP‑3009 and signs the USD₮0 authorization automatically; Permit2-capable clients may select the fallback.
3. Retry the identical POST body with the base64 payment payload:

POST /mcp
Accept: application/json, text/event-stream
PAYMENT-SIGNATURE: <base64 x402 v2 payment payload>

Sigil verifies the selected rail, token, exact amount, recipient, validity window, nonce, and signature. EIP‑3009 settles through USD₮0's native transferWithAuthorization; the fallback settles through the OKX Permit2 facilitator. Only then does Sigil execute the MCP tool. A successful on-chain settlement returns a base64 PAYMENT-RESPONSE receipt header containing the transaction hash, network, payer, and amount.

The registered endpoint is dual-transport for buyer interoperability. Native MCP clients send tools/call; OKX marketplace task buyers may send the business arguments as plain JSON. Both receive the same challenge before payment. The paid plain-JSON replay returns the sealed verdict directly, and malformed input is rejected before settlement.

Verify the public endpoint

onchainos agent x402-check \
  --endpoint https://sigil-asp.archonaudit.xyz/mcp \
  --agent-id 4943

The current endpoint reports valid: true, x402 version 2, 20,000 atomic USD₮0, and eip155:196, with EIP‑3009 at accepts index 0. A real OKX payment quote/pay replay settled tx 0x21a3e677a5edb568213572bf7e54a142aba018f68282b9c46636c23673adb4dc and returned sealed receipt sgl_01kxrq9mce3dpgj1am2dbj.