Process x402 agent payments

npm start

// Per-payment cap. Pair with a Dfns Wallets:Sign policy for server-side enforcement.
const MAX_PAYMENT_AMOUNT = 5_000_000n; // 5.00 USDC

async signPayment(req: PaymentRequirement): Promise<PaymentSignature> {
  if (BigInt(req.amount) > MAX_PAYMENT_AMOUNT) {
    throw new Error(`Payment exceeds per-payment cap of ${MAX_PAYMENT_AMOUNT}`);
  }

  const message = {
    from: ethers.getAddress(this.customerWalletAddress),
    to: ethers.getAddress(req.recipient),
    value: req.amount,
    validAfter: req.validAfter,
    validBefore: req.validBefore,
    nonce: req.nonce,
  };

  let sigResult = await this.dfnsApi.wallets.generateSignature({
    walletId: this.customerWalletId,
    body: {
      kind: 'Eip712',
      types: {
        ReceiveWithAuthorization: [
          { name: 'from', type: 'address' },
          { name: 'to', type: 'address' },
          { name: 'value', type: 'uint256' },
          { name: 'validAfter', type: 'uint256' },
          { name: 'validBefore', type: 'uint256' },
          { name: 'nonce', type: 'bytes32' },
        ],
      },
      domain: {
        name: 'USDC',
        version: '2',
        chainId: req.chainId,
        verifyingContract: ethers.getAddress(req.contract),
      },
      message,
    },
  });

  while (sigResult.status !== 'Signed') {
    if (sigResult.status === 'Failed' || sigResult.status === 'Rejected') {
      throw new Error(`Signature ${sigResult.status}: ${sigResult.reason}`);
    }
    await new Promise((r) => setTimeout(r, 2000));
    sigResult = await this.dfnsApi.wallets.getSignature({
      walletId: this.customerWalletId,
      signatureId: sigResult.id,
    });
  }

  const signedData = sigResult.signedData ?? sigResult.signature?.encoded;
  return { protocol: 'EIP-3009', signature: signedData!, message };
}

async settlePayment(contractAddress: string, payment: PaymentSignature): Promise<string> {
  const selector = '0xef55bec6'; // receiveWithAuthorization(...)
  const sig = ethers.Signature.from(payment.signature);

  const params = ethers.AbiCoder.defaultAbiCoder().encode(
    ['address', 'address', 'uint256', 'uint256', 'uint256', 'bytes32', 'uint8', 'bytes32', 'bytes32'],
    [
      payment.message.from,
      payment.message.to,
      payment.message.value,
      payment.message.validAfter,
      payment.message.validBefore,
      payment.message.nonce,
      sig.v, sig.r, sig.s,
    ],
  );

  const result = await this.dfnsApi.wallets.broadcastTransaction({
    walletId: this.merchantWalletId,
    body: {
      kind: 'Eip1559',
      to: contractAddress,
      data: selector + params.slice(2),
      value: '0',
      gasLimit: '200000',
      maxFeePerGas: '5000000000',
      maxPriorityFeePerGas: '1000000000',
    },
  });

  return result.txHash!;
}