@reckon402/buyer-sdk

Buyer-side helpers for the Reckon402 x402 v2 flow.

Exports

• signPayment(input) — sign an EIP-3009 TransferWithAuthorization against USDC's EIP-712 domain and build the full PaymentPayload. Returns { payload, paymentId, requirements, authorization }.
• encodeXPaymentHeader(payload) — base64-JSON encode for the PAYMENT-SIGNATURE header.
• decodeXPaymentResponse(headerValue) — parse a PAYMENT-RESPONSE header back into a SettlementResponse (with any Reckon402 X35 extensions carried through as extra fields).
• computePaymentId(authorization) — deterministic keccak256(encodePacked(from, to, value, validAfter, validBefore, nonce)). The facilitator worker imports this same function to guarantee the buyer and the facilitator agree on the paymentId byte-for-byte (no cross-impl divergence is possible — single source of truth).

Usage

import { signPayment, encodeXPaymentHeader, decodeXPaymentResponse }
  from '@reckon402/buyer-sdk'

const { payload, paymentId } = await signPayment({
  privateKey: process.env.BUYER_PK as `0x${string}`,
  recipient: SPLITTER_ADDRESS,
  amount: '10000',                         // 0.01 USDC
  network: 'eip155:84532',
  usdcAddress: '0x036CbD53842c5426634e7929541eC2318f3dCF7e',
  chainId: 84532,
})

const headerValue = encodeXPaymentHeader(payload)
const res = await fetch('https://agent.reckon402.com/research?q=test', {
  headers: { 'PAYMENT-SIGNATURE': headerValue },
})

const settlement = decodeXPaymentResponse(res.headers.get('PAYMENT-RESPONSE')!)
console.log(settlement.transaction, settlement.paymentId)  // on-chain tx + X35 id

No runtime deps beyond viem and @reckon402/types.