x402 Payment Protocol

PEAC integrates with HTTP 402-based payment protocols through two packages: @peac/rails-x402 for payment evidence extraction and @peac/adapter-x402 for evidence carrier transport. Together they enable payment receipts that are verifiable, portable, and transport-neutral.

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Install

Terminal

# Payment evidence extraction
pnpm add @peac/rails-x402

# Evidence carrier for x402 responses
pnpm add @peac/adapter-x402

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How it works

The x402 payment flow uses HTTP 402 (Payment Required) responses to negotiate payment before granting access. PEAC attaches evidence of the completed payment to a receipt.

x402 Payment Flow

Client -> Server: GET /api/data
Server -> Client: 402 Payment Required (payment details)
Client -> Payment: Complete payment
Client -> Server: GET /api/data + X-PAYMENT header
Server -> Client: 200 OK + PEAC-Receipt header

The receipt captures the settlement reference, amount, currency, and payment rail without storing sensitive payment details.

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Payment evidence

Extract the signed settlement receipt from x402 response headers:

x402-evidence.ts

import { extractReceiptFromHeaders } from '@peac/adapter-x402';

// x402 settlement responses carry the receipt in X-PAYMENT-RESPONSE
const receipt = extractReceiptFromHeaders(res.headers);

The extractReceiptFromHeaders function returns the upstream signed receipt, or null when no settlement receipt is present:

RawSignedReceipt

{
  format: 'jws',          // 'jws' or 'eip712'
  signature: '0xabc123...',
  // ...remaining upstream receipt fields as sent by the x402 facilitator
}

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Evidence carrier

The @peac/adapter-x402 package implements the Evidence Carrier Contract for x402 responses, placing receipt evidence in HTTP response headers.

x402-carrier.ts

import { toPeacCarrier, extractReceiptFromHeaders } from '@peac/adapter-x402';

// Build a PEAC evidence carrier from a signed receipt JWS, then send it
// in the PEAC-Receipt response header
const carrier = await toPeacCarrier(receiptJws);
res.setHeader('PEAC-Receipt', carrier.receipt_jws);

// Extract the signed receipt from an incoming x402 response
const receipt = extractReceiptFromHeaders(incomingResponse.headers);

Transport limits

Constraint	Value
Header size limit	8 KB
Header name	PEAC-Receipt
Format	Compact JWS

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Issuing a payment receipt

Wire 0.1

issue-x402-receipt-wire01.ts

import { issue } from '@peac/protocol';
import { extractReceiptFromHeaders } from '@peac/adapter-x402';

// Confirm the x402 settlement response carries a signed receipt before
// recording the observed payment as a PEAC record
const settlement = extractReceiptFromHeaders(res.headers);
if (!settlement) throw new Error('No x402 settlement receipt present');

const receipt = await issue({
  privateKey: process.env.PEAC_PRIVATE_KEY,
  kid: 'peac-2026-03',
  claims: {
    iss: 'https://api.example.com',
    aud: 'https://client.example.com',
    amt: '0.001',
    cur: 'USDC',
    rail: 'x402',
    reference: 'tx_abc123...',
  },
});

Interaction Record format

issue-x402-receipt-wire02.ts

import { issue } from '@peac/protocol';

const receipt = await issue({
  privateKey: process.env.PEAC_PRIVATE_KEY,
  kid: 'peac-2026-03',
  claims: {
    iss: 'https://api.example.com',
    kind: 'evidence',
    type: 'org.peacprotocol/payment',
    pillars: ['commerce'],
    extensions: {
      'org.peacprotocol/commerce': {
        payment_rail: 'x402',
        amount_minor: '1000',
        currency: 'USDC',
      },
    },
  },
});

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Supported payment methods

The x402 adapter works with any payment method supported by the x402 ecosystem:

Category	Examples
Stablecoins	USDC, USDT, DAI
Cryptocurrencies	ETH, BTC (via bridge)
Fiat wrappers	Stripe-backed x402

Category-first

PEAC extracts settlement evidence from the x402 response headers regardless of the underlying payment method. The same extractReceiptFromHeaders function handles all x402-compatible payment flows.

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Implementation examples

Express middleware with x402

express-x402.ts

import express from 'express';
import { peacMiddleware } from '@peac/middleware-express';

const app = express();

app.use(peacMiddleware({
  privateKey: process.env.PEAC_PRIVATE_KEY,
  kid: 'peac-2026-03',
  issuer: 'https://api.example.com',
}));

app.get('/api/data', (req, res) => {
  // x402 payment already completed at this point
  // PEAC middleware auto-attaches receipt to response
  res.json({ data: 'protected content' });
});

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Next steps

Payment Rails

All supported payment rails: MPP / paymentauth, x402, Stripe, Razorpay, card networks.

MPP / paymentauth

HTTP Auth payment challenges and receipts via draft-ryan-httpauth-payment. Complements x402.

Evidence Carrier

How payment evidence travels across HTTP, MCP, and A2A transports.

Express Middleware

Auto-attach receipts to Express.js responses.