@harpd/x402-logging-middleware

Drop-in x402 payment logging middleware for Node HTTP / Express apps. Every x402 interaction — a 402 Payment Required response, a client-attached X-Payment header, a settled transaction — is emitted as one structured, audit-friendly event. Zero runtime dependencies.

Part of Harpd's open-source agent-commerce toolkit. Pairs well with @harpd/agent-transaction-audit-schema (the event shape is compatible) and @harpd/observe (swap the sink to forward events to the Harpd collector).

Install

npm install @harpd/x402-logging-middleware

Express / Connect usage

import express from 'express'
import { createX402LoggingMiddleware } from '@harpd/x402-logging-middleware'

const app = express()
app.use(createX402LoggingMiddleware({ direction: 'inbound' }))

// A paid endpoint — the 402 + the eventual settlement both get logged.
app.get('/paid-resource', (req, res) => {
  res.setHeader('www-authenticate', JSON.stringify({
    accepts: [{ scheme: 'exact', asset: 'eip155:8453/erc20:0xUSDC', price: '10000', maxTimeoutSeconds: 60 }],
  }))
  res.status(402).end()
})

Every request through the middleware emits a JSON event:

{
  "middleware": "x402-logging-middleware",
  "version": "0.1.0",
  "direction": "inbound",
  "timestamp": "2026-07-29T...Z",
  "method": "GET",
  "path": "/paid-resource",
  "statusCode": 402,
  "asset": "eip155:8453/erc20:0xUSDC",
  "amountUsd": 0.01,
  "txHash": null,
  "x402": { "requirement": { "accepts": [ ... ] }, "payment": null, "paymentResponse": null }
}

Custom sink (forward to Harpd / your own store)

import { createX402LoggingMiddleware } from '@harpd/x402-logging-middleware'

const forwardToHarpd = (event) =>
  fetch('https://api.harpd.com/v1/events', {
    method: 'POST',
    headers: { authorization: `Bearer ${process.env.HARPD_KEY}` },
    body: JSON.stringify(event),
  })

app.use(createX402LoggingMiddleware({ sink: forwardToHarpd }))

Manual logging (clients, background jobs)

import { logX402Payment } from '@harpd/x402-logging-middleware'

logX402Payment({
  direction: 'outbound',
  resource: 'https://paid-api.example.com/v1/weather',
  asset: 'eip155:8453/erc20:0xUSDC',
  amountUsd: 0.01,
  txHash: '0xabc...',
  status: 'settled',
})

Why a middleware (not just console.log)

A payment audit trail that lives only in your app logs is fragile. This middleware gives you a single, normalized event for every x402 hop so the same record can feed dashboards, alerts, and compliance — see the audit schema package.

Further reading

• x402 vs MPP vs AP2: agent payment protocols compared — which protocol to build on (x402, Stripe MPP, Mastercard Agent Pay, Google AP2), and how they compose.
• Spending limits for x402 payments — per-agent budget control without holding funds or keys.

License

MIT © Harpd. Issues and PRs welcome at github.com/harpd-dev/x402-logging-middleware.

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About Harpd

Harpd is the AI Cost Intelligence platform for the agent era — measure, optimize and control production AI spend, from cost per successful task to agent-payment budgets (x402 / USDC).

• Website: https://harpd.com
• GitHub org: https://github.com/harpd-dev
• Contact: mailto:harpdsupport@gmail.com