MPP Payment Proxy

Before writing code

Fetch live docs:

1. Fetch https://www.npmjs.com/package/mppx for the proxy handler API and configuration
2. Web-search mppx proxy payment 402 existing API monetize for proxy implementation patterns
3. Web-search site:github.com stripe-samples machine-payments proxy for official proxy sample code
4. Fetch https://developers.cloudflare.com/agents/agentic-payments/mpp/ for Cloudflare Workers proxy patterns

Conceptual Architecture

What the Proxy Does

The MPP proxy wraps an existing upstream API with a payment gate, requiring clients to pay before their requests are forwarded. The upstream API is unmodified — it never sees the payment flow.

Client → MPP Proxy (402 challenge-response) → Upstream API
         ↑ payment gate                       ↑ unmodified

Use Cases

• API monetization — Charge for access to your own internal APIs
• Third-party reselling — Add payment layer on top of third-party APIs (with permission)
• API marketplace — Proxy multiple upstream APIs through a single payment gateway
• Rate-limiting replacement — Replace API key + rate limits with pay-per-use
• Value-added wrapper — Add caching, transformation, or enrichment on top of upstream APIs

Proxy Architecture

┌──────────────┐     ┌──────────────────────┐     ┌──────────────┐
│  AI Agent    │────→│    MPP Proxy          │────→│  Upstream    │
│  (Client)    │←────│  1. 402 Challenge     │←────│  API         │
│              │     │  2. Verify Payment    │     │  (Unmodified)│
│              │     │  3. Forward Request   │     │              │
│              │     │  4. Return + Receipt  │     │              │
└──────────────┘     └──────────────────────┘     └──────────────┘

mppx Proxy Handler

The mppx SDK provides a dedicated proxy handler:

import { Mppx, tempo } from 'mppx/server';

const mppx = Mppx.create({
  secretKey: process.env.MPP_SECRET_KEY,
  methods: [tempo.charge({ /* config */ })],
});

// Proxy all requests to upstream with payment gate
app.all('/api/*', mppx.charge({ amount: '100' }), async (c) => {
  const upstreamUrl = `https://upstream-api.com${c.req.path}`;
  const response = await fetch(upstreamUrl, {
    method: c.req.method,
    headers: c.req.headers,
    body: c.req.method !== 'GET' ? await c.req.text() : undefined,
  });
  return new Response(response.body, {
    status: response.status,
    headers: response.headers,
  });
});

Cloudflare Workers Proxy

For edge-deployed proxies:

// Cloudflare Workers + MPP proxy pattern
export default {
  async fetch(request, env) {
    // MPP middleware handles 402 challenge-response
    // On successful payment, forwards to upstream
  }
};

Pricing Strategies

Request/Response Handling

• Headers — Forward client headers to upstream (strip Authorization: Payment)
• Body — Forward request body unchanged
• Response — Add Payment-Receipt header to upstream response
• Errors — Distinguish between payment errors (402) and upstream errors (pass through)

Multi-Upstream Proxy

Route to different upstreams based on the path:

const upstreams = {
  '/ai/': 'https://api.openai.com',
  '/data/': 'https://api.data-vendor.com',
  '/search/': 'https://api.search-engine.com',
};

Best Practices

• Strip payment headers before forwarding to upstream
• Add service discovery (/openapi.json) describing proxied endpoints
• Implement caching for idempotent GET requests to reduce upstream calls
• Add response transformation if needed (but keep it minimal)
• Monitor upstream availability and return appropriate errors
• Set up health checks that verify both proxy and upstream availability
• Consider latency — the proxy adds the payment negotiation round-trip

Fetch the latest mppx proxy documentation and Cloudflare MPP integration guide for exact proxy API, configuration options, and edge deployment patterns before implementing.