MPP Charge Flow (One-Time Payments)

Before writing code

Fetch live docs:

1. Fetch https://www.npmjs.com/package/mppx for the charge middleware API and configuration
2. Fetch https://paymentauth.org/ for the canonical charge intent specification
3. Web-search site:github.com stripe-samples machine-payments charge for charge flow sample code
4. Fetch https://docs.stripe.com/payments/machine/mpp for Stripe charge integration details

Conceptual Architecture

What Charge Intent Is

The charge intent implements immediate, per-request settlement. Each API call triggers a single payment. The flow is:

Client: GET /api/data
Server: 402 Payment Required
        WWW-Authenticate: Payment <challenge with intent="charge">
Client: Fulfills payment (on-chain tx or card charge)
Client: GET /api/data
        Authorization: Payment <credential with proof>
Server: 200 OK
        Payment-Receipt: <receipt>

When to Use Charge

• API monetization — Pay per call (e.g., $0.01 per request)
• Data access — Pay for each data query or download
• File downloads — Pay per file or per MB
• Model inference — Pay per inference call
• Fixed-price resources — Content behind a paywall

Server-Side Implementation

// Protect a route with a charge gate
app.get('/api/data', mppx.charge({ amount: '100' }), async (c) => {
  // Only reached after successful payment
  return c.json({ data: 'premium content' });
});

The amount is specified in the smallest unit of the payment method's currency.

Dynamic Pricing

For routes where the price depends on the request:

app.get('/api/data/:size', async (c, next) => {
  const size = c.req.param('size');
  const amount = calculatePrice(size);
  return mppx.charge({ amount: String(amount) })(c, next);
}, async (c) => {
  return c.json({ data: 'variable-price content' });
});

Challenge Lifecycle

1. Generation — Server creates challenge with unique ID, HMAC-bound to secret key
2. Delivery — Challenge sent in WWW-Authenticate header with 402 status
3. Expiration — Challenge is time-limited (configurable, typically minutes)
4. Fulfillment — Client pays and constructs credential
5. Verification — Server verifies payment proof and HMAC binding
6. Consumption — Challenge is consumed (single-use)

Amount Conventions

Payment Method	Unit	Example: $0.01
Tempo (USDC)	Smallest token unit	Verify in SDK docs
Stripe	Cents (minor currency unit)	100 (1 USD cent = 100)
Lightning	Millisatoshis	Varies

Always verify the exact unit convention in the SDK documentation for your payment method.

Error Scenarios

Scenario	Server Response
No payment header	402 with payment-required challenge
Payment amount too low	402 with verification-failed
Payment to wrong address	402 with verification-failed
Expired challenge	402 with payment-expired
Duplicate credential (replay)	402 with verification-failed
Successful payment	200 with Payment-Receipt

Best Practices

• Set amounts that reflect the actual value of the resource
• Use dynamic pricing for variable-cost resources (compute, bandwidth)
• Set reasonable challenge expiration times (long enough for payment settlement)
• Monitor payment success rates and adjust pricing if needed
• Provide clear pricing documentation in your service discovery

Fetch the latest mppx SDK docs and payment method documentation for exact charge configuration options and amount unit conventions before implementing.