---
name: mpp-client-fetch
description: >
  Implement MPP client-side payment handling with mppx.fetch(). Use when
  building AI agents or automated clients that consume 402-protected APIs,
  handling the transparent challenge-response payment flow.
---

# MPP Client — mppx.fetch()

## Before writing code

**Fetch live docs**:
1. Fetch `https://www.npmjs.com/package/mppx` for the current client API and wallet configuration
2. Web-search `mppx client fetch payment wallet configuration` for client-side setup patterns
3. Web-search `site:github.com stripe-samples machine-payments client` for official client sample code
4. Fetch `https://mpp.dev/overview` for the client-side protocol flow description

## Conceptual Architecture

### What mppx.fetch() Does

`mppx.fetch()` is a drop-in replacement for the standard `fetch()` API that transparently handles the full HTTP 402 payment flow:

1. **Request** — Client sends a normal HTTP request
2. **402 Detection** — If server responds with 402, the client parses the `WWW-Authenticate: Payment` challenge
3. **Payment Fulfillment** — Client automatically pays using the configured wallet/payment method
4. **Credential Submission** — Client retries the request with `Authorization: Payment` credential
5. **Receipt Capture** — Client captures the `Payment-Receipt` header from the response
6. **Transparent Return** — Returns the final 200 response as if the payment never happened

The developer's code sees only the final successful response — all payment negotiation is handled internally.

### Client Configuration

```typescript
import { mppx } from 'mppx/client';

// Configure once
const client = mppx.create({
  wallet: {
    privateKey: process.env.WALLET_PRIVATE_KEY,  // For Tempo/crypto payments
  },
  // OR for Stripe SPT payments:
  stripe: {
    sharedPaymentToken: sptToken,  // Obtained from SPT provisioning
  },
});

// Use like fetch()
const response = await client.fetch('https://api.example.com/paid-resource');
const data = await response.json();
```

### Payment Method Selection

When a server supports multiple payment methods, the client selects based on:
- Configured wallet capabilities
- Method preference order
- Available balance (for crypto methods)
- SPT availability (for Stripe methods)

### Spending Controls

Clients should implement spending controls:
- **Per-request limit** — Maximum amount for a single payment
- **Session budget** — Maximum total spend per session
- **Daily/hourly caps** — Rate-based spending limits
- **Allowlisted domains** — Only pay specific services

### Receipt Handling

After a successful payment, the `Payment-Receipt` header contains:
- Payment status
- Method used
- Timestamp
- Reference identifier

Clients should log receipts for accounting and dispute resolution.

### Error Handling

| Scenario | Behavior |
|----------|----------|
| Insufficient balance | Throw error with details |
| Unsupported payment method | Throw error listing supported methods |
| Challenge expired | Retry with fresh request |
| Network error during payment | Do not retry payment (risk of double-pay) |
| Server returns non-402 error | Pass through as normal fetch error |

### CLI Usage

The `mppx` CLI provides a quick way to test payments:

```bash
npx mppx https://api.example.com/paid-resource
```

### Multi-Method Client

A client can be configured with multiple payment methods:

```typescript
const client = mppx.create({
  methods: [
    { type: 'tempo', wallet: { privateKey: process.env.WALLET_KEY } },
    { type: 'stripe', spt: { token: sptToken } },
  ],
  preference: ['tempo', 'stripe'],  // Try Tempo first, fall back to Stripe
});
```

### Best Practices

- Never hardcode wallet private keys or SPTs — use environment variables
- Implement spending limits to prevent runaway costs
- Log all payments with amounts and service URLs for auditing
- Handle payment failures gracefully — distinguish between "can't pay" and "won't pay"
- Cache successful payment sessions where the protocol supports it
- Implement circuit breakers for services that consistently fail

Fetch the latest mppx client documentation for exact configuration options, wallet setup, and error types before implementing.


---
Source attribution: OrcaQubits
Original project: https://github.com/OrcaQubits/agentic-commerce-skills-plugins/blob/30cd25c74c8aa7dc49b3ec4847d463f6104439f6/dist/codex/stripe-mpp/skills/mpp-client-fetch/SKILL.md
License: MIT
License information: https://opensource.org/license/mit
Original upstream instructions; downloaded from Mega AI Hub.

## Upstream license notice

MIT License

Copyright (c) 2024-2026 OrcaQubits AI (https://orcaqubits-ai.com)

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.



# Mega AI Hub resource metadata

Creator / publisher: OrcaQubits
Original project: https://github.com/OrcaQubits/agentic-commerce-skills-plugins/blob/30cd25c74c8aa7dc49b3ec4847d463f6104439f6/dist/codex/stripe-mpp/skills/mpp-client-fetch/SKILL.md
Attribution basis: Source repository record

Fields of work: Development & Engineering, Marketing, Sales
Roles: Backend Developer, Customer Success Manager, Growth Marketer, Product Marketer, Sales Engineer
Specialties: API, Backend
Tools: Mppx, Stripe
Country applicability: Country not specified
Classification basis: Source catalog labels
Instruction SHA-256: c8ea919e164283ee43dc64c3189f0e03db88a2f24215df9f926df1951284bfce