@bursar/mpp

Robinhood Chain for the Machine Payments Protocol. MPP is the HTTP 402 payment protocol from Stripe and Tempo: a server answers 402 with a challenge, the client retries with a signed credential, the server returns a receipt. Its SDK, mppx, runs EIP-3009 stablecoin charges on any EVM chain it is told about. This package tells it about USDG on Robinhood Chain and points it at Bursar's facilitator, so a server holds no key and a payer holds no ETH.

One route serves both protocols. The 402 carries an MPP challenge and an x402 one, and the server takes either credential back: a client built on mppx and a client built on the reference x402 packages pay the same endpoint.

Charge agents

import express from 'express';
import { Mppx } from 'mppx/express';
import { usdgCharge } from '@bursar/mpp/server';

const mppx = Mppx.create({
  methods: [usdgCharge({ recipient: '0xYourAddress' })],
  secretKey: process.env.MPP_SECRET_KEY,
});

const app = express();
app.get('/quote', mppx.evm.charge({ amount: '0.01' }), (_request, response) => {
  response.json({ quote: 'the koi swims upstream' });
});

usdgCharge is mppx's evm.charge with the chain, the token and its signing domain filled in and Bursar's keyless facilitator behind it. Everything else evm.charge accepts, it accepts. mppx has the same middleware for Hono, Next.js and Elysia, and an MCP transport for paid tool calls.

Pay as an agent

import { Fetch } from 'mppx/client';
import { usdgPayer } from '@bursar/mpp/client';

const fetch = Fetch.from({ methods: [usdgPayer({ account, maxAmount: '1.00' })] });
const response = await fetch('https://api.example.com/quote');

The account signs an EIP-3009 authorization under USDG's domain; the server's facilitator broadcasts it and pays the gas. maxAmount is the most one charge may take, in dollars.

What is in it

| Export | What it is | |---|---| | USDG | USDG on Robinhood Chain as an mppx asset: address, six decimals, eip155:4663, and the EIP-712 domain Global Dollar version 1, proved against the token's own separator in @bursar/core. | | FACILITATOR | https://facilitator.bursar.world/x402, the keyless facilitator for the exact scheme in USDG. | | usdgCharge | The server method, from @bursar/mpp/server. | | usdgPayer | The client method, from @bursar/mpp/client. |

A runnable pair, server and client, is in examples/mpp-rhc.

Limits

The facilitator settles the exact scheme in USDG on Robinhood Chain and nothing else; an MPP charge in another token or on another chain goes to that chain's own facilitator. x402 route binding defaults to resource, which is what admits clients built on the reference x402 packages; pass x402: { routeBinding: 'required' } to serve mppx clients alone.