MPP Example

Minimal implementation of the Machine Payments Protocol (MPP) charge intent.

Built to understand MPP from the inside. Implements both sides: a merchant server and an agent client.

What's here

• src/server.ts — MPP merchant (TicketShop), selling TOKEN2049 VIP passes via charge intent
• src/client.ts — AI agent that discovers payment methods and submits a charge
• src/types.ts — MPP types (ChargeRequest, AcceptPayment, etc.)
• OBSERVATIONS.md — What we learned implementing this from scratch

Run it

npm install

# Terminal 1: start the merchant server
npm run server

# Terminal 2: run the agent
npm run client

Key observations

See OBSERVATIONS.md for the full findings. The most important:

MPP is the simplest direct payment protocol. Two HTTP calls: discover payment methods (Accept-Payment header), submit a charge intent. No session, no cart, no Allowance. The tradeoff: no per-transaction spending constraints at the protocol level.

The cross-merchant budget gap persists. MPP has no budget enforcement mechanism. An agent with a charge intent can pay any amount to any recipient. Budget tracking is entirely external to the protocol.

Protocol

MPP defines payment intents (charge, authorize, subscription, stream) over HTTP. This implementation covers the charge intent — the simplest: pay and get a receipt.

Compare with acp-example (REST checkout), ucp-example (MCP checkout), ap2-example (authorization), and x402-example (pay-per-request).