Machine Payments Protocol: How AI Agents Pay Your API (2026)

The five-step 402 flow

MPP standardizes what an agent and a server say to each other when money is involved — the same HTTP status code browsers have ignored since the 1990s, finally given a workload. The protocol defines the challenge, the credential, and the receipt as HTTP headers, so it works over plain REST as well as MCP:

Source: the MPP protocol overview at mpp.dev and Stripe’s MPP documentation (docs.stripe.com/payments/machine/mpp). Challenges carry time-sensitive data and must be sent with Cache-Control: no-store; the spec lives as an IETF Internet-Draft, which is the standards-track path x402 never took.

Three intents: charge, session, subscription

The intent is what kind of payment you are asking for. A server can advertise several in one challenge response and let the agent choose:

MPP vs x402: same status code, different jobs

Both protocols revive HTTP 402 Payment Required. x402 came first (Coinbase open-sourced it in May 2025; the x402 Foundation took governance under the Linux Foundation in July 2026 with 40 members including Visa, Mastercard, Google, AWS, and Coinbase, and the ecosystem had processed roughly 165 million transactions across 69,000 active agents by April 2026 at a $0.028 median price). MPP arrived in March 2026 and covers the cases x402 deliberately left out:

The practical rule the protocols themselves publish: x402 for simple per-request stablecoin payments to crypto-native agents; MPP when you need multiple payment methods on one endpoint, sessions for high-frequency metered usage, fiat buyers without wallets, or an existing Stripe account you want the money to land in. And they are not rivals: MPP is backward-compatible with x402 — the mppx SDK emits both challenges on the same 402 response and accepts either credential, so one route can serve both agent populations.

Charge for an MCP tool call in a handler

Stripe’s Node SDK path is one middleware call. The handler returns the 402 challenge when unpaid and the resource with a receipt when paid — everything else (wallet plumbing, settlement, dashboard entries) is Stripe’s:

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

const mppx = Mppx.create({
  methods: stripeMachinePayments.defaultMethods(), // SPT cards + Tempo stablecoins
  secretKey: process.env.MPP_SECRET_KEY,
});

export async function handler(request) {
  const response = await mppx.charge({ amount: '0.50' })(request);
  if (response.status === 402) return response.challenge;   // unpaid: price + how to pay
  return response.withReceipt(Response.json({ data: '...' })); // paid: resource + receipt
}

Where the money lands — and how agents find you

MPP payments record through POST /v1/payment_intents and settle into your existing Stripe balance, in your default currency, on your standard payout schedule — no new billing system, no separate crypto treasury. Discovery runs through Stripe Directory, launched June 22 2026 as a CLI-first, structured search registry: stripe directory search "web scraping api" --format json --mcp-supported returns machine-readable listings — display name, description, URL, profile slugs, published apps, and a machine-payments flag — with a ranking boost for listings that expose MCP or MPP endpoints. Stripe opened the search to login-free use in July 2026, turning the directory into de-facto public infrastructure for agent procurement.

Cost structure, stated plainly: MPP is an open protocol, not a marketplace — there is no platform cut. You pay standard card and stablecoin processing on Stripe’s side, and Stripe can sponsor your customers’ Tempo network fees (private preview). That is the difference between the protocol layer and the marketplace layer: hosted MCP marketplaces like MCPize take 15–20% and hand you 80–85% monthly via Stripe Connect, directories and registries pay $0 and exist for discovery, and x402 rails settle per-call straight to your wallet with only on-chain gas between you and the money. Pick per rail per revenue stream — the full fee math lives in the MCP monetization guide.

Be honest: what the others do better

x402 is the simpler protocol if your buyers are crypto-native agents making one-off calls: one signed USDC payment per request, zero protocol fees beyond gas, multi-language SDKs, and an ecosystem that has already settled tens of millions of transactions. Hosted marketplaces — MCPize, Apify Store — solve the problem MPP leaves open: they bring buyers. A per-call endpoint nobody has discovered earns nothing, while a marketplace listing gets you in front of browsing buyers on day one (at the 15–20% take). The Stripe stack’s edge is reach and trust: fiat cards for agents whose operators never touched a wallet, sessions for high-frequency metered billing, dispute and fraud tooling you already trust, and a directory anchored to verified payment relationships. The VibeFuse Marketplace plays the storefront layer those options do not: an open-source marketplace where buyers browse, install, and auto-update your work inside a running harness — and you keep a flat, documented 80% of every sale.

Marketplace vs payment protocol: they compose

The 2026 creator mistake is treating these as competing choices. A marketplace is storefront and distribution; a payment protocol is metering and checkout. The monetization stack that actually earns looks like this:

Creator-side deep dives: the agent marketplace fee comparison, why the open-source marketplace pays differently, and the widget monetization playbook.