What Is MPP? The Machine Payments Protocol for AI Agents

MPP (the Machine Payments Protocol) is an open standard, co-authored by Stripe and Tempo, that lets an AI agent pay for a service inside a single HTTP request. Like x402, it revives the HTTP 402 status code. But MPP adds a session model and multiple payment rails. An agent authorizes once, then pays across stablecoins, cards and more with no human at the checkout.

What is MPP (the Machine Payments Protocol)?

MPP is an open, IETF-proposed standard co-authored by Stripe and Tempo that turns the HTTP 402 "Payment Required" status code into a machine-to-machine payment handshake. It launched in March 2026. An agent calls a priced endpoint, and MPP completes the payment inside that request, no account and no human at the keyboard.

Picture an agent pulling data from an API it's never used. The server answers with a price. The agent pays over MPP and gets the data back, all in one exchange.

Two things set MPP apart from a plain 402 payment: the session and the rails. Both are covered next. For the open spec, see mpp.dev.

How does MPP work? The session model

MPP's model has two moving parts: a session that sets the spending rules once, and a Shared Payment Token (SPT) that carries those rules to each merchant. Together they replace x402's sign-every-call flow, which is what lets MPP handle high-frequency, recurring and streaming payments, not just one-shot calls.

For the full step-by-step payment handshake, see Payments for AI Agents. This section stays on what makes MPP's model different.

Sessions: authorize once, pay continuously

A session is a signed permission with three limits: a spending cap, an expiry and a scope of what it covers. The agent signs it once, then spends inside those bounds with no fresh signature per call. Each call accumulates as an off-chain voucher, and only the net total settles on-chain, periodically. Avoiding both a signature and an on-chain transaction on every call is what makes sub-cent micropayments viable.

Because that authorization persists, recurring subscriptions and streaming usage fall out of it naturally. An agent can hold an ongoing plan and pay continuously, as long as it stays under the cap. To hold a session and its funds, an agent needs its own wallet.

Shared Payment Tokens

A Shared Payment Token is the scoped, time-limited credential an agent spends instead of your real card or wallet keys. It carries the session's authorization across rails, and it's what a merchant sees, never your underlying details.

It works like a virtual card number. Your bank can issue a stand-in number that completes a purchase but caps the exposure and hides your real account. An SPT does the same for an agent: the merchant charges a token scoped to the session's limits and never sees the wallet or card behind it. That indirection is what lets MPP reach card and buy-now-pay-later rails without ever exposing raw card numbers.

Which payment rails does MPP support?

MPP is multi-rail. Through SPTs it can move stablecoins, cards, buy-now-pay-later and Bitcoin. x402, by contrast, settles stablecoins only. One session authorization spends across any of them.

This is where MPP's design really diverges from a pure crypto rail. Agentic commerce isn't only stablecoins. Stablecoins came first because that's what x402 and MPP shipped on, but MPP is built to be far more flexible than a single-rail protocol.

Say your agent needs to buy a $200 dataset. It can put that on a card rail through MPP, while paying two cents per API call in stablecoins on the same session.

How is MPP different from x402?

MPP and x402 differ on three axes. x402 settles a stablecoin payment on-chain every call; MPP signs one session up front and settles the batch. x402 is stablecoin-only; MPP is multi-rail. x402 is per-call only; MPP adds recurring and streaming.

Every comparison post frames this as two paths, pick a side. That's the wrong frame. MPP is built to be compatible with x402, so x402's exact-payment flows map onto MPP's charge mechanism, and many services run both.

For the full x402 vs MPP vs card-networks side-by-side, see What Is x402?.

Who built MPP, and how widely is it used?

Stripe and Tempo co-authored MPP and launched it in March 2026, alongside Tempo's mainnet. Tempo is a payment-tuned Layer 1 blockchain. Early backers span payments and AI: Visa and Mastercard, Shopify, Anthropic and OpenAI. More than 50 services went live in the first week.

Two payment powerhouses now sit behind these agent rails. Coinbase built x402. Stripe built MPP.

Here's the proprietary read. mppscan, AgentCash's MPP explorer, shows 4,836 services accept MPP. That's across 1.93 million transactions from about 156,000 paying agents, settling roughly $277K on Tempo.

Now the honest part. MPP is far younger and smaller than x402. x402scan, AgentCash's x402 explorer, shows x402 at 242,481 services. This is early breadth, not scale. The supply side is real, but MPP only launched in March 2026, so treat these numbers as a starting line. For the x402 side of that comparison, see What Is x402?.

Payment giants moving into agent rails is a tailwind, not a threat. A bigger wave lifts the smaller ones riding it, and none of this works without the rails these companies are laying. AgentCash speaks both MPP and x402, so an agent doesn't have to pick a protocol before it can pay. Give an agent a balance and let it pay for one real call: you can get started.