Protocol guide · updated 2026-08-20What is MPP?
MPP — Machine Payments Protocol — is Stripe’s open protocol for AI agents to pay for APIs and content machine-to-machine. When an agent requests a paid resource, the server answers with HTTP 402 Payment Required and a WWW-Authenticate: Payment header describing what’s owed. The agent constructs and settles a payment — in USDC on Base — and retries with credentials attached. Same idea as x402, same settlement rail, different challenge format and a different company behind it.
Stripe launched MPP together with Tempo in 2026, and it moved to the center of the agent-payments conversation in August 2026 after Stripe’s reported $7B+ acquisition of OpenRouter put one company in control of both a major model-routing layer and a settlement rail. Read our full take: three different people just started writing the rules for how agents pay →
Why trust this page: ForgeMesh Labs runs its entire fleet of paid endpoints dual-stack — answering both x402 and MPP on the same 402 — and ran the first independent health census of MPP adoption across the x402 Bazaar. This is written from first-hand operation, not a summary of a press release.
How does an MPP payment actually flow?
Like x402, an MPP payment is a four-step HTTP exchange that completes in a few seconds — but the challenge lives in a standard HTTP header rather than a custom envelope:
1. The agent requests a resource
A client sends an ordinary HTTP request to a paid endpoint, with no credentials of any kind.
2. The server answers 402 with a WWW-Authenticate: Payment challenge
The response carries HTTP 402 Payment Required plus a WWW-Authenticate: Payment header — the machine-readable instructions for what to pay and how.
3. The agent constructs and settles a payment
The client builds a payment from the challenge — an EIP-3009-style authorization — and settles it in USDC on Base, the same rail x402 uses.
4. The agent retries, the server verifies and serves
The client retries the original request with payment credentials attached. The server verifies settlement and returns the resource.
MPP vs x402
In practice these are complementary rails, not rivals. Both trigger on HTTP 402 and both settle in USDC on Base. What differs is who governs the spec, how a server advertises the challenge, and how agents find you in the first place.
| x402 | MPP | |
|---|---|---|
| Challenge format | Base64 payment-required header envelope, defined by the x402 spec | HTTP 402 plus a WWW-Authenticate: Payment header, following standard HTTP challenge conventions |
| Discovery | Bazaar catalog — crawlable, ~25,000 listings from 1,200+ sellers | No public catalog — distribution is SDK-embedded, built directly into agent frameworks |
| Governance | x402 Foundation under the Linux Foundation, 40 member organizations | Stripe |
| Settlement | USDC on Base (and other networks), atomic and on-chain | USDC on Base — the same settlement rail x402 uses |
| How it reaches sellers | Open spec — any facilitator or server can implement it independently | Ships inside Stripe and Tempo tooling, and the agent-framework SDKs Stripe integrates with |
The practical answer is dual-stack. A server can answer both challenge types on the same 402 response — one endpoint reaching agents built for either protocol, with no separate infrastructure to run. Compare the full picture, including API-key alternatives, on our x402 guide →
How much of the x402 ecosystem already speaks MPP?
On August 19, 2026 we ran the first independent health census of every seller in the x402 Bazaar catalog — probing one live endpoint per seller, all 1,225 of them, for exactly this question. Nobody had measured dual-stack adoption before.
180 of 1,225 x402 Bazaar sellers already answer both an x402 envelope and an MPP WWW-Authenticate: Payment challenge on the same 402 response
Rail Pulse on the ForgeMesh homepage tracks MPP transaction and volume snapshots every day, alongside our x402 catalog crawl
Full methodology and the rest of the census — dead listings, unenforced paywalls, and more — in We health-checked every seller in the x402 Bazaar → For public MPP rail data outside our own crawl, see MPPscan, the public dashboard for MPP transaction and volume activity, and our Rail Pulse tracker.
Make your endpoint speak MPP
The fastest way to know where you stand is to test the endpoint you already run. Our free scanner checks any URL for the WWW-Authenticate: Payment header and tells you whether it’s MPP-ready, x402-ready, both, or neither.
Technical Build package · $49
Knowing the four steps is not the same as shipping them. Adding a valid WWW-Authenticate: Payment challenge alongside your existing x402 envelope — with correct EIP-3009 settlement, nonce and validity handling, and a facilitator that verifies both rails — is where sellers stall. Our Technical Build package ships the dual-stack middleware, the settlement wiring, and the exact envelope both stock clients accept, so one endpoint reaches both agent populations. We built our own fleet this way.
Our own fleet runs dual-stack across every endpoint precisely so we don’t have to guess which rail wins. Read what is x402? for the other half of the story, or subscribe to The Brief for the next move in agent payments before it’s old news.
MPP: frequently asked questions
What is MPP in one sentence?
MPP — Machine Payments Protocol — is an open protocol, launched by Stripe together with Tempo in 2026, that lets AI agents pay for APIs and content machine-to-machine: a server challenges an unpaid request with HTTP 402 and a WWW-Authenticate: Payment header, the agent constructs and settles a payment, and retries with credentials.
Who created MPP?
Stripe, working with Tempo, launched MPP in 2026 as an open protocol for agent-native payments. It moved to the center of the conversation in August 2026 after Stripe’s reported $7B+ acquisition of OpenRouter put the same company in control of both a major model-routing layer and a settlement rail.
How does an MPP payment actually work?
An agent requests a resource with no credentials. The server answers 402 Payment Required with a WWW-Authenticate: Payment header describing what’s owed. The agent constructs a payment from that challenge — an EIP-3009-style authorization — settles it in USDC on Base, and retries the same request with payment credentials attached. The server verifies settlement and returns the resource.
Is MPP the same as x402?
No, but they’re close cousins rather than rivals in practice. Both trigger on HTTP 402 and both settle in USDC on Base — the same rail. They differ in challenge format and governance: x402 uses a base64 payment-required header envelope with Bazaar catalog discovery, under Linux Foundation governance since July 2026. MPP uses a WWW-Authenticate: Payment header and is pushed by Stripe directly into agent-framework SDKs, with no public discovery catalog.
What does "dual-stack" mean?
A server that answers a single 402 response with both an x402 payment envelope and an MPP WWW-Authenticate: Payment challenge in the same reply. One endpoint, two protocols — it reaches agents built for either rail without maintaining two separate services. In our August 2026 census of all 1,225 x402 Bazaar sellers, 15% (180 sellers) already do this.
Do I need a Stripe account to receive MPP payments?
MPP is Stripe-managed on the seller side — receiving payments over the rail runs through a Stripe relationship, unlike x402, where a funded wallet alone is enough to receive USDC. That’s the practical trade-off: x402 has no account requirement, MPP has Stripe’s infrastructure and distribution behind it.
How many x402 sellers already support MPP too?
In our August 19, 2026 health census of every seller in the x402 Bazaar catalog, 180 of 1,225 sellers — 15% — answered with both an x402 envelope and an MPP challenge on the same 402 response. Nobody was measuring dual-stack adoption before that census; we were the first to check.
Where can I see live MPP rail activity?
MPPscan (mppscan.com) is the public dashboard for MPP transaction and volume data. ForgeMesh’s own Rail Pulse tracks MPP transaction and volume snapshots daily, alongside our x402 Bazaar catalog crawl, on the ForgeMesh homepage.
Go deeper: what is x402? · the dual-stack census · ForgeMesh blog