MPP (Machine Payments Protocol) is an open, IETF-track HTTP payment scheme: the server answers 402 with WWW-Authenticate: Payment, the client replies with Authorization: Payment, and a settled response carries a signed Payment-Receipt. Spec and tooling: tempoxyz/mpp · mpp.dev · client/server library mppx.

Every paid endpoint on Agent402.Tools is dual-stack: the same 402 carries an x402 offer and an MPP challenge. Same URL, same price - the buyer's client picks the wire. MPP is one of the two wires underneath Agentic Finance, agents that pay and get paid on their own; Agent402 is its applied layer.

The 402 lists the challenges in a fixed order: tempo (one per currency, USDC.e first), then evm (Base, then Celo), then stripe. A stock mppx client pays the first challenge it has a method for and does not fall back to the next one, so a wallet holding Tempo funds pays over Tempo. A client whose Tempo credential the server has just refused (an empty Tempo balance is the usual cause) sees the evm challenges first for a while, so it can pay on Base instead.

• evm is offered on every paid route.
• tempo is offered on every paid route except the identity-bound ones (wallet-keyed memory, usage history, attestations, feedback), whose identity is the signed EIP-3009 payer, and the long-running ones (report products, video, the fast and pro image tiers, seller payability checks), which can outlive a Tempo credential.
• stripe is offered on routes priced $0.50 or more, never on identity-bound routes.

Each paid operation in /openapi.json lists the same offers in the same order under x-payment-info.offers (method, currency, chain, amount in the currency's smallest unit), and /.well-known/x402 summarises them under mpp.

An evm/charge credential is an EIP-3009 TransferWithAuthorization signed under the token's EIP-712 domain, and that domain's name differs by chain: Base USDC is "USD Coin", while Celo, Monad and Sei USDC each report "USDC". Hardcoding one of them produces a signature no facilitator and no contract can accept on the chains that use the other, and it fails quietly, looking like a rejected payment rather than a wrong one.

There is nothing to guess: the challenge carries the verbatim x402 accepts entry it was minted from, and that entry's extra.name / extra.version are the domain to sign under, read from the token on chain.

If a credential arrives signed under a different known name, we recover the signer and recognise it before spending a facilitator round trip. The reply is a 402 with an RFC 9457 application/problem+json body naming both the name you signed under and the one the token uses. That response deliberately carries no WWW-Authenticate header, so a client whose manager prefers MPP has nothing to select and falls through to the x402 offer in the same 402, which the same wallet can pay. The hold is short, self-clearing, and never applied to a request that presents a credential.

The MPP marketplace lists other MPP sellers we can verify live and ranks them on the MPP leaderboard: inbound USDC.e transfers on Tempo to the recipient each seller's live challenge names, read from the chain by us over the most recent window (transfers, distinct payers, volume; rows at or above the router's floor are marked routable). Machine-readable at /api/mpp-index and /api/mpp-leaderboard. The transactions page shows every MPP-wire settlement per rail with explorer links, led by transaction counts (adoption) with external revenue underneath.

import { Mppx, tempo, evm } from "mppx/client";
import { privateKeyToAccount } from "viem/accounts";

const account = privateKeyToAccount(process.env.AGENT_KEY);
// Offer both methods; the client picks the one the 402 advertises that it can pay.
const client = Mppx.create({ methods: [tempo.charge({ account }), evm.charge({ account })] });

const res = await client.fetch("https://agent402.tools/api/uuid");
console.log(res.status, res.headers.get("payment-receipt"));
console.log(await res.json());

For evm you need USDC on Base or Celo in the paying wallet; for tempo you need USDC.e (or PathUSD) on Tempo mainnet. No API key, no signup: the wallet is the account.

The hosted connector at https://agent402.tools/mcp speaks MPP's MCP wire too: a wallet-only tool called through catalog.call (or a flagship such as web.search) answers with a readable isError tool result carrying the challenges in _meta["org.paymentauth/payment-required"] (-32043 when a presented credential was refused), the client retries with the credential in _meta["org.paymentauth/credential"], and the paid result carries _meta["org.paymentauth/receipt"]. mppx's McpClient.wrap over a stock MCP SDK client handles it. The connector replays the call as a loopback request to its own paid HTTP route, so the real gates verify and settle and the same invariants hold (src/mcp-mpp.js). See MCP Connector.

• evm: the Payment-Receipt reference is the on-chain tx hash on Base or Celo.
• tempo: the reference is the Tempo tx hash, viewable at https://explore.tempo.xyz/tx/<hash>. A rejected MPP credential answers 402 with fresh challenges and an RFC 9457 application/problem+json body naming the reason.
• stripe: the receipt references the Stripe PaymentIntent.
• Aggregate, machine-readable: /api/revenue/mpp (counts, per-rail hashes, no buyer data).

If you already speak x402, MPP can be added without touching settlement: emit an additional WWW-Authenticate: Payment challenge derived from your existing offer, and re-encode inbound Authorization: Payment credentials into your existing verification path. Agent402's implementation is open source in this repository (src/mpp-shim.js for the evm translation, src/mpp-tempo.js for native Tempo). Set MPP_SECRET_KEY to enable the shim on your own instance, TEMPO_API_KEY (plus a recipient; TEMPO_CURRENCY picks the currencies) to offer native Tempo, and STRIPE_SECRET_KEY + STRIPE_PROFILE_ID to offer cards over MPP (src/mpp-stripe.js). The same three settle the agent402-tollbooth gate on your own site, including native Tempo with split payments since 0.9.0 (see Pay-per-crawl).

• Paying with x402 - the other wire on the same 402
• Paying with Compute - the free proof-of-work tier
• What is MPP - the longer explainer