Software agents transacting on their own: discovering a service, reading a machine-readable price, paying per request from a non-custodial wallet over an open protocol such as x402 or MPP, receiving a verifiable receipt, and, on the other side, earning per request for what they serve. Agentic payments are the wire; agentic finance is the economy that forms on top of it - discovery, routing, pricing, reliability, receipts and transparency. Agent402 is its applied layer.
The Agentic Finance glossary
Every term the agentic-finance stack uses, defined once. From the HTTP status code that started it to the facilitators, receipts and routers built on top. Each entry links to the page that goes deep, and each has its own anchor so any page can point at a definition instead of restating it.
Start with what Agentic Finance (AIFI) is, then the two wires: x402 and MPP.
23 TERMS
Agentic Finance (AIFI)Agentic paymentsx402MPP (Machine Payments Protocol)HTTP 402 Payment RequiredPayment requirements (x402 offer)Payment challenge (MPP)FacilitatorEIP-3009 (transferWithAuthorization)Payment receiptSettlementNon-custodial wallet (wallet as identity)Rails (stablecoin rails)Dual-stack (x402 + MPP)Proof-of-work tier (free tier)Smart Order Router (SOR)Seller index and leaderboardTollbooth (pay-per-crawl)Pay-per-call (pay-per-request)Deterministic toolIdempotency keyMCP (Model Context Protocol)Tempo
The plumbing that lets one program pay another per request without an account: an HTTP 402 response names a price, the client answers with a signed stablecoin payment, and the server verifies, settles and delivers. x402 and MPP are the two open, HTTP-native standards for it. Distinct from agentic commerce (agents buying goods for humans through checkout flows).
An open protocol, originally from Coinbase, that turns HTTP 402 Payment Required into a working payment flow: the server sends machine-readable payment requirements (price, asset, chain, recipient) in a PAYMENT-REQUIRED header, the client retries with a signed authorization in a PAYMENT-SIGNATURE (formerly X-PAYMENT) header, a facilitator verifies and settles it in USDC, and the response carries a PAYMENT-RESPONSE receipt.
MPP (Machine Payments Protocol)
also: Machine Payments Protocol · Payment HTTP authentication scheme
#mppAn open, IETF-track protocol that carries pay-per-request payments through HTTP's standard authentication headers: a 402 response challenges with WWW-Authenticate: Payment, the client pays with an Authorization: Payment credential, and the settled response returns a signed Payment-Receipt. Its evm method settles like x402 (EIP-3009 via a facilitator); its tempo method settles natively on Tempo.
The HTTP status code reserved since 1997 for exactly this and left unused for decades: the server is saying the resource exists but costs money. Both x402 and MPP use it as the price quote; the payment rides on the retry. Agent402 answers 402 on every paid route and 200 once a payment settles.
The machine-readable price quote in an x402 402: a list of accepted ways to pay (called accepts), each naming the scheme, network, asset, amount, recipient and a validity window. A dual-stack server carries the same terms in an MPP challenge alongside it. It lives in the response HEADER, so a 402 with an empty body is still a full offer.
MPP's price quote: a WWW-Authenticate: Payment header on the 402 naming the method (evm or tempo), the amount, the asset, the chain and a one-time challenge id. The client answers the challenge with an Authorization: Payment credential on the retry - the same mechanism (RFC 9110) behind every login prompt on the web.
A service that verifies a signed payment authorization and broadcasts the on-chain transfer on the seller's behalf, so the seller needs no node, no gas and no key management. On x402 the facilitator settles EIP-3009 authorizations; different facilitators cover different chains, and a seller can hold several. Agent402 keeps its settlement ordering fixed: the tool runs first and settlement happens only on a successful response, so a failed call is never charged.
The token standard that makes x402 and MPP's evm method possible: the payer signs an off-chain authorization for an exact amount to an exact recipient within a validity window, and anyone (the facilitator) can submit it on chain. The payer pays no gas and never hands over a key. USDC implements it on every EVM rail Agent402 accepts.
The proof that a specific payment bought a specific answer: on x402 the settled response carries a PAYMENT-RESPONSE header with the on-chain transaction; on MPP a signed Payment-Receipt header. Agent402 mirrors both on dual-stack routes and links every revenue figure to its receipt on chain.
The moment a payment authorization becomes an on-chain transfer. Settlement is what turns a signed promise into money in the seller's wallet; a payment can verify (the signature is good, the funds exist) and still fail to settle. Agent402 settles AFTER the tool runs and only on a successful response, publishes settlement counts by rail and wire, and ledgers the rare charged-but-failed case as a debt to refund.
The agent's own key pair holding stablecoins. In agentic finance the payment IS the identity: there is no account to create, and the payer address on a settled payment is what a seller keys memory, usage history and quotas to. Keys never leave the client - the wallet signs an authorization and the facilitator broadcasts it.
The chains and stablecoins a payment can settle on. Agent402 accepts USDC on Base, Solana, Polygon, Arbitrum, Monad, Celo, Avalanche, Sei, Optimism, Stellar and Algorand, USDG on Robinhood Chain, and PathUSD/USDC natively on Tempo - a buyer picks any rail listed in the 402 at the same list price (fee-charging rails carry the fee in the quote).
A server that answers ONE 402 with both an x402 offer and an MPP challenge for the same resource at the same price, and accepts either credential on the retry. A stock x402 client and a stock mppx client both work unmodified against it. Every paid route on Agent402 is dual-stack, and the open-source tollbooth adds it to any site.
Agent402's no-wallet path: a pure-CPU tool can be paid for with a short proof-of-work solve instead of money. The solve is signed, single-use and scoped to the tool, so it costs the caller a few hundred milliseconds of compute and the operator nothing. Tools that spend money upstream (search, LLM gateway, browser) are wallet-only.
One call that resolves a task to the best seller across the whole ecosystem - Agent402's own catalog or any indexed external seller - pays that seller on the agent's behalf on the same chain the agent paid on, and relays the result with a receipt. Only sellers with proven on-chain settlement are routable, so a routed call cannot land on a seller that has never been paid.
Agent402's live map of who sells what over x402 and MPP: origins crawled from public manifests and facilitator registries, live-verified, and ranked by settlements actually observed on chain rather than by self-reported claims. The leaderboard is what makes routing safe; the index is what makes discovery possible.
The open-source middleware or reverse proxy that puts a price on a site or API for AI agents while humans browse free: it answers 402 over both x402 and MPP, verifies the payment, and lets the request through. Non-custodial, no signup, adaptive proof-of-work for the free path, deploy templates for Cloudflare, Next.js and Docker.
Pricing where every request carries its own payment - fractions of a cent to a few cents - instead of a subscription, a quota or an API key. It is what makes an agent's spend legible (one receipt per call) and what lets a seller earn from a single request by a stranger with no onboarding.
A tool whose output is a pure function of its input: same request, same answer, every time, with no language model in the serving path. Determinism is what lets an agent cache, retry safely and verify a paid answer, and what lets a catalog be tested end to end (every Agent402 tool answers its own documented example in CI).
A caller-chosen header that lets a paid request be retried without paying twice: the seller stores the first settled answer under the key and replays it to an identical retry. Agent402 commits the cached body only after settlement succeeds, so an unsettled attempt is never replayed as if it were paid.
The open standard AI assistants use to discover and call tools. Agent402 exposes its catalog as a hosted MCP connector and an npm stdio server, so an assistant can find a tool, read its price and call it - paying over x402 or MPP, or with a proof-of-work solve on the free tier - without a human creating an account.
The payments-focused chain that is MPP's native settlement method (chain id 4217, TIP-20 tokens such as PathUSD and USDC.e). Its tempo/charge credentials are validated and broadcast through Tempo's own relay rather than an EIP-3009 facilitator, so a server that speaks it natively holds no signing key of its own. Agent402 accepts it and, through the router, pays other Tempo sellers over it.