x402 is an open protocol that lets software pay for a single HTTP request in stablecoins. The server answers 402 Payment Required with a price, the client signs a payment, and the same request completes. MPP, the Machine Payments Protocol, is the IETF-track version of that same handshake, carried in standard HTTP authentication headers.
No subscription, no checkout page, no account. That matters because an AI agent cannot open one: paying per call is the only purchase an autonomous program can actually make on its own.
When HTTP/1.1 was written in 1997 its authors set aside a response code for the case where a client had to pay before it could have something: 402 Payment Required. The spec marked it reserved for future use, and there it sat for roughly thirty years, one of the few status codes almost no server ever sent.
The reason was not neglect. Card payments cannot fit inside one HTTP response: they need a redirect to a hosted page, a session, a stored credential, and a human to approve the charge. A protocol-level "pay me for this request" was unimplementable with the payment rails of the time.
Stablecoins changed the arithmetic. A signed transfer authorization is small enough to travel in a header, cheap enough to be worth a tenth of a cent, and needs no merchant account. x402 is what the 402 status code looks like once a payment system exists that can settle inside a single round trip.
Four steps, one round trip from the caller's point of view. Nothing here requires a browser, a human, or an account.
A plain HTTP request, with no credentials attached. There is no account to authenticate against, so nothing to send.
The response names the amount, the asset, the chain, and where to pay. This is machine-readable, so the caller does not need documentation to understand the terms.
It signs an EIP-3009 stablecoin transfer authorization with its own key and repeats the identical request, carrying the authorization in a header. Funds are never handed to the seller's server.
A facilitator checks and settles the authorization on chain, then the server returns the real response plus a receipt. The caller experiences one request that cost a fraction of a cent.
# 1. the agent asks, without paying POST /api/edgar-filing-text # 2. the server quotes a price HTTP/1.1 402 PAYMENT REQUIRED WWW-Authenticate: Payment realm="agent402", amount="0.004", asset="USDC", network="base" # 3. the agent signs and retries POST /api/edgar-filing-text Authorization: Payment <EIP-3009 authorization> # 4. verified, settled, delivered HTTP/1.1 200 OK Payment-Receipt: 0x8f2a…c41d { "filing": "10-K", "text": "…" }
The payment settles only alongside a successful response, so a failed call is never charged. Because settlement lands on a public chain, both sides can check afterwards what was actually paid instead of trusting an invoice.
They differ in wire format, not in economics. Same signed authorization, same facilitator, same price, same guarantee. A server can answer both on one route, which is what Agent402 does, so the buyer's client decides which dialect to speak.
| x402 | MPP | |
|---|---|---|
| Full name | x402 | MPP, the Machine Payments Protocol |
| Origin | Open standard published by Coinbase | IETF-track specification (tempoxyz/mpp) |
| Challenge | 402 with an x402 JSON body of payment requirements | 402 with a standard WWW-Authenticate: Payment header |
| Credential | X-PAYMENT header carrying the signed authorization | Authorization: Payment, the normal HTTP auth slot |
| Receipt | Settlement details returned in the response body | Payment-Receipt response header |
| Settlement | EIP-3009 stablecoin authorization, verified by a facilitator | Identical: EIP-3009, same facilitator, same price |
| Client | @x402/fetch and other x402 clients | mppx and any HTTP client that speaks the Payment scheme |
| On Agent402 | Served on every paid route | Served on the same routes, no configuration either side |
Agentic payments are purchases made by software rather than people. An AI agent working a real task keeps running into things it cannot answer from memory: fetch a live page, pull a filing, convert a file, check an address on a chain.
Signing up for twenty APIs is not something it can do. It has no email inbox to confirm, no credit card, no authority to accept terms of service, and no way to rotate the keys it would collect. Every one of those is a human step wedged in front of a machine task.
Paying a fraction of a cent per call, from a wallet it controls, is a step it can take unattended. That inverts the model: instead of registering for access, the agent simply pays for the one thing it needs, and the payment is the identity. Nothing to sign up for means nothing to leak.
The practical test of an agentic payment: could the software complete the purchase with no human awake? If a human has to approve, register, or paste a key, it is not agentic.
The protocol is chain-agnostic; each server picks what it accepts. Agent402 settles on twelve rails, so a buyer pays on whichever chain it already holds stablecoins on. Gas is sponsored by the facilitator on EVM chains, which means a caller needs only the stablecoin and no native gas token. Counts below are real settled calls per rail.
| chain | asset | caip-2 | calls settled |
|---|---|---|---|
| Base | USDC | eip155:8453 | 7,820 |
| Solana | USDC | solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp | 3,031 |
| Algorand | USDC | algorand:wGHE2Pwdvd7S12BL5FaOP20EGYesN73ktiC1qzkkit8= | 3,028 |
| Polygon | USDC | eip155:137 | 155 |
| Arbitrum | USDC | eip155:42161 | 122 |
| Robinhood Chain | USDG | eip155:4663 | 118 |
| Stellar | USDC | stellar:pubnet | 103 |
| Celo | USDC | eip155:42220 | 95 |
| Monad | USDC | eip155:143 | 80 |
| Avalanche | USDC | eip155:43114 | 52 |
| Sei | USDC | eip155:1329 | 35 |
| Optimism | USDC | eip155:10 | 34 |
live from GET /api/stats · 87 of these settled over the MPP wire rather than x402
Not a forecast: a ranking of real sellers by Base USDC settled volume, built by crawling public x402 sellers and reading the chain. Agent402 is excluded from this view, because a neutral index should be checkable against itself.
| # | seller | usdc settled | calls | buyers |
|---|---|---|---|---|
| 01 | BlockRun.AI | $27,958.07 | 2,572,489 | 167 |
| 02 | api.botpay.network | $1,155.47 | 14,868 | 2,319 |
| 03 | StableEnrich | $664.42 | 23,881 | 179 |
| 04 | api.m2mcent.com | $188.25 | 1,034 | 2 |
| 05 | x402.twit.sh | $182.28 | 30,230 | 47 |
| 06 | AgentUtility.ai | $128.44 | 4,790 | 22 |
x402 has two sides, and both are open. Charge for what you serve, or pay for what you need.
Put an x402 paywall in front of your endpoint, then register the origin so buyers and routers can find you. Free to list, no signup, and nothing deducted from your price: buyers pay straight into your wallet.
# we probe, you appear
curl -X POST https://agent402.tools/api/index/register -H 'content-type: application/json' -d '{"origin":"https://api.you.com"}'
LIST YOUR API →
Paste one MCP URL into Claude, Cursor, or any streamable-HTTP client and your agent can buy from 500+ tools. Pure-CPU tools run free over proof-of-work, so you can try it without a wallet.
# zero install
claude mcp add --transport http agent402 https://agent402.tools/mcp
ADD TO CLAUDE →
x402 is an open payment protocol that finally uses HTTP status code 402 Payment Required. A client asks for a resource, gets a price back, pays in a stablecoin, and the same request goes through. It lets a program buy one thing in one round trip with no subscription, no checkout page and no account.
MPP is the IETF-track standard that gives HTTP a native Payment authorization scheme. The server answers 402 with a WWW-Authenticate: Payment challenge, the client retries with Authorization: Payment credentials, and the settled response returns a Payment-Receipt header. It is a second wire for the same idea as x402, with the same EIP-3009 USDC settlement underneath.
They differ in wire format, not in economics. x402 carries payment requirements in its own JSON body and an X-PAYMENT header; MPP uses standard HTTP authentication headers, WWW-Authenticate: Payment and Authorization: Payment, plus a Payment-Receipt on success. Both settle the same EIP-3009 USDC authorization through the same facilitator at the same price. A server can answer both on one route, which is what Agent402 does, so the buyer's client chooses.
Agentic payments are purchases made by software rather than people. An AI agent cannot sign up for twenty APIs, because it has no email, no credit card and no way to accept terms, but it can pay a fraction of a cent per call from its own wallet. The wallet is the identity, so there is nothing to register and no key to rotate.
HTTP reserved 402 Payment Required in 1997 as a placeholder for a digital cash system that never arrived. Card payments needed a redirect, a session and a human, none of which fit inside a single HTTP response. Stablecoins made a one-round-trip machine payment practical, so the status code finally has a payment system to describe.
Not always. Some servers, including Agent402, offer a proof-of-work tier where your own machine solves a single-use sha256 puzzle instead of paying, costing about a second of CPU. A wallet is needed only for calls that cost the operator real money, and those quote their price in the 402 challenge before anything is charged.
It depends on the server. Agent402 settles on twelve rails: USDC on Base, Solana, Polygon, Arbitrum, Monad, Celo, Avalanche, Sei, Optimism, Stellar and Algorand, plus USDG on Robinhood Chain. Gas is sponsored by the facilitator on EVM chains, so callers need only the stablecoin. Base carries the most volume and is the chain most public x402 indexes measure.
Put an x402 paywall in front of your endpoint so unpaid requests answer 402 with your price and payment terms, then register the origin with a public index so buyers and routers can find you. See /sell for Agent402's own free, no-signup listing flow and how it prices routed calls.
On a correctly implemented server the payment only completes alongside a successful response, so a failed call is never charged. Because settlement happens on a public chain, both sides can verify independently what was actually paid rather than relying on an invoice.
Agent402 is the applied layer for both protocols: an open index, a neutral router, and an on-chain ranking of every x402 seller. Free to list, free to browse.