# What is x402? The HTTP payment protocol AI agents use

x402 is an open protocol that lets a client pay for an HTTP request at the moment it makes it. The server answers 402 Payment Required with machine-readable terms, the client signs a stablecoin authorisation, and it resends the request with an X-PAYMENT header. No account, no API key, no human.

Published: 2026-10-14
Last reviewed: 2026-10-14
Publisher: Roundhouse AI Ltd.
Source: https://roundhouseai.io/resources/what-is-x402

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## What problem does x402 solve?

Every existing way to pay for an API assumes a person. Someone signs up, enters a card, accepts terms and copies an API key into a configuration file. That works when software is operated by a human on a timescale of days.

It does not work when the buyer is an agent that found your endpoint thirty seconds ago and wants one call. The sign-up is the blocker, and it is a blocker no amount of documentation removes.

x402 removes the account. The buyer proves it can pay, in the same request where it asks for the thing.

## How does the x402 flow work?

Five steps, all inside one HTTP exchange.

1. The client requests a resource with no payment attached.
2. The server replies **HTTP 402 Payment Required** with an `accepts` array: how much, in which asset, on which network, and to which address.
3. The client picks one of those terms and signs an authorisation to transfer that amount. On EVM chains this is an [EIP-3009](https://eips.ethereum.org/EIPS/eip-3009) `TransferWithAuthorization`, which is a signature rather than a transaction.
4. The client resends the original request with the signed authorisation in an `X-PAYMENT` header.
5. The server verifies the authorisation, settles it, and returns the resource along with an `X-PAYMENT-RESPONSE` header naming the settlement.

> [!NOTE]
> The buyer signs; it does not broadcast. A facilitator submits the transaction and pays the gas, which is why an agent holding only USDC, and no native token, can still pay.

## What is a facilitator, and why does it matter?

A facilitator is the service that checks a payment authorisation is valid and then puts it on-chain. It sits between the buyer and the seller and does two jobs the seller would otherwise have to build: `verify` and `settle`.

This matters for a reason that is easy to miss. The facilitator is the party that submits the transaction, so **the transaction's sender is the facilitator, not the payer**. That is what makes x402 payments identifiable on-chain at all. It is the subject of [how the settlement record is built](https://roundhouseai.io/resources/how-agents-discover-paid-services).

## What does x402 actually cost?

The protocol does not set a price; the resource server does. In practice the prices are small, because the point is to make a single call purchasable.

| | Typical x402 call | Traditional API |
| --- | --- | --- |
| Price point | $0.001 – $1 per request | Monthly plan or committed tier |
| Onboarding | None. Pay on the first request | Account, card, key provisioning |
| Settlement | Seconds, on-chain | Invoiced monthly |
| Buyer needs | A funded wallet | A human with a card |
| Refusal signal | HTTP 402 with terms | HTTP 401/403 with a sign-up link |

The cost that is easy to overlook is gas. Because a facilitator relays the transfer, the buyer does not pay it directly. The facilitator does, and prices it in.

## Which networks and assets does x402 use?

In practice today: **USDC on Base**. The protocol is chain-agnostic and the `accepts` array can offer several options at once, but the overwhelming majority of live settlements are USDC on Base, because that is where the facilitators and the catalogues are.

Roundhouse indexes Base mainnet. Over the 90 days to 23 September 2026 it recorded 26,671,518 x402-pattern settlements on Base, within about 1% of the 26.4M that [Token Terminal](https://tokenterminal.com) reported for the same chain and window, which is the closest thing the ecosystem has to an independent check.

## How is x402 different from just sending a crypto payment?

A bare transfer tells the seller nothing. It does not say which request it was for, it does not arrive inside the request, and the seller has to reconcile it afterwards.

x402's contribution is that the payment is **part of the HTTP exchange**. The 402 response carries the terms, the retry carries the authorisation, and the 200 carries the receipt. A seller never has to match a payment to an order, because the payment arrived attached to the order.

## Frequently asked questions

### Is x402 a blockchain or a token?

Neither. x402 is an HTTP protocol. It describes how a server states its price and how a client attaches a payment, and it leaves the settlement to whichever chain and asset the two agree on. There is no x402 token.

### Do I need an account to pay with x402?

No. That is the point of it. A client needs a funded wallet and the ability to sign; it does not register, authenticate or hold an API key. A seller that wants per-customer limits can still issue keys on top, but the protocol does not require one.

### Does the buyer need ETH for gas?

No, on the common path. The buyer signs an EIP-3009 authorisation and a facilitator broadcasts it, paying the gas itself. This is what lets an agent operate holding only a stablecoin.

### Is an x402 payment reversible?

No. It settles on a public blockchain and there is no chargeback. A seller that needs to refund does so by sending a new payment. This is a genuine difference from card rails and it should shape what you sell this way.

### How can I see real x402 payments?

They are public, and [how much of that volume is real](https://roundhouseai.io/resources/how-much-x402-volume-is-real) is worth reading before using any headline figure. Roundhouse indexes them into a queryable record: the [live flow of funds](https://roundhouseai.io/flows) shows settlements as they land, [services](https://roundhouseai.io/services) lists the endpoints being paid, and the [data API](https://roundhouseai.io/docs/api) answers the same questions programmatically.

## Where to check this yourself

Every claim above about live activity is checkable against the index rather than taken on trust. The [data API](https://roundhouseai.io/docs/api) serves the same rows these figures come from. `GET /v0/endpoints` and the capability layer answer without a credential, and the rest take a key you can mint yourself for $0.01 over x402 at `GET /v0/test/x402`, no account. Each figure here names the chain and the window it was measured over.

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Roundhouse indexes x402 settlements and ERC-8004 identities into one public record.

- Every article: https://roundhouseai.io/resources.md
- Every surface on this site: https://roundhouseai.io/llms.txt
- The index, endpoints and schema: https://api.roundhouseai.io/v0
- A key of your own, $0.01 over x402, no account: https://api.roundhouseai.io/v0/test/x402
- Limits and the SQL surface: https://roundhouseai.io/docs/api