# x402 glossary: 44 terms for HTTP 402, agent payments and USDC

## x402 glossary: 44 terms for HTTP 402, agent payments and USDC

**This glossary defines the terms used in writing about x402, the protocol that lets software pay for a web request by answering HTTP 402 "Payment Required".** It is adapted from the glossary of the Group's book [*The x402 Economy*](https://services.561.group/the-x402-economy-book/) and extended with terms from the Group's [market](https://market.561.group/). Definitions are written for readers who use the web but do not build it. If you are new to the subject, start with [What is x402?](https://services.561.group/what-is-x402/).

### Protocol and web terms

**HTTP.** The Hypertext Transfer Protocol, the rules web browsers and servers follow to request and deliver pages and data.

**HTTP status code.** The three-digit number a web server returns with every answer, such as `200` (success), `404` (not found) or `402` (payment required). See [What does HTTP 402 Payment Required mean?](https://services.561.group/what-http-402-payment-required-means/)

**HTTP 402 Payment Required.** A status code reserved in the HTTP standard since the 1990s and described as "reserved for future use" in every specification through RFC 9110 (2022). x402 gives it a meaning.

**x402.** The open protocol, started by Coinbase in 2025 and now stewarded by the x402 Foundation at the Linux Foundation, in which a server answers with a price, the client pays in a stablecoin and retries, and the server delivers.

**x402 Foundation.** The body under the Linux Foundation that governs the x402 specification; launched operationally on 14 July 2026 with 40 members. See [Who built x402 and who governs it](https://services.561.group/who-built-x402-and-who-governs-it/).

**Header.** A labeled line of extra information attached to an HTTP request or response. x402 version 2 uses `PAYMENT-REQUIRED`, `PAYMENT-SIGNATURE` and `PAYMENT-RESPONSE`.

**`PAYMENT-REQUIRED`.** The response header, base64-encoded JSON, that carries the price and the accepted ways to pay with a `402`.

**`PAYMENT-SIGNATURE`.** The request header, base64-encoded JSON, that carries the client's signed payment on the retry.

**`PAYMENT-RESPONSE`.** The response header that carries the settlement receipt on success, including the transaction identifier.

**`accepts`.** The list inside the payment-required message of the ways the server will take payment; each entry names a scheme, network, amount, asset, payee and timeout.

**`payTo`.** The field naming the address that will receive the money. A `payTo` read from the seller's own response is not independent evidence of who the seller is.

**Scheme (`exact`, `upto`, `batch-settlement`).** x402 payment kinds. `exact` pays a fixed amount now. `upto` authorizes a maximum, with the seller charging what was used. `batch-settlement` gathers many payments into one blockchain transaction.

**Resource server.** In x402, the server that sells something and answers unpaid requests with 402.

**Facilitator.** The optional party that verifies an x402 payment and submits it to the blockchain for a seller, often screening the payer against sanctions lists. It exposes verify and settle operations.

**Session.** In x402 version 2, a wallet-based arrangement that lets a client that has paid skip the full exchange for a while.

**Extension.** An optional, schema-declared add-on to an x402 message, such as the Bazaar discovery information or sign-in-with-x.

**X-Payment-Recovery.** A request header the Group's market accepts on recoverable offers: a token of 32 random bytes as 64 lowercase hex characters, saved before paying, which lets a repeated signed request return the exact purchased response instead of charging twice.

### Money and blockchain terms

**Blockchain.** A shared public ledger, maintained by many computers, that records transfers of tokens. Records are permanent and anyone can read them.

**Base.** A low-fee blockchain network built by Coinbase on top of Ethereum; one of the two main networks for x402 payments, and the one the Group's market uses.

**Solana.** An independent blockchain designed for speed and low fees; the other main network for x402 payments.

**CAIP-2.** A standard format for naming blockchain networks, such as `eip155:8453` for Base. x402 version 2 uses it.

**Stablecoin.** A token on a blockchain whose issuer promises to redeem it at a fixed value, usually one US dollar, and holds reserves to back the promise.

**USDC.** A dollar stablecoin issued by Circle. Nearly all x402 payments are made in it: 99.6% of the value in one 2026 analysis of about 199 million settlements. It has six decimal places, so `10000` atomic units is one cent.

**Atomic units.** The smallest indivisible unit of a token. For USDC, one millionth of a dollar. x402 amounts are given as integers of atomic units, for example `9000000` for $9.00.

**EIP-3009 (transfer with authorization).** A standard that lets the holder of a token such as USDC sign a transfer that someone else submits to the blockchain. x402's `exact` scheme uses it on Base and similar networks.

**Wallet.** Software that holds the secret key for a blockchain address and signs transfers from it.

**Address.** The public identifier of a wallet on a blockchain. An address is a pseudonym: it does not name its owner, but every payment it makes is public.

**Chargeback.** A reversal of a card payment ordered by the card issuer after a dispute. x402 payments have no chargeback.

**GENIUS Act.** The US law, enacted 18 July 2025, that defines payment stablecoins and who may issue them. See [Is x402 legal?](https://services.561.group/is-x402-legal-the-genius-act-and-stablecoin-payments/)

### Agent and protocol-ecosystem terms

**Agent (AI agent).** A program, usually built on a large language model, that pursues a goal by choosing steps and calling other programs and services on a person's behalf.

**MCP (Model Context Protocol).** A protocol AI assistants use to call external tools and data sources. x402 version 2 names it as a way to carry payment messages; the Group's market offers a read-only MCP endpoint for discovery.

**A2A (Agent2Agent protocol).** A protocol for AI agents to exchange messages and hand tasks to one another, introduced by Google. The Group's market publishes an A2A agent card.

**ACP (Agentic Commerce Protocol).** The protocol OpenAI and Stripe published in September 2025 for checkout through an AI assistant, with the merchant charging a scoped payment token.

**AP2 (Agent Payments Protocol).** Google's protocol, published in September 2025, for proving that a person authorized an agent's purchase.

**Mandate.** In AP2, a signed record of what a person has authorized an agent to buy.

**MPP (Machine Payments Protocol).** The protocol Stripe and Tempo published in March 2026 for payments carried by requests, in stablecoins or cards.

**L402.** Lightning Labs' protocol for HTTP 402 payments over Bitcoin's Lightning Network, first published in 2020 as LSAT. See [x402 vs MPP, AP2, ACP and L402](https://services.561.group/x402-vs-mpp-ap2-acp-and-l402/).

**Prompt injection.** Text written to steer an AI agent against its instructions, for example by telling it to pay an attacker.

### Market and measurement terms

**Bazaar.** Coinbase's catalog of x402 services, which lists an endpoint after a payment to it settles through Coinbase's facilitator. (The Group also operates a separate directory called the Bazaar; see [Market, Bazaar and Shop](https://services.561.group/market-bazaar-and-shop-explained/).)

**Micropayment.** A payment small enough, typically under a dollar and often under a cent, that processing costs and the buyer's attention have historically outweighed it.

**Calls per payer.** The number of paid requests each distinct buyer makes to a service. Near one suggests sampling; tens or more suggest recurring use.

**Self-payment.** A payment from a seller to itself, for testing or to make a service look used. It inflates raw counts of x402 activity.

**Wash trading.** Payments between addresses controlled by the same party, made to create the appearance of activity. See [How big is the x402 market?](https://services.561.group/how-big-is-the-x402-market/)

**Four-valued logic (Belnap).** A logic in which a statement can be told true, told false, told both, or told neither, so that contradiction and ignorance are ordinary values. The Group sells it per call: [Four-valued logic, sold per call](https://services.561.group/four-valued-logic-api-sold-per-call/).