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* and extended with terms from the Group's market. 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?.
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?
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.
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?
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.
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.)
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?
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.