Glossary

The vocabulary of agent payments, each term defined from its primary sources and linked to what the record shows.

Terms

A2A: A2A — the Agent2Agent protocol — is an open standard for communication between AI agents that know nothing about each other's internals.
accepts array: The accepts array is the list of payment options in an x402 payment challenge: what an endpoint returns with HTTP 402 when a request arrives unpaid.
ACP: ACP — the Agentic Commerce Protocol — is an open standard for commerce flows between buyers, AI agents and businesses, published under an open-source licence.
agent: An agent is software that acts for somebody and can pay for what it uses.
agent wallet: An agent wallet is where the money an agent spends is held and where the limits on that spending are set.
agentic commerce: Agentic commerce is an agent completing a purchase for somebody through a merchant's own checkout.
AP2: AP2 — the Agent Payments Protocol — is an open protocol for authorising an agent's purchase and recording what the person actually asked for.
buyer: A buyer is the party whose money is spent.
CAIP-2: CAIP-2 is a way of naming a blockchain in one string: a namespace, a colon, and a reference.
catalogue: A catalogue is a published list of x402 endpoints that a buyer or an agent can read to discover what is payable.
custody: Custody is holding somebody else's money.
discovery: Discovery is how an x402 endpoint becomes findable at all: the endpoint describes itself in its own payment challenge, and a facilitator collects those descriptions into a catalogue.
EIP-2612: EIP-2612 adds a permit function to an ERC-20 token, so a holder can grant a spending allowance with a signed message instead of a transaction.
EIP-3009: EIP-3009 is an Ethereum token standard extension that lets a token holder sign a transfer authorization off-chain, which anyone can then submit on-chain.
EIP-712: EIP-712 is the Ethereum standard for hashing and signing typed structured data rather than an opaque string.
ERC-20: ERC-20 is the Ethereum standard interface for a token: how it is transferred, and how a holder lets somebody else spend on their behalf.
extension: An extension is an optional piece of x402 functionality outside the core payment mechanics.
facilitator: A facilitator is a service that verifies and settles x402 payments on behalf of a seller.
gas: Gas is the fee a blockchain charges for doing work, paid in that network's own token.
HTTP 402: HTTP 402 is the response status code named Payment Required.
KYA: KYA — Know Your Agent — is the idea that verifying a party in an agent payment means verifying two things: the person or organization behind the agent, and the agent itself.
L402: L402 is an HTTP payment scheme that pairs a bearer token with a Lightning invoice.
machine payment: A machine payment is a payment made by software for a resource it is using, with no checkout page and nobody entering card details.
MCP: MCP — the Model Context Protocol — is an open standard for connecting AI applications to external systems: data sources, tools and workflows.
micropayment: A micropayment is a payment small enough that the cost of collecting it matters.
MPP: MPP — the Machine Payments Protocol — is an open protocol for paying for an HTTP request with no checkout and nobody present.
on-chain vs off-chain: On-chain and off-chain describe where a step of a payment happens.
pay per crawl: Pay per crawl lets a site owner charge AI crawlers for access instead of blocking them.
pay-per-call API: A pay-per-call API is one that charges for each request rather than through an account or a subscription.
payment authorization: A payment authorization is the signed instruction a payer produces that permits a payment, before any money has moved.
payment challenge: A payment challenge is what a paid endpoint returns when a request arrives without payment: a machine-readable statement of what it would cost and how to pay it.
payment intent: A payment intent is the kind of obligation a payment challenge is asking for: a one-off charge, metered usage, or a recurring subscription.
payment receipt: A payment receipt is a statement issued after a machine payment saying that it happened.
PAYMENT-REQUIRED header: The PAYMENT-REQUIRED header is where a current x402 endpoint states its price.
payTo: payTo is the field in an x402 payment quote that names the address a payment is sent to.
rate limiting vs payment gating: Rate limiting and payment gating are two different answers to the same request.
replay protection: Replay protection is what stops a signed payment authorization being presented more than once.
request example: A request example is the sample call a catalogue publishes beside a listed endpoint, showing what a buyer sends to it.
RFC 9421: RFC 9421 is the IETF standard for HTTP Message Signatures: signing chosen parts of an HTTP request so a server can check who sent it and that it arrived unaltered.
robots.txt: robots.txt is the file at the root of a site that tells automated clients which parts of it they are asked not to fetch.
RSL: RSL — Really Simple Licensing — is an open standard for stating machine-readable licensing terms for content, including attribution, pay per crawl and pay per inference compensation.
scheme: A scheme is x402's word for how a payment is formed, checked and settled.
seller: A seller is the party that charges for a resource.
settlement: Settlement is the step where an authorised x402 payment is submitted to the blockchain and the transfer actually happens.
stablecoin: A stablecoin is a cryptocurrency whose value is pegged to another asset, usually a national currency, so that it does not move the way an ordinary token does.
status class: A status class is the group an HTTP status code belongs to, identified by the digit it starts with.
tamper evidence: Tamper evidence is a property of a record: not that it cannot be altered, but that an alteration can be detected afterwards by somebody who was not there when it was written.
Tempo: Tempo is a blockchain purpose-built for stablecoin payments at scale, which its own site describes as putting payments ahead of everything else.
transport: A transport is the carrier a protocol's messages travel on.
UCP: UCP — the Universal Commerce Protocol — describes itself as a common language for platforms, agents and businesses: building blocks for agentic commerce from discovery through checkout.
UMA: UMA — a Universal Money Address — is a human-readable address for receiving money, described by its publishers as being like an email address but for money.
upto scheme: The upto scheme is the x402 scheme for a payment whose final amount is not known when the client signs.
USDC: USDC is a stablecoin issued by regulated affiliates of Circle and redeemable one-for-one for US dollars.
verify and settle: Verify and settle are the two operations an x402 facilitator exposes.
Visa Intelligent Commerce: Visa Intelligent Commerce is a card-network programme for letting an agent pay at ordinary merchants.
Visa Trusted Agent Protocol: The Visa Trusted Agent Protocol is a specification for letting a merchant recognise that an approved agent with commerce intent is the thing visiting its site, rather than a crawler or something worse.
Web Bot Auth: Web Bot Auth is a proposal for letting an automated HTTP client sign its requests so a server can tell who is asking.
x402: x402 is an open standard for paying for an HTTP request itself.

Parties

402index: 402index is a directory of paid APIs for AI agents.
Assay: Assay is a paid-probe rating service for x402 services, published by Nominal Labs.
Coinbase Developer Platform (CDP): Coinbase Developer Platform (CDP) is Coinbase's developer product suite.
Dexter: Dexter is an account product for agent payments.
x402 Foundation: The x402 Foundation is the organization that stewards the x402 specification.
x402.org facilitator: The x402.org facilitator is the public facilitator served from the protocol's own project site.
x402scan: x402scan is an explorer for the x402 ecosystem.

House rules

Sources: Each entry is pinned to primary sources — a specification, an RFC, an EIP, or a party's own published documents — listed on the entry.
Numbers: Entry text carries links rather than figures. Where a quantity matters, the entry links the source that holds it, so nothing here can go stale in silence.
Named parties: A party is described as what it is and does, per its own documents. No ranking, no endorsement, and no comparison between parties.