x402 Glossary: The Eight Terms That Matter

This glossary defines the eight terms that carry the x402 ecosystem, in plain language and with links into the live directory. Each definition is short enough to quote and precise enough to build on; together they cover the protocol, the payment rail, the pricing model, and the agent tooling layer.

x402

An open payment protocol, originated by Coinbase, that activates the HTTP 402 status code for machine-payable web requests. The server states a price in a 402 response, the client attaches a signed stablecoin payment and retries, and a facilitator settles the transfer on-chain. No accounts, API keys, or subscriptions are involved. The full x402 definition covers its origins and boundaries.

HTTP 402

The "Payment Required" status code, reserved in the early HTTP specifications but left without agreed semantics for decades. x402 finally gives it concrete meaning: the response carries machine-readable payment terms, and a client that pays and retries is served normally. It behaves as a challenge rather than an error. The history of HTTP 402 explains why the reservation sat unused for so long.

Facilitator

The service in an x402 payment that verifies a client's signed payment against the server's terms and settles the transfer on-chain, exposing both steps as simple API calls. Servers integrate a facilitator instead of running nodes; clients get fulfillment at HTTP speed. Seven facilitators settle the listings in this directory, with adoption concentrated in one operator. The facilitator hub compares all seven by live listing count.

Stablecoin

A cryptocurrency designed to hold a steady value against a reference asset, typically the US dollar. Stablecoins give x402 prices a familiar unit of account — a $0.01 call costs a cent, not a fluctuating token amount — while settling on-chain without banks or card networks. In this directory every listing accepts USDC, the dollar-pegged stablecoin, on Base or Solana.

Micropayment

A payment too small to be practical on traditional rails, where fixed per-transaction fees would exceed the amount moved. On-chain stablecoin transfers make micropayments routine: the largest price band in this directory, micro at $0.01 or less per request, holds 32 listings, and listed prices reach $0.00001. Micropayments are what let APIs sell single calls instead of monthly plans.

Pay-per-request

The pricing model in which each individual API call or resource fetch carries its own price, paid at the moment of use. It is the only pricing model in this directory: all 100 listings charge per request, with no subscriptions or commitments. The model matches how autonomous agents consume services — opportunistically, in bursts, across many vendors. Our guide to monetizing an API for AI agents covers price-setting in practice.

MCP

The Model Context Protocol, an open convention for connecting AI agents to external tools and data sources through a standard interface. An MCP server exposes capabilities an agent can discover and invoke; combined with x402, each tool call can carry its own per-call price, turning hosted tools into paid services without accounts. This directory tracks four such MCP tools, three of them settling on Solana.

Settlement

The moment a payment actually moves on-chain: the transfer of USDC from the client's wallet to the server's address, broadcast and confirmed on the named network. In x402 the facilitator performs settlement after verification, sparing the server from running nodes or watching for confirmations. Verification asks "is this payment good?"; settlement answers "has the money moved?" The facilitator guide details both steps.

Read together, the eight terms form one sentence: an agent using x402 answers an HTTP 402 challenge with a stablecoin micropayment, priced pay-per-request, which a facilitator settles — and MCP is how tool-using agents find endpoints worth paying.

Put the vocabulary to work: browse the x402 Looker directory, or follow the terms into the full guide collection.