Glossary

Technical terms used across x402, MCP, and adjacent protocols

16 terms

402: HTTP status code "Payment Required". Reserved in RFC 7231 since 1999, unused at scale until x402 — payment-aware servers return 402 with payment instructions in headers.
x402: Open protocol that uses HTTP 402 plus standardized headers to make any web resource payable per request. Specified at github.com/coinbase/x402.
agent: Autonomous program — typically LLM-driven — that issues HTTP requests, receives 402 challenges, signs payments, and resubmits with proof.
MCP: Model Context Protocol. Open spec for agents to discover and invoke tools. MCP servers may charge for invocations via x402.
facilitator: Service that observes chain settlement and issues attestations to API operators. Removes the requirement for operators to run their own indexer.
verifier: Endpoint a server hits to confirm an X-Payment header is valid. Often hosted by the same entity as the facilitator.
X-Payment-Required: HTTP response header set by a server alongside a 402. Carries amount, currency, payee, and chain.
X-Payment: HTTP request header carrying a signed payment receipt. Server forwards the value to a verifier; on success it returns 200.
USDC: USD-backed stablecoin issued by Circle on multiple chains. Most x402 payments settle in USDC because of its multi-chain coverage and ~$0 transfer cost on Solana / L2s.
stablecoin: Token whose price is pegged to a fiat currency, typically USD. x402 is currency-agnostic but stablecoins are the dominant settlement asset.
split: Multi-recipient settlement where one payment is divided across two or more wallets atomically. Supported on-chain via splitter programs (e.g. Chest Gate on Solana).
referrer: Wallet that originated traffic to a paid resource. With splits, the referrer receives a share of every payment they originated.
attestation: Cryptographically signed claim from a facilitator that a payment cleared. Servers cache attestations to avoid hot-loop chain reads.
chain: The blockchain on which a payment settles. x402 is chain-agnostic; the X-Payment-Required header advertises the chain a server expects.
gate: Server-side wrapper that turns a non-payable endpoint into a payable one by emitting 402 challenges, verifying X-Payment headers, and forwarding cleared requests to the upstream.
Bazaar: x402 server registry maintained by a facilitator. The CDP Bazaar (api.cdp.coinbase.com) and PayAI Bazaar are the two largest public indexes today.