x402 Glossary

Every x402 protocol term and quirk, defined in plain English — including every documented bug class from the live catalog.

Protocol terms

• x402 — An open protocol built on the HTTP 402 "Payment Required" status code for per-request, account-less payments.
• PAYMENT-REQUIRED — The response header a 402 challenge carries: scheme, network, amount, asset, payTo, and timeout.
• PAYMENT-RESPONSE — The header returned with a successful 200 response: a settlement receipt including the on-chain transaction hash.
• Facilitator — The service that verifies a signed payment authorization and settles it on-chain.
• Verify — The facilitator step that checks a signed payment authorization is well-formed and sufficient.
• Settle — The facilitator step that submits the verified payment on-chain.
• Compliant endpoint — An x402 endpoint that follows the spec exactly.
• Quirk endpoint — An x402 endpoint that deviates from the spec in exactly one documented way.
• radius-cli — The reference command-line x402 client for the Radius testnet.
• MPP (Machine Payments Protocol) — A related protocol for agent-to-agent payments using WWW-Authenticate: Payment.

Bug classes (from the live catalog)

• Encoding — 4 example endpoint(s) in the catalog.
• Header format — 15 example endpoint(s) in the catalog.
• Network mismatch — 8 example endpoint(s) in the catalog.
• Amount / price — 5 example endpoint(s) in the catalog.
• Verify / settle — 8 example endpoint(s) in the catalog.
• Replay / idempotency — 4 example endpoint(s) in the catalog.
• Fail-open / fail-closed — 9 example endpoint(s) in the catalog.
• Edge / CDN — 11 example endpoint(s) in the catalog.
• Client policy — 4 example endpoint(s) in the catalog.
• Scheme variants — 11 example endpoint(s) in the catalog.

Browse the full catalog · Example endpoints