# Glossary > Short definitions for payment and agent protocols. Not investment advice. JSON-LD: https://agentpaywire.com/glossary.json ## L402 An HTTP and gRPC authentication scheme from Lightning Labs. The server answers with HTTP 402 and a WWW-Authenticate challenge that carries a macaroon and a Lightning invoice. The client retries with Authorization: L402 :. Library: [L402 Protocol Specification](https://docs.lightning.engineering/the-lightning-network/l402/protocol-specification) (lightning-labs-l402-protocol-specification) ## x402 An HTTP payments protocol that uses a 402 response so a client can pay for a resource. It is a different protocol from Lightning Labs L402. Library: [x402: An open standard for internet-native payments](https://www.x402.org/x402-whitepaper.pdf) (x402-whitepaper) ## Macaroon The credential in an L402 challenge. On this API it is base64 and is sent back, with the payment preimage, in the Authorization header. Library: [L402 Protocol Specification](https://docs.lightning.engineering/the-lightning-network/l402/protocol-specification) (lightning-labs-l402-protocol-specification) ## Preimage The secret revealed when a Lightning invoice is paid. sha256 of the preimage is the payment hash. L402 sends it as hex after the macaroon. Library: [L402 Protocol Specification](https://docs.lightning.engineering/the-lightning-network/l402/protocol-specification) (lightning-labs-l402-protocol-specification) ## Payment hash The hash committed in a Lightning invoice. Paying the invoice reveals the preimage that hashes to it. BOLT 11 is the invoice encoding this site cites for that request format. Library: [BOLT #11: Invoice Protocol for Lightning Payments](https://github.com/lightning/bolts/blob/master/11-payment-encoding.md) (bolt-11-invoice-protocol) ## BOLT11 The Lightning invoice encoding: a bech32 payment request a payer can pay once. This API puts a BOLT11 invoice in the L402 challenge. Library: [BOLT #11: Invoice Protocol for Lightning Payments](https://github.com/lightning/bolts/blob/master/11-payment-encoding.md) (bolt-11-invoice-protocol) ## BOLT12 The Lightning offers specification. It defines reusable payment requests and related invoice messages, and it is written to address limits of BOLT 11 invoices. This site publishes a BOLT12 offer for voluntary tips. Library: [BOLT #12: Negotiation Protocol for Lightning Payments](https://github.com/lightning/bolts/blob/master/12-offer-encoding.md) (bolt-12-offers-spec) ## LNURL A family of HTTPS protocols used by some Lightning wallets, often carried as a bech32-encoded URL. This library has no entry whose page is the LNURL specification. A Lightning address commonly resolves through LNURL-pay. This library has no entry that defines this term. ## Lightning address An identifier that looks like an email address and resolves to a Lightning payment request. The library entry for Alby Hub v1.24.3 mentions a node operator Lightning address. Tips on this site use the BOLT12 offer and the on-chain address in support.json, not a Lightning address. Library: [Release v1.24.3 Julian Assange ยท getAlby/hub](https://github.com/getAlby/hub/releases/tag/v1.24.3) (getalby-hub-v1-24-3-release) ## NWC Nostr Wallet Connect, defined by NIP-47. It lets a client send commands such as pay_invoice and make_invoice to a remote Lightning wallet over Nostr. Library: [NIP-47: Nostr Wallet Connect (NWC)](https://github.com/nostr-protocol/nips/blob/master/47.md) (nostr-nip-47-wallet-connect) ## Inbound liquidity The amount a Lightning channel can currently receive. The library lists the Lightning Network paper for the channel model. That paper summary on this site does not use the phrase inbound liquidity. Library: [The Bitcoin Lightning Network: Scalable Off-Chain Instant Payments](https://lightning.network/lightning-network-paper.pdf) (lightning-network-paper) ## Facilitator In x402, a service that can verify and settle a payment, and that can catalog payable endpoints for discovery. This API settles its own Lightning invoices. USDC on the x402 rail is verified and settled by the facilitator named in the worker configuration. The worker holds no key for that rail. L402 is listed first. Library: [Bazaar (Discovery Layer)](https://docs.x402.org/extensions/bazaar) (x402-bazaar-extension-docs) ## Stablecoin A token meant to track a reference currency. Machine payment protocols in the library, including MPP, name stablecoins as one payment method alongside cards and Lightning. Library: [MPP: Machine Payments Protocol](https://mpp.dev/) (mpp-dev) ## USDC A US-dollar stablecoin issued by Circle. A library entry on Circle CPN uses a per-call USDC example. This site lists USDC deposit addresses for voluntary tips on Base, Ethereum, Arbitrum One, and Solana. L402 on the paid API is 100 sats for 100 credits over 30 days. The x402 rail is 0.01 USDC per request on Base and does not sell credits. Library: [Turn Agent Traffic Into Revenue With CPN Managed Payments](https://www.circle.com/blog/turn-agent-traffic-into-revenue-with-cpn-managed-payments) (circle-cpn-managed-payments) ## Base An Ethereum layer-2 network. Library entries that describe x402 directories name USDC on Base as one accepted payment. On this site, Base is the primary USDC tip network. The paid API x402 rail is 0.01 USDC per request on Base and does not sell credits. Library: [satring: Curated Paid API Directory for AI Agents](https://satring.com/) (satring) ## MCP Model Context Protocol. It defines how a client talks to a tool server, including transports and the lifecycle. This site runs a read-only MCP server at https://mcp.agentpaywire.com/mcp. Library: [Specification - Model Context Protocol (Version 2026-07-28)](https://modelcontextprotocol.io/specification/2026-07-28) (mcp-specification-2026-07-28) ## A2A Agent2Agent, an open protocol for communication between independent agent systems. This site publishes an Agent Card for discovery. It does not implement A2A task methods. Library: [Agent2Agent (A2A) Protocol Specification](https://a2a-protocol.org/latest/specification/) (a2a-protocol-specification) ## Agent Card The JSON document A2A uses to describe an agent. The discovery guidance names /.well-known/agent-card.json as one way to find it. This site serves that path. Library: [Agent Discovery (A2A Protocol)](https://a2a-protocol.org/latest/topics/agent-discovery/) (a2a-agent-discovery) ## Server card A JSON description of a remote MCP server for discovery before a client connects. SEP-2127 describes server cards advertised from a domain. This site serves one at /.well-known/mcp/server-card.json. Library: [SEP-2127: MCP Server Cards - HTTP Server Discovery](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2127) (mcp-sep-2127-server-cards) ## llms.txt A proposed Markdown file at /llms.txt that tells an agent how to use a site. This site publishes /llms.txt and /llms-full.txt. Library: [The /llms.txt file, v2](https://llmstxt.org/) (llms-txt) ## HTTP 402 The status a paid endpoint returns when the client still needs to pay. The L402 specification uses it with a WWW-Authenticate challenge. The free files on this site do not return 402. Library: [L402 Protocol Specification](https://docs.lightning.engineering/the-lightning-network/l402/protocol-specification) (lightning-labs-l402-protocol-specification) ## WWW-Authenticate The response header that carries an authentication challenge. For this paid API the value starts with the L402 scheme and includes a macaroon and a BOLT11 invoice. Library: [L402 Protocol Specification](https://docs.lightning.engineering/the-lightning-network/l402/protocol-specification) (lightning-labs-l402-protocol-specification) ## API credit One paid request on this site's convenience API. 100 sats buys 100 credits for 30 days. The current numbers are also in /.well-known/l402. Library: [L402 Protocol Specification](https://docs.lightning.engineering/the-lightning-network/l402/protocol-specification) (lightning-labs-l402-protocol-specification) ## BIP21 The bitcoin: URI scheme for a payment request. This library has no BIP21 entry. support.json publishes a BIP21 URI for the on-chain tip address, and /api/protocol-facts.json cites the BIP. This library has no entry that defines this term. ## MPP Machine Payments Protocol, an HTTP 402 standard for machine payments. Its documentation describes challenges, credentials, and receipts across more than one payment method. It is not the protocol this paid API uses. Library: [MPP: Machine Payments Protocol](https://mpp.dev/) (mpp-dev) ## AP2 Agent Payments Protocol, from the google-agentic-commerce/AP2 repository. The library lists that repository. It is not the protocol this paid API uses. Library: [google-agentic-commerce/AP2](https://github.com/google-agentic-commerce/AP2) (google-ap2-github) ## ACP Agentic Commerce Protocol, an interaction model for buyers, their agents, and businesses. The library lists the specification repository maintained by OpenAI and Stripe. It is not the protocol this paid API uses. Library: [agentic-commerce-protocol/agentic-commerce-protocol](https://github.com/agentic-commerce-protocol/agentic-commerce-protocol) (acp-github) ## UCP Universal Commerce Protocol. The library lists the ucp.dev site. It is not the protocol this paid API uses. Library: [Universal Commerce Protocol (UCP)](https://ucp.dev/) (ucp-dev) ## OpenAPI A machine-readable description of an HTTP API. The paid API serves its OpenAPI 3.1 document at /openapi.json and /v1/openapi.json on api.agentpaywire.com. The static site serves a separate document at /openapi.json. This library has no entry that defines this term. ## RSS RSS 2.0 is an XML feed format. This site publishes article RSS at /rss.xml and a changelog feed at /library/rss.xml. An Atom feed of the same changelog is at /library/atom.xml. This library has no entry that defines this term. ## CC BY 4.0 The license on this site's metadata and summaries. Linked pages keep their publishers' rights. The license URL is https://creativecommons.org/licenses/by/4.0/. This library has no entry that defines this term. ## Invoice A Lightning payment request. BOLT 11 defines a one-time invoice. This paid API creates a BOLT11 invoice in the 402 challenge. Voluntary tips can use the BOLT12 offer instead. Library: [BOLT #11: Invoice Protocol for Lightning Payments](https://github.com/lightning/bolts/blob/master/11-payment-encoding.md) (bolt-11-invoice-protocol) ## Skill An instruction file for an agent. This site publishes Agent Skills at /skills//SKILL.md. The index is /skills/index.json. This library has no separate entry for the Agent Skills specification. This library has no entry that defines this term. ## Satoshi The integer unit used by price_sats. One bitcoin is 100,000,000 satoshis. The paid API price is a count of that unit. It is not a market quote. Library: [The Bitcoin Lightning Network: Scalable Off-Chain Instant Payments](https://lightning.network/lightning-network-paper.pdf) (lightning-network-paper)