35 terms covering the x402 payment protocol, agentic commerce, and the surrounding chain and stablecoin ecosystem.
A blanket term for any protocol that governs payments between autonomous software agents. Concrete implementations in production today include x402 and L402; several other proposals circulate in research.
An HTTP API designed to be called primarily by AI agents, typically exposing x402 payment requirements and structured, machine-readable responses.
Transactions where one or both parties are autonomous software agents, typically AI agents buying or selling API access, data, or services without human intervention.
An x402-native inbox and delivery service for AI agents, one of several early consumer-facing products built on the protocol.
The practice of charging for API access - x402 turns this into a per-request, account-free flow that works for both humans and agents.
The process by which an AI agent detects a 402 response, selects a funding source, signs a payment payload, and retries the request without any human in the loop - relying on a facilitator to settle on-chain.
Coinbase's Layer 2 optimistic rollup, the primary settlement network used by x402 because of its low fees and native USDC support.
Chain Agnostic Improvement Proposals - a family of standards (CAIP-2, CAIP-10, CAIP-19) used by x402 to identify chains, accounts, and assets in a portable way.
A facilitator mode that validates a payment proof's structure without submitting it on-chain - used for integration testing.
The Layer 1 blockchain whose ecosystem hosts the majority of x402-compatible chains and stablecoin liquidity.
A stateless service that verifies and settles x402 payments on behalf of a resource server so the server never needs to hold funds or integrate a chain client.
The HTTPS endpoint that a resource server advertises in its 402 response for clients to use when settling a payment.
The transaction fee paid to validators on a blockchain - referred to as gas on EVM chains (Base, Polygon, Arbitrum) and as compute units / lamports on Solana. On x402's preferred chains these fees are a small fraction of a cent, enabling micropayments.
The reserved HTTP status code 'Payment Required' first defined in RFC 2068 (1997), carried forward in RFC 2616 and RFC 7231, and revived as the foundation for on-demand payment negotiation on the modern web.
A Lightning Network payment protocol developed by Lightning Labs that predates x402 and targets Bitcoin-settled API payments.
Lightning Service Authentication Tokens - a macaroon-based auth format for binding Lightning payment proofs to API access. Lightning Labs formalised the rename from LSAT to L402 in 2023 via bLIP-0026; most documentation now refers to the combined protocol as L402.
Any payment initiated and settled between two software systems - x402's target use case for B2B agent interactions.
Model Context Protocol - Anthropic's open standard for connecting AI models to tools. x402 can be layered on top when an MCP server gates calls behind payment, but this is an emerging integration pattern and is not part of the MCP spec itself.
A very small payment, typically under one cent, enabled by low-fee chains and stablecoins - the core unit of x402 pay-per-call billing.
A one-time value included in a payment proof to prevent replay attacks against resource servers.
A pricing model where the client pays a fixed amount for each API request rather than a flat monthly subscription.
A signed payment payload submitted with a retried HTTP request authorising the facilitator to settle the required amount on the client's behalf.
The machine-readable description of what a resource server demands for access - chain, asset, amount, recipient, facilitator, and expiry.
The HTTP response header defined by the x402 spec that carries structured payment requirements alongside a 402 status code. Sometimes loosely called the '402 header'; the canonical name in the spec is PAYMENT-REQUIRED.
A server-signed acknowledgement returned after a successful x402 payment, useful for audit trails and refund flows.
Any HTTP service that exposes a paid endpoint under x402 - returning 402 until a valid payment proof is supplied.
The on-chain finalisation of an x402 payment, confirmed by a facilitator before the resource server serves the paid response.
A high-throughput Layer 1 blockchain supported by x402 facilitators for USDC micropayments.
A crypto asset pegged to a fiat currency, used in x402 so both sides know the exact value of each payment - USDC is the dominant choice.
USD Coin - a dollar-pegged stablecoin issued by Circle and the default settlement asset for most x402 deployments.
An open payment protocol built on top of HTTP 402 that lets servers demand stablecoin payment before serving a resource, and lets clients (including autonomous AI agents) pay programmatically.
The Linux Foundation-hosted standards body coordinating x402 specification work, reference implementations, and ecosystem grants. Launched on 2 April 2026 with 22 founding participants including Google, Microsoft, AWS, Visa, Mastercard, Stripe, Cloudflare, Circle, Coinbase, Solana Foundation, and Polygon Labs.
A reverse-proxy or edge service that adds x402 enforcement to existing APIs without requiring the origin to implement the protocol directly.
Official and community SDKs that implement the x402 client and server flows for languages such as TypeScript, Python, and Go.
A Node.js library that wraps axios so any outbound HTTP call automatically handles 402 responses - reading requirements, paying, and retrying.
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