37 words you’ll meet in the course, in plain English.
An open standard, maintained by OpenAI and Stripe, for AI agents to complete purchases from businesses using secure payment tokens. It doesn’t use x402.
See also MPP (Machine Payments Protocol) and AP2 (Agent Payments Protocol).
A wallet an AI agent can pay from, funded by its owner and limited by rules the owner sets, such as a maximum per call.
Examples as of October 2026: Coinbase’s Agentic Wallet, a Coinbase-managed USDC wallet with owner-set limits per call and per session, and AWS Bedrock AgentCore Payments, which caps each payment session. Cloudflare Wallets were announced in August 2026, but so far you can only reserve a handle, and it can’t hold funds.
See also Wallet, Spend permission and AI agent.
Used in: Who funds the agent.
Software that uses an AI model to carry out tasks for someone, such as finding data, calling tools or making small purchases within set limits.
See also Client, Agent wallet and Crawler.
Used in: The status code that waited; Why an agent pays when free exists.
Google’s standard for letting agents pay based on instructions a user has approved in advance. Google donated it to the FIDO Alliance in April 2026.
Google also published an x402 extension for its agent-to-agent protocol, A2A, so agents can charge each other with onchain payments.
See also MPP (Machine Payments Protocol), ACP (Agentic Commerce Protocol) and x402.
A blockchain network built by Coinbase. Most x402 dollars settle on Base.
In x402 messages Base is written eip155:8453, and its test network, Base Sepolia, is eip155:84532. Base says its transactions settle in under a second and cost less than one cent. Over the 30 days to 11 October 2026, Base carried 66% of the x402 dollars that x402scan counted on Base and Solana.
See also Blockchain, Solana and Testnet.
Used in: The status code that waited; Who funds the agent; The real numbers; and 2 more lessons.
A way of writing any data using only letters, digits and a few symbols, so it travels safely in an HTTP header. x402 headers carry base64-encoded JSON.
See also x402.
An x402 scheme where many signed payments are settled together in one onchain claim, cutting the per-payment cost of very small prices.
It was merged into the x402 repository in April 2026. It matters because a facilitator fee of $0.001 can equal the whole price of a cheap call. As batching spreads, onchain payment counts can fall without demand falling.
See also Scheme and Settlement.
Used in: One payment, step by step.
Coinbase’s searchable catalogue of x402 endpoints and paid MCP tools, which agents use to find services to pay for.
It launched in September 2025 and is labelled as in early development. A service is catalogued after a facilitator processes a paid request that carries the bazaar extension. On 11 October 2026 it listed 32,169 resources, and the median one had 2 calls in the previous 30 days.
See also Facilitator, MCP (Model Context Protocol) and CDP (Coinbase Developer Platform).
A shared public record of transactions kept in step by many computers. Once a payment is recorded, anyone can look it up.
See also Base, Solana and Settlement.
Used in: The status code that waited.
Coinbase’s tools for developers, including the most used x402 facilitator, the Bazaar and the Agentic Wallet.
See also Facilitator, Bazaar and Agent wallet.
The software that asks for a resource and pays for it. In x402 it is often an AI agent, but it can be any program, or a person’s browser.
The x402 spec defines a client as “any application or agent that requests access to protected resources”. It reads the price in the 402, checks it against its owner’s limits, signs a payment and sends the request again.
See also AI agent, Wallet and Resource server.
Used in: One payment, step by step.
A program that visits web pages automatically to collect them, for a search engine, an AI training set or an agent answering a question.
Cloudflare lets sites set separate rules for search, agent and training crawlers. Its pay per crawl can charge crawlers that identify themselves by signing their requests.
See also Pay per crawl and AI agent.
Used in: Why an agent pays when free exists.
A feature built into USDC that lets you sign a transfer for someone else to submit. x402 uses it so agents can pay without paying network fees.
It is published as ERC-3009. The signed message fixes the sender, recipient, amount, a time window and a one-time random number, so the transfer can’t be altered or used twice. The facilitator submits it by calling transferWithAuthorization on the USDC contract.
See also USDC, exact (scheme) and Facilitator.
Used in: One payment, step by step.
A draft Ethereum standard for finding and trusting agents through three onchain registries: identity, reputation and validation.
The x402 scheme for a fixed price. The client signs a payment for exactly the stated amount, such as $0.001 for one reading.
By default the payment is verified before the server does the work and settled afterwards, so a failed request leaves the buyer uncharged. With USDC on Base it uses EIP-3009, and the facilitator pays the network fee.
See also Scheme, upto (scheme) and EIP-3009 (Transfer With Authorization).
Used in: Why an agent pays when free exists.
A service that checks x402 payments and submits them to the blockchain for the seller. It can’t change the amount or who gets paid.
Facilitators offer two main endpoints: /verify, which only reads and checks a signed payment, and /settle, which submits it to the chain. Coinbase’s facilitator handled 91% of Base x402 payments from April to October 2026. Its first 1,000 settlements a month are free, then $0.001 each. Other facilitators exist. In the exact scheme on Ethereum-style chains such as Base, the facilitator pays the network fee.
See also Settlement, CDP (Coinbase Developer Platform) and Gas.
Used in: The status code that waited; One payment, step by step; Myths from the comments; and 8 more lessons.
The fee a blockchain charges to process a transaction. In x402’s exact scheme on Base, the facilitator pays it, so the buyer only needs USDC.
See also Settlement and Facilitator.
The HTTP status code for “Payment Required”. Every HTTP standard since 1997 has called it reserved for future use. x402 gives it a job.
Status codes are the three-digit numbers at the start of every web response, such as 200 OK and 404 Not Found. A 1992 W3C draft sketched 402 as a way to tell the client which payment methods were acceptable. RFC 2068 (1997) reserved it for future use, and RFC 9110 (2022), the current standard, still does. x402 builds on it without changing HTTP. Cloudflare’s pay per crawl also answers with 402, using its own headers.
See also x402 and Pay per crawl.
An open standard for connecting AI apps such as Claude or ChatGPT to tools and data. x402 can charge for MCP tools.
The MCP site compares it to a USB-C port for AI applications. Over MCP, a paid tool returns the same x402 payment requirements inside its result, and the client calls the tool again with the payment attached.
Used in: One payment, step by step.
Cloudflare’s product for charging agents per request through x402. In closed beta since 30 September 2026, for US buyers and sellers only.
A seller chooses which requests to charge, an audience (everyone, or only verified bots), a price from $0.001 to $100 and a wallet. Payments settle in USDC on Base through Coinbase’s facilitator, using x402 version 2. Cloudflare’s docs listed no Cloudflare fee as of 14 October 2026.
See also Pay per crawl, Facilitator and x402.
Used in: Why an agent pays when free exists; Myths from the comments.
A rival standard for machine payments over HTTP 402, from Stripe and Tempo, launched in March 2026.
Stripe’s docs list MPP on the Tempo and Solana networks and x402 on Base. MPP’s software can also accept x402 exact payments at the same endpoint. Visa reported that MPP settled about $25,000 across roughly 115,000 transactions in its first few weeks.
See also x402, AP2 (Agent Payments Protocol) and ACP (Agentic Commerce Protocol).
Cloudflare’s beta, launched July 2025, that answers AI crawlers with a 402 and a price and lets publishers allow, charge or block each one.
It uses its own price headers, such as crawler-price, and Cloudflare acts as merchant of record, billing the crawler and paying the publisher. It is separate from the x402-based Monetization Gateway.
See also Crawler, Monetization Gateway and HTTP 402 Payment Required.
Used in: Why an agent pays when free exists; Myths from the comments.
The seller’s server. It sets the price, answers 402 to unpaid requests and serves the data once the payment checks out.
It usually works with a facilitator, which verifies and settles payments for it, so the seller doesn’t need to run blockchain software. In the default flow it has the payment verified before doing the work and settled after, so a failed request costs the buyer nothing.
See also Facilitator and Client.
Used in: One payment, step by step.
The rule for how an x402 payment is formed and settled. The main ones are exact, a fixed price, and upto, which pays for actual use up to a cap.
Each entry in a 402’s accepts list names a scheme. The x402 spec also defines batch-settlement, where many signed payments share one onchain claim, and auth-capture, which holds funds in escrow before capturing them.
See also exact (scheme), upto (scheme) and Batch settlement.
Used in: Pricing and the honest calculator; Risks; The stewards.
Paying wallets you own or control, so money goes round in a loop without buying anything from anyone else.
On Base from April to October 2026, one facilitator’s round trips between the same wallets made up about 36% of all x402 dollars. Counting tools such as x402scan can’t tell these payments from real purchases without sorting recipients by hand.
See also Wash trading and x402scan.
Used in: The real numbers.
The step where money actually moves: the facilitator submits the signed payment to the blockchain and the USDC goes from buyer to seller.
Settlement produces a transaction hash, a public ID anyone can look up, which the server returns in the PAYMENT-RESPONSE header. Signing and verifying move nothing. Only settlement does.
See also Facilitator and Gas.
Used in: One payment, step by step.
Another blockchain network that x402 supports. It passed Base on x402 payment counts for a few weeks in August and September 2026.
Artemis data put Solana at 76% of x402 payments in the four weeks to 22 September 2026. On dollars Base still led, with 66% over the 30 days to 11 October by x402scan’s count.
See also Base.
Used in: The real numbers.
A permission you sign once that lets an app or agent spend up to a set amount from your wallet each period, such as 10 USDC a month.
Coinbase’s Spend Permissions work with its smart wallets on Base and other chains. The spender can only move tokens within the allowance and period you approved. It can’t make other calls from your account.
See also Agent wallet and Wallet.
Used in: Who funds the agent.
A digital token designed to hold a steady value, usually one US dollar, so that it works for payments.
See also USDC.
Used in: The status code that waited; Myths from the comments; Risks; and 1 more lesson.
A practice copy of a blockchain where tokens have no real value, used to test payments for free. Base’s testnet is Base Sepolia.
The x402 docs suggest the free facilitator at x402.org/facilitator, which works on Base Sepolia and Solana devnet, for testing before switching to a production facilitator.
See also Base and Facilitator.
The x402 scheme for usage-based prices. The client authorizes a maximum, and the seller settles the actual amount, which can be less, or zero.
It suits work whose cost isn’t known in advance, such as paying per AI token generated. Each authorization can be settled once, expires, and is tied to one recipient. Cloudflare’s Monetization Gateway uses it for variable prices.
See also Scheme and exact (scheme).
Used in: Why an agent pays when free exists.
A stablecoin issued by Circle and redeemable one for one for US dollars. It is the default currency for x402 payments on Base.
Circle says USDC is fully reserved, backed by cash and cash-equivalent assets. It counts in millionths of a dollar, so an x402 amount of 1000 is $0.001. USDC supports EIP-3009, which lets someone else submit a transfer you have signed.
See also Stablecoin and EIP-3009 (Transfer With Authorization).
Used in: The status code that waited; One payment, step by step; Who funds the agent; and 7 more lessons.
Software that holds the keys to a blockchain address. Its private key signs payments, and its address is where money is received.
In x402 the seller only needs an address to be paid into, called payTo. The buyer needs a wallet holding USDC that can sign. A wallet can be run by a person, an app or a provider such as Coinbase.
See also Agent wallet and Spend permission.
Used in: The status code that waited; Who funds the agent; How to accept x402 today; and 1 more lesson.
Paying or trading back and forth between related wallets to make activity look bigger than it really is.
In x402 data it shows up as large volumes between a small set of wallets. Visa’s analysis with Artemis, which removed identified wash and test activity, found about $15.0M of x402 volume from launch to April 2026.
See also Self-dealing and x402scan.
Used in: The real numbers.
An open standard that lets software pay for something on the web inside an ordinary HTTP request, using the 402 Payment Required status code.
Announced in May 2025 and created at Coinbase, x402 is now looked after by the Linux Foundation’s x402 Foundation. A server answers 402 with a price in a PAYMENT-REQUIRED header. The client signs a payment and repeats the request with a PAYMENT-SIGNATURE header. The server returns the resource with a PAYMENT-RESPONSE header as a receipt. Version 2, released in December 2025, is the current one. Payments are usually in USDC, most often on Base or Solana.
See also HTTP 402 Payment Required, Facilitator, Scheme and x402 Foundation.
The Linux Foundation body that looks after x402. Announced in April 2026, it listed 58 member organisations in October 2026.
Coinbase contributed the protocol. A Technical Steering Committee with seats for Coinbase, Cloudflare and Stripe makes technical decisions, and the code is open source under the Apache-2.0 licence. Michael Hursta was named executive director on 1 October 2026.
See also x402.
Used in: The map; The stewards.
An open-source explorer from Merit Systems that counts x402 payments by chain, facilitator, buyer and seller.
It counts USDC transfers made by the facilitators it knows about, on Base, Solana, Polygon and Optimism. It doesn’t count web requests, and it can’t tell a purchase from a payment between related wallets. It is the main free source of x402 numbers.
See also Wash trading and Self-dealing.
Used in: The real numbers.