# Causeway Services > An x402 market where software pays in USDC over HTTP 402, plus guides, books, Koios News and games from the 561 Group. Causeway Services is the public-services company of the 561 Group. It runs market.561.group, an x402 market where 130 offers (as of 2026-10-21) are bought per request in USDC on Base over HTTP 402, and publishes plain-language guides to x402, a book about it, Koios News and games. Numbers are dated and sourced, and the market's own results are reported as they are: the market is small and demand is unmeasured. ## Articles - [Four-valued logic, sold per call: Belnap's logic as an x402 API](https://services.561.group/four-valued-logic-api-sold-per-call/): Eight endpoints at $0.001 to $0.002 sell Belnap's four-valued logic over HTTP 402: join, meet, negate, conflate and count true, false, both and neither. - [How to buy from an x402 market as a software agent: a quickstart](https://services.561.group/how-to-buy-from-an-x402-market-as-a-software-agent/): Discover offers, read a live 402, decode payment-required, sign an EIP-3009 authorization, retry with PAYMENT-SIGNATURE and keep the receipt. - [Causeway Services: x402 guides, a market where software pays, books, news and games](https://services.561.group/): Causeway Services runs an x402 market of 130 offers payable in USDC over HTTP 402, and publishes guides, books, Koios News and games. Start here. - [The 130-offer x402 market explained: what software can buy at market.561.group](https://services.561.group/the-130-offer-x402-market-explained/): 130 machine-payable offers in USDC on Base, from $0.001 data conversions to $12 study books: every family, price range and what each does, as of 2026-10-21. - [What is x402? HTTP 402 Payment Required for AI agents and APIs](https://services.561.group/what-is-x402/): x402 is an open protocol that lets software pay for a web request in USDC by answering HTTP 402 Payment Required. How it works, who runs it, who uses it. - [Koios News: an AI-assisted newsroom with RSS feeds and a $0.25 x402 briefing](https://services.561.group/koios-news-and-its-x402-briefing/): Koios.News publishes source-bound, disclosed AI-assisted reporting in ten sections, with RSS, Atom and JSON feeds, and sells a $0.25 briefing over x402. - [How big is the x402 market? What the careful numbers say, October 2026](https://services.561.group/how-big-is-the-x402-market/): Filtered x402 commerce is tens of millions of dollars, not hundreds; raw counts include wash trading and tests. Each figure with its source and date. - [Market, Bazaar and Shop: how the 561 Group's three storefronts relate](https://services.561.group/market-bazaar-and-shop-explained/): market.561.group sells by x402, bazaar.561.group searches and compares 143 listings, shop.561.group sells books. Why shop-in-bazaar-in-market is a claim. - [Play: 21 browser games from the 561 Group that show you the odds](https://services.561.group/play-561-group-games-with-visible-odds/): Pig, Yacht, Farkle, Fair Spin, Kelly's Wager and 16 more at play.561.group: practice-money games that display bust probability, expected value and house edge. - [How an x402 payment works, step by step: one request, one payment](https://services.561.group/how-an-x402-payment-works-step-by-step/): A single x402 payment in five steps: the 402 response, the signed EIP-3009 authorization, facilitator verify and settle, and the receipt. Version 2 headers. - [Is x402 legal? The GENIUS Act, sanctions and open questions for stablecoin payments](https://services.561.group/is-x402-legal-the-genius-act-and-stablecoin-payments/): The GENIUS Act regulates US stablecoin issuers, not x402 sellers; sanctions rules still apply to every payment. What the law settles and what remains open. - [The x402 Economy: a book sold for $9 in USDC over HTTP 402](https://services.561.group/the-x402-economy-book/): The x402 Economy is a 63-page book on HTTP 402 and the x402 market, sold for $9 in USDC through the protocol it explains. Contents and how to buy. - [What does HTTP 402 Payment Required mean? A history of the web's unused code](https://services.561.group/what-http-402-payment-required-means/): HTTP 402 Payment Required was reserved in the web's standard in the 1990s and left unused for decades. Its history, why micropayments failed, and what changed. - [Who built x402 and who governs it? From Coinbase to the Linux Foundation](https://services.561.group/who-built-x402-and-who-governs-it/): Coinbase released x402 in May 2025; the Linux Foundation's x402 Foundation launched 14 July 2026 with 40 members. Timeline, members, and what it doesn't govern. - [x402 glossary: 44 terms for HTTP 402, agent payments and USDC](https://services.561.group/x402-glossary/): Plain definitions of x402, HTTP 402, facilitator, EIP-3009, CAIP-2, USDC, MCP, A2A, AP2, MPP, L402, calls per payer and more, each with its first-use context. - [x402 security: risks and defenses for AI agent buyers and API sellers](https://services.561.group/x402-security-risks-for-buyers-and-sellers/): Spending limits outside the model, payments bound to requests, settle before delivery: what 2026 research found wrong with x402 and how to defend. - [What happens when you sell an API on x402? Lessons from the Bazaar and one small seller](https://services.561.group/what-happens-when-you-sell-an-api-on-x402/): Median x402 price: one cent. A few sellers earn nearly everything. What sold, calls per payer, and our own ephemeris gateway that earned about fifty cents. - [x402 vs MPP, AP2, ACP and L402: agent payment protocols compared](https://services.561.group/x402-vs-mpp-ap2-acp-and-l402/): x402, MPP and L402 let one HTTP request carry its payment; AP2 and ACP prove a person approved a purchase. Each protocol sorted by the question it answers. ## Machine interfaces - [x402 discovery](https://market.561.group/.well-known/x402): Every payable resource on the market with method, price, asset and network. - [Market catalog](https://market.561.group/v1/catalog): Input contracts, examples, output schemas and recovery rules for all 130 offers. - [Market llms.txt](https://market.561.group/llms.txt): The market's offers in llms.txt form. - [A2A agent card](https://market.561.group/.well-known/agent-card.json): One agent-to-agent skill per offer. - [Read-only MCP](https://market.561.group/mcp): POST JSON-RPC tools list_apis, search_apis and get_api; never pays. - [Bazaar discovery](https://bazaar.561.group/discovery.json): Search, detail and comparison over the Shop and Market catalogs. - [Shop catalog](https://shop.561.group/catalog.json): Original book data with prices and file digests. ## 561 Group - [561 Group](https://561.group/): The parent organization. - [Science](https://science.561.group/): Research notes behind the claims published here. - [Engineering](https://engineering.561.group/): The software the services are built from. - [Operations](https://operations.561.group/): How the services are run. - [Market](https://market.561.group/): Machine-payable offers priced in USDC on Base. - [Koios.News](https://koios.news/): General news with disclosed AI-assisted sourcing, feeds and a $0.25 x402 briefing. - [Shop](https://shop.561.group/): Books as EPUB and PDF. ## Feeds and discovery - [Sitemap](https://services.561.group/sitemap.xml): every page with last-modified dates - [RSS feed](https://services.561.group/feed.rss.xml) - [Atom feed](https://services.561.group/feed.atom.xml) - [JSON Feed](https://services.561.group/feed.json) - [Full text](https://services.561.group/llms-full.txt): every article as Markdown in one file - [Search](https://services.561.group/search/): query with ?q= - Fediverse: @services@services.561.group (https://services.561.group/actors/services)