The 130-offer x402 market explained: what software can buy at market.561.group

The 130-offer x402 market explained: what software can buy at market.561.group

As of 2026-10-21, the Group's market at [market.561.group](https://market.561.group/) lists 130 offers, each payable per call in USDC on Base through x402. Prices run from $0.001 to $15.00, and the median price is $0.0012: the typical purchase is a fraction of a cent. Seventeen offers are GET requests and 113 are POST requests with a JSON body. This article is derived from the market's live discovery documents, `/.well-known/x402` and `/v1/catalog`, and is meant as a map for people and for agents choosing what to call.

A market whose customers are software is strange to read. It has no shop window, no cart and no sign-up. Every offer is a route, a price and a contract for the input it accepts. The catalog says exactly that, and so do we.

What kind of market is this?

It is a seller's catalog: one seller, the Group's Causeway Services, offering operations and publications that an HTTP client can buy without an account. Each offer has a route, a method, a price, an input schema, an output schema, an example, and (for books and the briefing) a recovery rule. Every unpaid request to an offer returns an HTTP 402 with the live payment terms; the terms are authoritative and the prices in discovery are previews. The settlement is the same everywhere: the exact scheme, network eip155:8453 (Base), asset USDC.

The one honest caveat belongs up front. A listing and a 402 are not customers. The market's own documentation says an HTTP 402 or a listing does not establish a customer purchase, and that internal self-purchases are not external demand. We built and listed these offers ahead of demand, and the evidence on how much demand exists across all of x402 is thin. Read this catalog as supply.

How are the 130 offers grouped?

Together that is 130. Below, the families people ask about most.

What are the 69 data-conversion offers?

The largest family is the least glamorous and probably the most reusable: 69 small operations, all at $0.0012, that do one standards-defined transformation exactly. Examples, by standard:

• JSON: RFC 8785 canonicalization, RFC 6901 pointers, RFC 7396 merge patch, RFC 6902 patch apply, check and create, JSON Schema meta-validation and instance validation (drafts 4 through 2020-12).
• Other formats: CSV to JSON and back (RFC 4180), JSON Lines, TOML 1.1, YAML 1.2, XML to ordered nodes, HTML parsing with the native error-recovery rules, CBOR (RFC 8949) encoding, decoding, diagnostics and deterministic-encoding checks.
• Encodings: Base16, Base32, Base58 (Bitcoin alphabet), Base64 and Base64url (RFC 4648), strict UTF-8, Unicode normalization (NFC, NFD, NFKC, NFKD), gzip, zlib and raw DEFLATE.
• Names and fields: WHATWG URL normalization, form data, MIME types, RFC 6570 URI templates, RFC 9110 HTTP dates, RFC 9651 structured fields, RFC 9530 content digests, RFC 6920 content names, SHA-2 multihashes and CIDs.

They share a design rule that matters to a paying machine: input that cannot be processed is refused before any payment is requested. Duplicate JSON keys, invalid Unicode, non-finite numbers, malformed Base64 and similar faults return 4xx with no 402. Most accept up to 64 KiB. A worked call to the canonical-JSON offer:

curl -s -X POST https://market.561.group/v1/data/json-canonicalize \
  -H 'content-type: application/json' \
  -d '{"text":"{\"z\":2,\"a\":1}"}'

That unpaid request returns 402 with the price ($0.0012, which is 1200 atomic units of USDC). With payment, the documented response is {"text":"{\"a\":1,\"z\":2}","byteLength":13,"standard":"RFC8785"}. A thousand such calls would cost $1.20.

Why would software pay for these when libraries exist? Sometimes it should not. The case for the offers is an agent in a sandbox without the right runtime, a pipeline that needs a second opinion from an independent implementation, or a task where a signed receipt for the transformation is worth the price.

What are the four-valued-logic offers?

Eight operations at $0.001 or $0.002 implement Belnap's four-valued logic over named points. They are strange enough to get their own article: Four-valued logic, sold per call.

What do the SEN offers do?

SEN, the Semiotic Expression Notation, is the Group's own formal notation for claims. Eleven offers expose its pipeline in steps or in one call (/v1/sen/induce, $0.01): admit a token and a claim as an utterance, articulate it, generate its predication site, posit a valuation over it, and settle the result to a least fixed point, a coordination-free settlement of what was told. Others judge whether two payloads coincide, canonicalize a JSON carrier, and infer the type of a closed term (refused before payment if the term is open or malformed). These are research instruments offered as services; the Group's research on them is at science.561.group. Nobody should buy them expecting a mainstream use case.

What data does the market read live?

Three offers read the Base network directly from a public node, returning the snapshot block they read at:

These are the same kind of service that, in Coinbase's Bazaar catalog, drew the most distinct payers in August 2026, and they carry the same warning: those services were sampled once by many buyers and rarely revisited. See What happens when you sell an API on x402?

A separate offer, GET /v1/x402-market/observation at $0.10, returns a sampled measurement of Base USDC AuthorizationUsed events with provider provenance and an explicit extrapolation. Its own description says it does not identify x402 use or agent identity, which is a deliberate honesty about a hard measurement problem.

What are the ephemeris and astronomy offers?

Four offers compute where the planets are at a given moment. Two are $0.04 (a deterministic apparent tropical ephemeris with signs, motion, lunar phase and aspects; and apparent geocentric positions with lunar illumination), and two are $0.32 (a JPL DE440-backed precision moment with houses and angles; and DE440 apparent positions with light time and cross-engine measurement evidence). They are the Group's first x402 product and have a candid sales history, told in What happens when you sell an API on x402?

What do the translation offers do?

Six offers translate Mandarin Chinese into English (plain text or SRT and WebVTT subtitles, timings kept), translate short-drama episodes with retained cue identity and a private series key for approved terminology, record reviewer corrections, and read Mandarin at every level at once as an interlinear reading with segmented words, tone-marked pinyin and glosses from open lexicons. Prices are $0.001 to $0.01. Translation outputs are drafts with mechanical checks, not certified translations.

What are the books and the briefing?

Seven publications, priced $9.00 to $12.00, are delivered as one ZIP with an EPUB and a PDF: *The x402 Economy* and six independent study companions to professional certificates. The Koios intelligence briefing is $0.25 and returns source-led analysis as JSON with every section linked to its cited evidence. Books and the briefing accept an X-Payment-Recovery token so a dropped connection never means paying twice.

How do I find the offer I need?

Three ways, all free:

1. Read the catalog. GET /v1/catalog returns each offer's route, price, request.bodySchema, request.example, output.schema and description. The category and gateway fields group the families above.
2. Search through MCP. POST /mcp with tools/list, then search_apis with a query such as base gas, and get_api with an offer's SKU. It never pays.
3. Ask the agent card. /.well-known/agent-card.json lists one A2A skill per offer with examples.

Then follow the software-agent quickstart.

How is the market kept honest?

• Prices are previews; the 402 is authoritative. Some offers price by input size and show a starting price; their 402 quotes the exact amount.
• Rights claims are labeled. Offers may carry a sourceRightsClaim, and the catalog marks it seller testimony, not independently verified resale authority.
• Verification is independent. The market's operator procedure asks for payee, network and asset to be checked against an independently supplied address, and states that an HTTP 200, a 402 and a commit alone prove none of: delivery, settlement or customer demand.
• Demand is unknown, not zero. The market's own README says a missing demand signal means demand is unknown. We agree, and we do not claim revenue.

The Market also sits in a family of storefronts with the Bazaar and the Shop; that relationship, and why it is not as simple as "inside", is the subject of Market, Bazaar and Shop.

Where is the primary data?

• `/.well-known/x402`: x402 discovery, 130 resources
• `/v1/catalog`: contracts and examples
• `/llms.txt`, `/openapi.json`, `/.well-known/agent-card.json`, `/.well-known/api-catalog`

Counts and prices in this article were read from those documents on 2026-10-21. The catalog changes as offers are added or held; trust the live document over this page.