# 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`](https://market.561.group/.well-known/x402) and [`/v1/catalog`](https://market.561.group/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](https://services.561.group/how-big-is-the-x402-market/). Read this catalog as supply.

### How are the 130 offers grouped?

| Family | Offers | Price per call | What it does |
| --- | --- | --- | --- |
| Data and format conversion (`/v1/data/*`) | 69 | $0.0012 | Standards-exact conversions and checks for JSON, CSV, YAML, TOML, XML, HTML, CBOR, Base16/32/58/64, URLs, hashes, compression, HTTP fields |
| Four-valued logic (`/v1/four/*`) | 8 | $0.001 to $0.002 | Belnap's four-valued logic as an API: join, meet, negate, conflate, convert, count |
| SEN relating (`/v1/sen/*`) | 11 | $0.001 to $0.01 | The Group's Semiotic Expression Notation pipeline: utter, articulate, settle, relate, type-infer |
| Content addressing, schema, allocation | 3 | $0.002 to $0.02 | Content ids, JSON Schema validation, optimal budget allocation |
| Translation (`/v1/translate/*`) | 6 | $0.001 to $0.01 | Mandarin to English, subtitles, interlinear reading, reviewed-episode records |
| Base blockchain reads (`/v1/base-chain/*`) | 3 | $0.001 to $0.002 | Live balance, gas price and transaction lookup from a Base node |
| Ephemeris and astronomy | 4 | $0.04 or $0.32 | Planetary positions at a moment, with error bounds; JPL DE440 precision tier |
| Bibliography | 2 | $0.002 and $0.02 | ISBN book records; citation checking against CrossRef, OpenAlex and Open Library |
| Policy checks for transactions | 2 | $0.03 and $0.04 | Deterministic pass, fail or escalate receipts before a transaction is signed |
| x402 market observation | 1 | $0.10 | A sampled measurement of Base USDC authorization activity |
| Pacing intelligence | 2 | $0.01 and $0.05 | Analysis of caller-submitted biometric observations; stateless |
| Software supply-chain triage | 1 | $0.02 | Ranks packages as orphan-patch candidates from public inputs |
| Claim and metered-inference utilities | 2 | $0.001 and $0.01 | A structural-claim heuristic; a metered inference contract |
| Koios briefing | 1 | $0.25 | The current Koios intelligence briefing |
| Publications | 7 | $9.00 to $12.00 | Books as EPUB and PDF in one ZIP |
| Further analysis operations | 8 | $0.01 to $15.00 | Evidence packets and analysis outputs, listed in the catalog; we do not describe them here |

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:

```bash
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](https://services.561.group/four-valued-logic-api-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](https://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:

| Route | Price | Returns |
| --- | --- | --- |
| `GET /v1/base-chain/balance` | $0.001 | Native ETH balance of one address |
| `GET /v1/base-chain/gas-oracle` | $0.001 | Current gas price in wei and gwei |
| `GET /v1/base-chain/transaction` | $0.002 | One transaction by hash, or `found:false` (billed) for an unknown hash |

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?](https://services.561.group/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?](https://services.561.group/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*](https://services.561.group/the-x402-economy-book/) and six independent study companions to professional certificates. The [Koios intelligence briefing](https://services.561.group/koios-news-and-its-x402-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](https://services.561.group/how-to-buy-from-an-x402-market-as-a-software-agent/).

### 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](https://services.561.group/market-bazaar-and-shop-explained/).

### Where is the primary data?

- [`/.well-known/x402`](https://market.561.group/.well-known/x402): x402 discovery, 130 resources
- [`/v1/catalog`](https://market.561.group/v1/catalog): contracts and examples
- [`/llms.txt`](https://market.561.group/llms.txt), [`/openapi.json`](https://market.561.group/openapi.json), [`/.well-known/agent-card.json`](https://market.561.group/.well-known/agent-card.json), [`/.well-known/api-catalog`](https://market.561.group/.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.