discovery
Discovery is how an x402 endpoint becomes findable at all: the endpoint describes itself in its own payment challenge, and a facilitator collects those descriptions into a catalogue. The description is the mechanism; the catalogue is the list it produces.
The description travels in the challenge. x402's bazaar extension enables resource discovery and cataloging for x402-enabled endpoints and MCP tools: a resource server declares its endpoint specification — the HTTP method or MCP tool name, the input parameters and the output format — so that facilitators can catalog and index them in a discovery service. The core specification carries some of the same material in the resource object itself, including a service name and topical tags stated as being for discovery filtering.
The collecting is the facilitator's. Endpoints that settle through one are the population it can list, which is why a catalogue is that facilitator's own list rather than a view of the rail. Discovery is what fills a catalogue; the catalogue is what a reader gets to see.
This is a design where being findable is a property of the answer an endpoint gives. An endpoint that returns a payment challenge without the extension is payable and undescribed, so absence from a list is not evidence about whether an address answers.
That gap is the reason a record of what was listed, and when, is a different instrument from the list. A catalogue publishes its current state; the date an endpoint entered or left it cannot be recovered from the list afterwards.
→ probe402 reads the catalogues it derives targets from on a fixed clock and retains every capture; how that capture works is set out in the method.
Sources
- Extension: bazaarx402 Foundation
- The x402 specification (v2)x402 Foundation
- Seller: get discoveredCoinbase Developer Platform documentation
Related catalogue · facilitator · extension · MCP