POST /explore - optional introspection over facets (§5.3.3).

Explore does not federate; it is scoped to this registry's own index.

A bare body, or one without resultType.facets, returns every supported facet rather than a 400. The outside-failure monitor watched one agent read /openapi.json, find a request schema that said only "object", and then send seven plausible bodies here and be refused each time. Introspection that has to be introspected first is a contradiction; asked for nothing specific, it now answers with everything it has.

{
  "query": {
    "text": "payments"
  },
  "resultType": {
    "facets": [
      {
        "field": "type",
        "limit": 20
      },
      "publisher"
    ]
  }
}