> ## Documentation Index
> Fetch the complete documentation index at: https://retrievalcontextprotocol.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Catalog

> Discover the downstream engines an aggregator federates — the introspection method behind gateways and federation.

`catalog/list` is gated by **`catalog`**. It is advertised by an **aggregator** (a
gateway that fronts several downstream engines) and lets a client discover what
sits behind it — the introspection primitive behind [Federation](/features/federation).

<CodeGroup>
  ```json Request theme={null}
  { "jsonrpc": "2.0", "id": 1, "method": "catalog/list", "params": {} }
  ```

  ```json Response theme={null}
  {
    "jsonrpc": "2.0", "id": 1, "result": {
      "engines": [
        { "id": "docs", "info": { "name": "docs-engine", "version": "1.2" },
          "capabilities": { "retrieve": { "modes": ["hybrid"] } } },
        { "id": "web", "info": { "name": "web-engine", "version": "0.9" },
          "capabilities": { "retrieve": { "modes": ["dense"] }, "graph": { "ops": ["local"] } } }
      ]
    }
  }
  ```
</CodeGroup>

## Result

<ResponseField name="engines" type="object[]">
  One entry per downstream engine, each `{ id, info, capabilities }` — the same
  identity and capability shapes returned by
  [`initialize`](/concepts/initialization). A client can therefore reason about
  each backend's abilities without connecting to it directly.
</ResponseField>

## Aggregators vs single engines

<CardGroup cols={2}>
  <Card title="Single engine" icon="server">
    Advertises retrieval capabilities (`retrieve`, `rerank`, …) and serves them
    directly. No `catalog`.
  </Card>

  <Card title="Aggregator / gateway" icon="sitemap">
    Advertises `catalog`, fronts many engines, and either routes to one or fuses
    all of them. `catalog/list` enumerates the fleet.
  </Card>
</CardGroup>

<Info>
  `catalog` makes an aggregator **self-describing**: a client points at one
  endpoint, calls `catalog/list`, and learns the whole federated topology. This is
  how a client-side [Selector or Federation](/features/federation) can also be
  driven entirely server-side behind a single URL.
</Info>