# ARD Federation Federation is how [[Agent Registry (ARD)|Agent Registries]] cooperate instead of competing to be the one index that holds everything. No single registry will ever crawl the entire agentic web, so the [[Agentic Resource Discovery Specification (ARD)|ARD]] lets one registry pull in results from others. The client stays in control through a single `federation` parameter on the search request. ## Three modes - **`auto`** (the default) — the registry queries upstream registries on your behalf, merges their results with its own, and hands you one unified list. You see a single answer and don't think about where it came from. - **`referrals`** — the registry returns its own results plus a `referrals` array: catalog entries (of type `application/ai-registry`) pointing at other registries you might want to query yourself. You decide which ones to follow. - **`none`** — search this registry's index only. No fan-out. ## Why it's simple The trick is that ARD already mandates a standard HTTP REST interface on every registry. So registry-to-registry routing is just one registry calling another's `/search` over plain HTTP. There's no translation layer, no special peering protocol, no federation-specific format. A registry forwarding a query is doing the exact same thing a client does. One deliberate limit: `auto` and `referrals` apply to search, but the `/explore` aggregation endpoint does **not** federate. Counting facets across registries you don't control would produce numbers you can't trust, so explore stays scoped to the registry you asked. ## Where it shows up The motivating example in the spec is a request that spans two worlds: "Book me a flight to Tokyo and file the expense report." An orchestrator queries the enterprise registry with `referrals`, gets the internal expense agent plus a pointer to a public registry, follows it to find a flight-booking agent, and ends up with both capabilities (then invokes each over its own native protocol). One query model, results stitched across a private and a public index. ## My take This is the piece that keeps ARD from collapsing into "whoever runs the biggest registry wins." Federation over a mandatory REST baseline means the network can stay plural: enterprises run private registries, vendors run public ones, and a client can reach across them with a single flag. It mirrors how DNS resolvers and search engines already federate. Plumbing, done right, so the ecosystem doesn't have to centralize. ## References - https://agenticresourcediscovery.org/ - https://github.com/ards-project/ard-spec ## Related - [[Agentic Resource Discovery Specification (ARD)]] - [[Agent Registry (ARD)]] - [[AI Catalog (ai-catalog.json)]]