How AI Agents Find Tools: MCP Registries Explained
A tool the model cannot discover may as well not exist. Registries are the phone book for the agent era.
The Model Context Protocol (MCP) is the open standard that lets an AI assistant call external tools over a defined schema. It answers "how does the model use my tool." A registry answers the quieter but equally important question: "how does the model know my tool is there at all."
Two different problems
Building an MCP server gets your tool callable. But a callable tool sitting at an unknown URL is invisible. The registry is a searchable index of MCP servers — name, description, endpoint, and how to authenticate — that clients and assistants can query. Publishing to it is the difference between a tool your team pastes a URL for and a tool an agent can find on its own.
Callable is not the same as discoverable. One is a server; the other is a listing.
What a good registry entry says
A registry record names the server, describes what it does in plain language, points to the remote endpoint, and declares the auth method — often an OAuth connector key rather than a raw token. It should be honest about scope: what the tool touches, what it will not do without approval, and who operates it. A vague entry gets skipped; a precise one gets used.
Remote versus packaged
A registry entry can point at a hosted remote (the agent connects over streamable HTTP) or a package the user installs. Remote-first is friendlier for non-technical users: click connect, authorize, done. Packaged suits teams that want to self-host. The same tool can offer both, but the remote listing is usually the front door.
Why this matters for discovery
Search engines index web pages; registries index tools. As more work moves to agents, being in the registry is becoming as important as being on the first page of a search — it is where the software that acts on your behalf goes looking. AMH's Cloudflare Ops MCP is listed in the official registry precisely so agents can find it without a human handing over a link.
The takeaway
If you build an MCP tool, do not stop at "it works when I paste the URL." List it, describe it honestly, and declare its auth. Discovery is not automatic, and the registry is where it happens.