Notes
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Tool Catalog (MCP)

Before an agent can call a tool, it has to know the tool exists. Under the Model Context Protocol, a client asks each connected server to list what it offers, and what comes back is the tool catalog — the server's advertised set of callable capabilities, in a form the model can reason about.

What an entry contains
Each tool in the catalog is a small contract:

  • A name the client uses to invoke it;
  • A description in natural language, written for the model rather than for a developer — this is what the model reads when deciding whether the tool fits;
  • An input schema in JSON Schema, declaring the arguments and their types so a call can be validated before it is made;
  • Optional annotations, such as a hint that the tool only reads, or that it changes something irreversibly.

The catalog is fetched when the connection opens and cached. A server whose capabilities change notifies the client that the list has moved, which is what lets a tool set be assembled from several servers at once and still stay current.

Why it matters
The catalog is the agent's entire view of what it can do, and it is also prompt payload: every entry consumes context and competes for the model's attention, so an agent wired to a dozen servers selects worse than one with a focused set — the problem dynamic tool loading exists to solve. It is a trust boundary too. Those descriptions arrive from the server and land directly in the model's context, which makes a hostile server's wording an injection vector.

A tool catalog is the set of callable tools an MCP server advertises to a client, retrieved when the connection opens and refreshed when the server signals a change. Each entry carries a name, a natural-language description written for the model, and a JSON Schema for its arguments. It defines the entire space of actions an agent can take through that server — and because every entry occupies context, an oversized catalog degrades tool selection.

Walk into an unfamiliar workshop and the first useful thing is a list of what is on the shelves — what each tool is, what it is for, and how to hold it.

A tool catalog is exactly that, handed to an AI agent by a piece of software it has just connected to. The software lists everything on offer — search these files, send that email, look up a customer record — along with what each one needs to be given. The agent can only ever use what appears on that list, which makes the list, and the plain-English notes on it, surprisingly important.