> ## Documentation Index
> Fetch the complete documentation index at: https://docs.tai42.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Private tool sets

> Scoped sub-MCP servers.

A private tool set is a sub-MCP server mounted inside a running server. It exposes
a chosen subset of tools under its own slug and its own MCP endpoint, so one server
can present several scoped surfaces instead of one flat catalog.

## Sub-MCP apps

A sub-MCP app is registered on the live sub-app router under a slug, listing the
tools it exposes and the transport it serves. Each registration is a self-contained
MCP surface: a client that connects to a sub-MCP sees only that set of tools, not
the whole server. The registry keeps a JSON-safe view of every registered sub-MCP —
its slug, its tools, and its transport.

## Registering and tearing down

Sub-MCP apps are managed live, at run time, through three authed doors and their
CLI commands:

```bash theme={null}
tai sub-mcp list
tai sub-mcp register billing --tool invoice --tool refund   # register or reload a slug with its tool set
```

Registering a slug builds (or reloads) its sub-MCP app; unregistering it tears down
the sub-app's ASGI lifespan cleanly. An unknown slug fails loudly with a `404`
rather than silently succeeding.

## Why scope a tool set

A private tool set narrows what a given client can see and call. Pairing a sub-MCP
with [access control](/concepts/access-control) lets you route different callers to
different scoped surfaces of the same server, keeping each client's tool list
minimal and intentional.

<Note>
  The sub-MCP namespace is `app.sub_app`. It is separate from the raw FastMCP escape
  hatch on the [MCP-citizen surface](/concepts/mcp-citizen-surface), which is named
  `app.fastmcp` precisely so it does not read as the sub-MCP.
</Note>

See the [private-tool-set guide](/guides/private-tool-set) for exposing one and the
[HTTP API reference](/reference/api/index) for the `/api/sub-mcp` routes.