---
title: app
sidebarTitle: app
---

# `fastmcp.apps.app`


FastMCPApp — a Provider that represents a composable MCP application.

FastMCPApp binds entry-point tools (model calls these) together with backend
tools (the UI calls these via CallTool).  Backend tools are tagged with
``meta["fastmcp"]["app"]`` so they can be found through the provider chain
even when transforms (namespace, visibility, etc.) have renamed or hidden
them — the server sets a context var that tells ``Provider.get_tool`` to
fall back to a direct lookup for app-visible tools.

Usage::

    from fastmcp import FastMCP, FastMCPApp

    app = FastMCPApp("Dashboard")

    @app.ui()
    def show_dashboard() -> Component:
        return Column(...)

    @app.tool()
    def save_contact(name: str, email: str) -> str:
        return name

    server = FastMCP("Platform")
    server.add_provider(app)


## Classes

### `FastMCPApp` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/apps/app.py#L144" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>


A Provider that represents an MCP application.

Binds together entry-point tools (``@app.ui``), backend tools
(``@app.tool``), and the Prefab renderer resource.  Backend tools
are tagged with ``meta["fastmcp"]["app"]`` so ``Provider.get_tool``
can find them by original name even when transforms have been applied.


**Methods:**

#### `tool` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/apps/app.py#L168" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```python
tool(self, name_or_fn: F) -> F
```

#### `tool` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/apps/app.py#L180" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```python
tool(self, name_or_fn: str | None = None) -> Callable[[F], F]
```

#### `tool` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/apps/app.py#L191" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```python
tool(self, name_or_fn: str | AnyFunction | None = None) -> Any
```

Register a backend tool that the UI calls via CallTool.

Backend tools default to ``visibility=["app"]``.  Pass ``model=True``
to also expose the tool to the model (``visibility=["app", "model"]``).

Supports multiple calling patterns::

    @app.tool
    def save(name: str): ...

    @app.tool()
    def save(name: str): ...

    @app.tool("custom_name")
    def save(name: str): ...


#### `ui` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/apps/app.py#L258" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```python
ui(self, name_or_fn: F) -> F
```

#### `ui` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/apps/app.py#L273" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```python
ui(self, name_or_fn: str | None = None) -> Callable[[F], F]
```

#### `ui` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/apps/app.py#L287" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```python
ui(self, name_or_fn: str | AnyFunction | None = None) -> Any
```

Register a UI entry-point tool that the model calls.

Entry-point tools default to ``visibility=["model"]`` and auto-wire
the Prefab renderer resource and CSP. They are tagged with the app
name so structured content includes ``_meta.fastmcp.app``.

Supports multiple calling patterns::

    @app.ui
    def dashboard() -> Component: ...

    @app.ui()
    def dashboard() -> Component: ...

    @app.ui("my_dashboard")
    def dashboard() -> Component: ...


#### `add_tool` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/apps/app.py#L362" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```python
add_tool(self, tool: Tool | Callable[..., Any]) -> Tool
```

Add a tool to this app programmatically.

The tool is tagged with this app's name for routing.


#### `lifespan` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/apps/app.py#L418" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```python
lifespan(self) -> AsyncIterator[None]
```

#### `run` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/apps/app.py#L426" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```python
run(self, transport: Literal['stdio', 'http', 'sse', 'streamable-http'] | None = None, **kwargs: Any) -> None
```

Create a temporary FastMCP server and run this app standalone.

