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 ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/src/fastmcp/apps/app.py#L142" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

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 ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/src/fastmcp/apps/app.py#L164" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

tool(self, name_or_fn: F) -> F

tool ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/src/fastmcp/apps/app.py#L176" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

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

tool ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/src/fastmcp/apps/app.py#L187" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

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 ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/src/fastmcp/apps/app.py#L252" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

ui(self, name_or_fn: F) -> F

ui ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/src/fastmcp/apps/app.py#L267" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

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

ui ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/src/fastmcp/apps/app.py#L281" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

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 ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/src/fastmcp/apps/app.py#L355" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

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 ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/src/fastmcp/apps/app.py#L409" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

lifespan(self) -> AsyncIterator[None]

run ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/src/fastmcp/apps/app.py#L417" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

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

Create a temporary FastMCP server and run this app standalone.