---
title: FastMCPApp
sidebarTitle: FastMCPApp
description: Wire an interactive UI to backend tools with managed visibility and composition safety.
icon: puzzle-piece
tag: NEW
---

import { VersionBadge } from '/snippets/version-badge.mdx'
import PrefabPinWarning from '/snippets/prefab-pin-warning.mdx'
import { PrefabDemoFrame } from '/snippets/prefab-demo-frame.mdx'

<VersionBadge version="3.2.0" />

<PrefabPinWarning />

<PrefabDemoFrame demo="contacts" height="650px" title="Contacts app demo" />

Search a list, fill out a form, click save, the list updates. That pattern — UI that reads and writes data on the server — needs two things: backend tools that actually do the work, and a way to call them from the UI. `FastMCPApp` handles the wiring.

You'll build up to the contacts app above by the end of this page. Let's start with something smaller.

## A minimal interactive app

The smallest interactive app: a form that saves a note, and a list that updates when the user submits.

```python
from prefab_ui.actions import SetState, ShowToast
from prefab_ui.actions.mcp import CallTool
from prefab_ui.app import PrefabApp
from prefab_ui.components import (
    Badge, Button, Column, ForEach, Form, Heading,
    Input, Row, Separator, Text,
)
from prefab_ui.rx import RESULT
from fastmcp import FastMCP, FastMCPApp

app = FastMCPApp("Notes")
notes_db: list[dict] = []


@app.tool()
def add_note(title: str, body: str) -> list[dict]:
    """Save a note and return all notes."""
    notes_db.append({"title": title, "body": body})
    return list(notes_db)


@app.ui()
def notes_app() -> PrefabApp:
    """Open the notes app."""
    with Column(gap=6, css_class="p-6") as view:
        Heading("Notes")

        with ForEach("notes") as note:
            with Row(gap=2, align="center"):
                Text(note.title, css_class="font-semibold")
                Badge(note.body)

        Separator()

        with Form(
            on_submit=CallTool(
                "add_note",
                on_success=[
                    SetState("notes", RESULT),
                    ShowToast("Note saved!", variant="success"),
                ],
                on_error=ShowToast("Failed to save", variant="error"),
            )
        ):
            Input(name="title", label="Title", required=True)
            Input(name="body", label="Body", required=True)
            Button("Add Note")

    return PrefabApp(view=view, state={"notes": list(notes_db)})


mcp = FastMCP("Notes Server", providers=[app])
```

The model sees one tool: `notes_app`. Calling it opens the UI. When the user submits the form, `CallTool("add_note")` fires, the server saves the note, returns the updated list, and `SetState("notes", RESULT)` writes that list back into state. `ForEach("notes")` re-renders. The model never sees `add_note` — it's UI-only.

## Why not just `@mcp.tool(app=True)`?

A fair question. Any [Interactive Tool](/apps/prefab) can call a server tool — there's nothing stopping you from putting `CallTool("add_note")` inside a regular `@mcp.tool(app=True)`. It works for one or two tools. Things get harder once the app grows:

- Which tools should the model see, and which are UI-only?
- What happens to `CallTool("add_note")` when you mount this server under a namespace and the tool becomes `notes_add_note`?
- How do you keep it all wired correctly as you compose servers?

`FastMCPApp` owns these concerns. Entry points register as model-visible. Backend tools register as UI-only by default. Backend tools get globally stable identifiers that survive namespacing, and `CallTool` accepts function references, so references stay valid when you compose servers.

The rest of this page covers each piece in turn.

## `@app.ui()` — entry points

Entry points are what the model sees. They return a `PrefabApp` and default to `visibility=["model"]`, showing up in the LLM tool list but not callable from within the UI.

```python
@app.ui()
def dashboard() -> PrefabApp:
    """The model calls this to open the dashboard."""
    with Column(gap=4, css_class="p-6") as view:
        Heading("Dashboard")
        ...
    return PrefabApp(view=view)
```

`@app.ui()` supports the same options as `@mcp.tool`: `name`, `description`, `title`, `tags`, `icons`, `auth`, and `timeout`.

## `@app.tool()` — backend tools

Backend tools do the work. By default they're visible only to the UI (`visibility=["app"]`), not the model.

```python
@app.tool()
def save_contact(name: str, email: str) -> list[dict]:
    """Save a contact and return the updated list."""
    db.append({"name": name, "email": email})
    return list(db)
```

If you want a tool callable by both the model and the UI, pass `model=True`:

```python
@app.tool(model=True)
def list_contacts() -> list[dict]:
    """Both the model and the UI can call this."""
    return list(db)
```

Backend tools support `name`, `description`, `auth`, and `timeout`.

## `CallTool` — UI → backend

`CallTool` is how the UI invokes a backend tool. Pass the tool's name (or a direct function reference):

```python
from prefab_ui.actions.mcp import CallTool

CallTool("save_contact", arguments={"name": "Alice", "email": "alice@example.com"})

# Or a function reference — resolves to a stable global key
CallTool(save_contact, arguments={...})
```

Arguments can reference state with `Rx`:

```python
from prefab_ui.rx import STATE

CallTool("search", arguments={"query": STATE.search_term})
```

### Handling results

Server calls are async. Use `on_success` and `on_error` callbacks:

```python
from prefab_ui.actions import SetState, ShowToast
from prefab_ui.rx import RESULT

CallTool(
    "save_contact",
    on_success=[
        SetState("contacts", RESULT),
        ShowToast("Saved!", variant="success"),
    ],
    on_error=ShowToast("Something went wrong", variant="error"),
)
```

`RESULT` is a reactive reference to the tool's return value, available inside `on_success`. `ERROR` (from `prefab_ui.rx`) is the counterpart inside `on_error`. Callbacks can be a single action or a list; they execute in order and short-circuit on error.

### `result_key` shorthand

When a tool's return value should replace a state key, use `result_key`:

```python
CallTool("list_contacts", result_key="contacts")

# same as:
CallTool("list_contacts", on_success=SetState("contacts", RESULT))
```

## Actions

`CallTool` is one of several actions. Actions attach to handlers like `on_click`, `on_submit`, and `on_change`.

Client-side actions run instantly in the browser, no server round-trip:

```python
from prefab_ui.actions import SetState, ToggleState, AppendState, PopState, ShowToast

SetState("count", 42)
ToggleState("expanded")
AppendState("items", {"name": "New Item"})
PopState("items", 0)
ShowToast("Done!", variant="success")
```

Pass a list to chain actions:

```python
Button(
    "Reset",
    on_click=[
        SetState("query", ""),
        SetState("results", []),
        ShowToast("Cleared"),
    ],
)
```

### Loading states

A common pattern: disable a button and show a spinner while a call is in flight.

```python
from prefab_ui.rx import Rx

saving = Rx("saving")

Button(
    saving.then("Saving...", "Save"),
    disabled=saving,
    on_click=[
        SetState("saving", True),
        CallTool(
            "save_data",
            on_success=[
                SetState("saving", False),
                SetState("result", RESULT),
                ShowToast("Saved!", variant="success"),
            ],
            on_error=[
                SetState("saving", False),
                ShowToast("Failed", variant="error"),
            ],
        ),
    ],
)

# PrefabApp(view=view, state={"saving": False, ...})
```

## Forms

Forms collect input and submit it to a tool. When submitted, named input values become the tool's arguments.

### Manual forms

```python
from prefab_ui.components import Form, Input, Select, SelectOption, Textarea, Button

with Form(
    on_submit=CallTool(
        "create_ticket",
        on_success=ShowToast("Ticket created!", variant="success"),
    )
):
    Input(name="title", label="Title", required=True)
    with Select(name="priority", label="Priority"):
        SelectOption("Low", value="low")
        SelectOption("Medium", value="medium")
        SelectOption("High", value="high")
    Textarea(name="description", label="Description")
    Button("Create Ticket")
```

On submit, `CallTool` receives `{"title": ..., "priority": ..., "description": ...}`.

### Forms from Pydantic models

For structured input, `Form.from_model()` generates the whole form — inputs, labels, validation:

```python
from typing import Literal
from pydantic import BaseModel, Field

class BugReport(BaseModel):
    title: str = Field(title="Bug Title")
    severity: Literal["low", "medium", "high", "critical"] = Field(
        title="Severity", default="medium"
    )
    description: str = Field(title="Description")


@app.ui()
def report_bug() -> PrefabApp:
    with Column(gap=4, css_class="p-6") as view:
        Heading("Report a Bug")
        Form.from_model(
            BugReport,
            on_submit=CallTool(
                "create_bug",
                on_success=ShowToast("Bug filed!", variant="success"),
            ),
        )
    return PrefabApp(view=view)


@app.tool()
def create_bug(data: BugReport) -> str:
    return f"Created: {data.title}"
```

`str` becomes a text input, `Literal` becomes a select, `bool` becomes a checkbox. Field titles and defaults are respected.

## Composition and namespacing

The reason `FastMCPApp` exists — and why you'd pick it over plain `@mcp.tool(app=True)` with string-based `CallTool` — is composition safety.

When you mount a server under a namespace, tool names get prefixed:

```python
platform = FastMCP("Platform")
platform.mount("contacts", contacts_server)

# "save_contact" becomes "contacts_save_contact"
```

`CallTool("save_contact")` would now be broken. But `CallTool(save_contact)` with a function reference resolves to a globally stable identifier that bypasses the namespace. Your app works the same whether standalone or mounted.

### Mounting

`FastMCPApp` is a Provider. Add it to a server with `providers=` or `add_provider`:

```python
mcp = FastMCP("Platform", providers=[app])

# or
mcp = FastMCP("Platform")
mcp.add_provider(app)
```

Multiple apps can coexist; each gets its own global keys, so there's no collision even if two apps have a tool named `save`.

```python
mcp = FastMCP("Platform", providers=[contacts_app, inventory_app, billing_app])
```

### Running standalone

For development, `FastMCPApp` has a `run()` shortcut that wraps itself in a temporary `FastMCP` server:

```python
app = FastMCPApp("Contacts")
# ... register tools ...

if __name__ == "__main__":
    app.run()
```

## A full example: contact manager

This brings everything together — entry point, backend tools, Pydantic form, manual form, state, actions, and multi-visibility.

```python expandable
from __future__ import annotations

from typing import Literal

from prefab_ui.actions import SetState, ShowToast
from prefab_ui.actions.mcp import CallTool
from prefab_ui.app import PrefabApp
from prefab_ui.components import (
    Badge, Button, Column, ForEach, Form,
    Heading, Input, Muted, Row, Separator, Text,
)
from prefab_ui.rx import RESULT, Rx
from pydantic import BaseModel, Field
from fastmcp import FastMCP, FastMCPApp

contacts_db: list[dict] = [
    {"name": "Arthur Dent", "email": "arthur@earth.com", "category": "Customer"},
    {"name": "Ford Prefect", "email": "ford@betelgeuse.org", "category": "Partner"},
]


class ContactModel(BaseModel):
    name: str = Field(title="Full Name", min_length=1)
    email: str = Field(title="Email")
    category: Literal["Customer", "Vendor", "Partner", "Other"] = "Other"


app = FastMCPApp("Contacts")


@app.tool()
def save_contact(data: ContactModel) -> list[dict]:
    """Save a new contact and return the updated list."""
    contacts_db.append(data.model_dump())
    return list(contacts_db)


@app.tool()
def search_contacts(query: str) -> list[dict]:
    """Filter contacts by name or email."""
    q = query.lower()
    return [
        c for c in contacts_db
        if q in c["name"].lower() or q in c["email"].lower()
    ]


@app.tool(model=True)
def list_contacts() -> list[dict]:
    """Return all contacts. Visible to both the model and the UI."""
    return list(contacts_db)


@app.ui()
def contact_manager() -> PrefabApp:
    """Open the contact manager."""
    with Column(gap=6, css_class="p-6") as view:
        Heading("Contacts")

        with ForEach("contacts") as contact:
            with Row(gap=2, align="center"):
                Text(contact.name, css_class="font-medium")
                Muted(contact.email)
                Badge(contact.category)

        Separator()

        Heading("Add Contact", level=3)
        Form.from_model(
            ContactModel,
            on_submit=CallTool(
                "save_contact",
                on_success=[
                    SetState("contacts", RESULT),
                    ShowToast("Contact saved!", variant="success"),
                ],
                on_error=ShowToast("Failed to save", variant="error"),
            ),
        )

        Separator()

        Heading("Search", level=3)
        with Form(
            on_submit=CallTool(
                "search_contacts",
                arguments={"query": Rx("query")},
                on_success=SetState("contacts", RESULT),
            )
        ):
            Input(name="query", placeholder="Search by name or email...")
            Button("Search")

    return PrefabApp(view=view, state={"contacts": list(contacts_db)})


mcp = FastMCP("Contacts Server", providers=[app])

if __name__ == "__main__":
    mcp.run()
```

Also available as a runnable server at `examples/apps/contacts/contacts_server.py`.

## Next steps

- **[Interactive Tools](/apps/prefab)** — the building blocks: charts, tables, dashboards, reactive state
- **[Examples](/apps/examples)** — complete working servers
- **[Development](/apps/development)** — preview and test app tools locally
- **[Prefab UI docs](https://prefab.prefect.io)** — full component reference
