Meet [Prefect Horizon](https://prefect.io/horizon?utm_source=gofastmcp&utm_medium=docs&utm_campaign=docs_banner&utm_content=sitewide_banner), the enterprise MCP gateway built by the team behind FastMCP.

New in version `3.2.0`

`fastmcp dev apps` launches a browser-based preview for your app tools. It starts your MCP server and a local dev UI side by side — you pick a tool, fill in its arguments, and see the rendered result in a new tab. No MCP host client needed. This works with both [Prefab apps](/content/apps/prefab/index.html) and [custom HTML apps](/content/apps/low-level/index.html).

## Quick Start

```
fastmcp dev apps server.py
```

The dev UI opens at `http://localhost:8080`. Your MCP server runs on port 8000 with auto-reload enabled by default — save a file and the server restarts automatically.

## How It Works

The dev server does three things: The **picker page** connects to your MCP server, finds all tools with UI metadata, and renders a form for each one. The forms are auto-generated from the tool’s input schema — text fields, dropdowns, checkboxes, all wired up. When you submit a form, the dev server **calls your tool** via the MCP protocol and opens the result in a new tab. The result page loads the tool’s UI resource (the Prefab renderer or your custom HTML) inside an AppBridge — the same protocol that real MCP hosts use. A **reverse proxy** on `/mcp` forwards requests from the browser to your MCP server, avoiding CORS issues that would otherwise block the iframe-based renderer from talking to a different port.

## MCP Inspector

The dev UI includes an inspector panel on the left side that captures MCP traffic in real-time. It shows JSON-RPC messages flowing between the browser and your server — requests, responses, and AppBridge `postMessage` traffic. Each entry shows direction, method, timing, and a smart summary. Click any entry to expand the full JSON-RPC body. The panel auto-scrolls to new messages unless you’ve scrolled up to inspect older ones. The inspector is useful for debugging: you can see exactly what arguments your tool received, what it returned, and how the AppBridge communicated with the renderer.

## Options

```
fastmcp dev apps server.py:mcp --mcp-port 9000 --dev-port 9090 --no-reload
```

| Option       | Flag         | Default | Description                                   |
|--------------|--------------|---------|-----------------------------------------------|
| MCP Port     | `--mcp-port` | `8000`  | Port for your MCP server                      |
| Dev Port     | `--dev-port` | `8080`  | Port for the dev UI                          |
| Auto-Reload  | `--reload` / `--no-reload` | On | Watch files and restart the server on changes |

## Multiple Tools

If your server has multiple app tools, the picker shows a dropdown. Each tool gets its own form and launch button. The tool’s `title` is displayed when available, falling back to the tool name.

```
# Server with multiple app tools
fastmcp dev apps examples/apps/contacts/contacts_server.py
```