---
title: Development
sidebarTitle: Development
description: Preview and test your app tools locally without a full MCP host.
icon: flask
---

import { VersionBadge } from '/snippets/version-badge.mdx'

<VersionBadge version="3.2.0" />

<Frame>
  <img src="/apps/images/dev-app.png" alt="The dev UI showing a rendered Prefab app with the MCP inspector panel" />
</Frame>

`fastmcp dev apps` gives you a browser preview for your app tools without needing an MCP host client. It starts your server and a local dev UI side by side: you pick a tool, fill in its arguments, and the rendered result opens in a new tab.

Works with both [Interactive Tools](/apps/prefab) and [custom HTML apps](/apps/low-level).

## Quick start

```bash
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

```bash
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.

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