Development

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

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 and custom HTML apps.

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

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