URL: https://gofastmcp.com/apps/development
Title: Development - FastMCP

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Quick start
How it works
MCP inspector
Options
Multiple tools
Reference
Development
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Preview and test your app tools locally without a full MCP host.
Copy page
New in version
3.2.0
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
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
Form Input
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