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QR Code MCP App

examples/apps/qr_server/README.md

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QR Code MCP App

An MCP App server that generates QR codes with an interactive viewer UI. Ported from the ext-apps QR server example to demonstrate FastMCP's MCP Apps support.

What it demonstrates

  • Linking a tool to a ui:// resource via AppConfig
  • Serving embedded HTML with the @modelcontextprotocol/ext-apps JS SDK from CDN
  • Declaring CSP resource domains via ResourceCSP
  • Returning ImageContent (base64 PNG) from a tool

Setup

bash
cd examples/apps/qr_server
uv sync

Usage

bash
uv run python qr_server.py

Or install it into an MCP client:

bash
fastmcp install stdio fastmcp.json

How it works

The server registers one tool (generate_qr) and one resource (ui://qr-server/view.html). The tool generates a QR code as a base64 PNG image. The resource serves an HTML page that uses the MCP Apps JS SDK to receive the tool result and display the image in a sandboxed iframe.

The HTML loads the ext-apps SDK from unpkg, so the resource declares csp=ResourceCSP(resource_domains=["https://unpkg.com"]) to allow the host to set the appropriate Content-Security-Policy.