Practical MCP with FastMCP & LangChain

The MCP Inspector

The MCP Inspector is the official tool provided by the MCP team to help developers test and debug their MCP servers. It provides a user-friendly web interface to interact with a server, send requests, and view responses in real-time.

This tool is especially useful during development to quickly test your server's functionality without needing to set up a full client application.

Install the MCP Inspector:

npm i -g @modelcontextprotocol/inspector@0.20.0

You can run the following command in your terminal to start the tool and connect it to your server:

# cd $HOME/workspace/mcp-server
npx @modelcontextprotocol/inspector calculator_server.py

This will generate a URL in the terminal with a session token:

Starting MCP inspector...
⚙️ Proxy server listening on 127.0.0.1:6277
🔑 Session token: 8efcf5ecb60382471505f3f9cd9ee8c8ccc72213d2189e7551480327fd466e4d
Use this token to authenticate requests or set DANGEROUSLY_OMIT_AUTH=true to disable auth

🔗 Open inspector with token pre-filled:
   http://localhost:6274/?MCP_PROXY_AUTH_TOKEN=8efcf5ecb60382471505f3f9cd9ee8c8ccc72213d2189e7551480327fd466e4d

🔍 MCP Inspector is up and running at http://127.0.0.1:6274 🚀

If you're running a remote server and want to access the inspector from its public interface using your web browser, you need to use a command like HOST=0.0.0.0 ALLOWED_ORIGINS=npx @modelcontextprotocol/inspector to allow the inspector to accept connections from the server's public IP address.

Here is how to do it:

# Get public IP address
PUBLIC_IP=$(curl -s ifconfig.me)

# Run the inspector with the appropriate environment variables
HOST=0.0.0.0 \
  ALLOWED_ORIGINS=http://$PUBLIC_IP:6274 \
  npx @modelcontextprotocol/inspector