09 · Connecting Your Server to Claude Desktop

Everything so far has run through the MCP Inspector. This lesson connects the same server to a real host application — Claude Desktop — so you can use it in an actual conversation.

The config file

Claude Desktop reads its MCP server list from a JSON config file, claude_desktop_config.json, located at:

• macOS: ~/Library/Application Support/Claude/claude_desktop_config.json
• Windows: %APPDATA%\Claude\claude_desktop_config.json
• Linux: ~/.config/Claude/claude_desktop_config.json

If the file doesn't exist yet, create it. The relevant shape is an mcpServers object, where each key is a name you choose for the server and each value describes how to launch it:

{
  "mcpServers": {
    "demo-server": {
      "command": "python",
      "args": ["/absolute/path/to/server.py"]
    }
  }
}

command is the executable to run, args is the list of arguments passed to it — together they're exactly the subprocess invocation Claude Desktop will spawn, the same way the Inspector spawned python server.py in lesson 4. Use an absolute path to server.py; Claude Desktop doesn't necessarily launch the process from the directory you expect.

Environment variables, if your server needs them

If your server reads an API key or other config from the environment, add an env object alongside command and args:

{
  "mcpServers": {
    "demo-server": {
      "command": "python",
      "args": ["/absolute/path/to/server.py"],
      "env": {
        "SOME_API_KEY": "your-key-here"
      }
    }
  }
}

Your server doesn't need any special code to read this — it's set on the subprocess's environment exactly like any other environment variable, so plain os.environ["SOME_API_KEY"] in your server picks it up.

Restart and verify

Config changes aren't picked up live — fully quit and reopen Claude Desktop (not just close the window) for it to re-read the file and spawn your server. If the config is valid and the server starts cleanly, you'll see a small tools/MCP indicator in the Claude Desktop UI; clicking it should list demo-server and its tools.

If it doesn't show up, check Claude Desktop's MCP log file (also under the same per-OS config directory, typically named something like mcp-server-demo-server.log) — this captures anything your server wrote to stderr plus any startup failures, and is almost always where the actual error message is.

Use it for real

Open a new conversation and ask something that requires the tool you built, for example, if you still have the add tool from lesson 4: "What's 47 plus 128? Use the add tool." Claude Desktop's host will present your tool to the model, the model will issue a tools/call exactly as you saw in the Inspector, and you'll get a real answer computed by your own code running on your own machine — not by the model doing arithmetic itself.

Exercise

Register the notes/tasks server you'll build in lesson 10's project ahead of time by adding a second entry to mcpServers right now, pointing at a notes_server.py path that doesn't exist yet. Confirm Claude Desktop reports a startup failure for that entry in its log (rather than silently ignoring it) — this is exactly the error you'll want to recognize once you've actually built that server and are debugging why it isn't showing up.