Connect Claude Desktop to thirds.ai

Claude Desktop cannot connect directly to the thirds.ai MCP server today. thirds.ai requires an API key in a bearer header, but the native remote connector form does not accept a custom bearer header. You can connect through the third-party mcp-remote bridge.

Install the bridge

Install Node.js 18 or later. Open Settings → Developer → Edit Config in Claude Desktop. Add this entry to claude_desktop_config.json and replace YOUR_API_KEY with a key from the API keys page:

{
  "mcpServers": {
    "thirds": {
      "command": "npx",
      "args": [
        "-y",
        "mcp-remote",
        "https://thirds.ai/mcp",
        "--header",
        "Authorization:${THIRDS_AUTH_HEADER}"
      ],
      "env": {
        "THIRDS_AUTH_HEADER": "Bearer YOUR_API_KEY"
      }
    }
  }
}

Quit and reopen Claude Desktop. The thirds server and its tools should appear in the tools menu. The no-space header form also works around a known Claude Desktop argument issue on Windows.

mcp-remote is an experimental third-party adapter. It runs on your computer and changes Claude Desktop's local stdio messages into Streamable HTTP requests. Review its source and setup notes before you install it.

The key is stored in your local Claude Desktop configuration. Restrict that file to your user, never commit it, and revoke the key if it is exposed. If you do not want a local adapter or a key in this file, use Claude Code, Cursor, VS Code, Codex, Grok, ChatGPT, Warp, Zed, OpenCode, Antigravity, Lovable, Replit, or Hermes.

Do not add https://thirds.ai/mcp in Settings → Connectors → Add custom connector. That native path cannot send the required key. Anthropic also says that remote HTTP servers don't load directly from claude_desktop_config.json. The local adapter gives the file the stdio process it supports. See Anthropic's remote connector guide.

Read the main MCP guide before you start paid work.

FAQ

Why does the setup use a local command?

Claude Desktop does not connect to this bearer-protected remote server from its native connector form or a direct URL in the JSON file. mcp-remote gives Claude Desktop a local stdio server and forwards its requests.

Why does the server fail to start?

Check that Node.js 18 or later and npx are available to Claude Desktop. Then check the JSON syntax and restart the app. Keep debug logs private because local tools can report command details.

Can I avoid storing the key in the config file?

Use Claude Code, Cursor, VS Code, or Codex. Their setup can read the key from an environment variable or secure input store. The Claude Desktop bridge needs the key in its local process environment.