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The MCP Inspector can be run directly from the command line using npx, making it easy to quickly inspect and debug MCP servers without installing anything globally.

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Quick Start

Run the inspector with a single command:
This will:
  • Start the inspector server on an available port (default: 8080)
  • Automatically open your browser to the inspector interface
  • Display the URL in the terminal

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Command Options

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--url <url>

Auto-connect to an MCP server when the inspector starts.

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--port <port>

Specify the starting port number. The inspector will find the next available port if the specified port is already in use.
Default: 8080 Port Range: Must be between 1 and 65535

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--no-open

Prevent the inspector from automatically opening a browser tab when it starts. Useful in CI/CD pipelines, headless environments, or when you prefer to open the URL manually.
You can combine it with other flags:

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--help, -h

Display help information and available options.

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Environment Variables

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MCP_INSPECTOR_FRAME_ANCESTORS

Configure which origins can embed the inspector widget in iframes. This is useful when embedding the inspector into your own application or when testing widgets from different domains. Default behavior:
  • Development mode: * (allows all origins for easier development)
  • Production mode: 'self' (same-origin only for security)
Format: Space-separated list of origins or * for all origins Examples:
In production deployments, it’s recommended to explicitly list allowed origins rather than using * to prevent unauthorized embedding.
When running in development mode (npm run dev), the inspector automatically allows all origins by default, so you don’t need to set this variable unless you want to test production CSP policies.

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MCP_URL

Set the external base URL for your MCP server. This is useful when running behind a reverse proxy (ngrok, E2B sandboxes, Cloudflare tunnels) where the public URL differs from localhost. When set, the CLI will use this URL for widget asset URLs and Vite HMR WebSocket connections, ensuring everything works correctly through the proxy. Default behavior:
  • If not set, the CLI generates a localhost URL automatically
  • If set, the CLI preserves your value and does not overwrite it
Examples:
When using a reverse proxy, set MCP_URL to the public-facing URL. This ensures widget hot-reload (HMR) and asset loading work correctly through the proxy.