MCP Inspector

The MCP inspector is a developer tool for testing and debugging MCP servers.

Running the Inspector

Requirements

• Node.js: ^22.7.5

From an MCP server repository

To inspect an MCP server implementation, there's no need to clone this repo. Instead, use npx. For example, if your server is built at build/index.js:

npx @modelcontextprotocol/inspector node build/index.js

You can pass both arguments and environment variables to your MCP server. Arguments are passed directly to your server, while environment variables can be set using the -e flag:

# Pass arguments only
npx @modelcontextprotocol/inspector node build/index.js arg1 arg2

# Pass environment variables only
npx @modelcontextprotocol/inspector -e key=value -e key2=$VALUE2 node build/index.js

# Pass both environment variables and arguments
npx @modelcontextprotocol/inspector -e key=value -e key2=$VALUE2 node build/index.js arg1 arg2

# Use -- to separate inspector flags from server arguments
npx @modelcontextprotocol/inspector -e key=$VALUE -- node build/index.js -e server-flag

The inspector runs both an MCP Inspector (MCPI) client UI (default port 6274) and an MCP Proxy (MCPP) server (default port 6277). Open the MCPI client UI in your browser to use the inspector. (These ports are derived from the T9 dialpad mapping of MCPI and MCPP respectively, as a mnemonic). You can customize the ports if needed:

CLIENT_PORT=8080 SERVER_PORT=9000 npx @modelcontextprotocol/inspector node build/index.js

For more details on ways to use the inspector, see the Inspector section of the MCP docs site. For help with debugging, see the Debugging guide.

Authentication

The inspector supports bearer token authentication for SSE connections. Enter your token in the UI when connecting to an MCP server, and it will be sent in the Authorization header. You can override the header name using the input field in the sidebar.

Security Considerations

The MCP Inspector includes a proxy server that can run and communicate with local MCP processes. The proxy server should not be exposed to untrusted networks as it has permissions to spawn local processes and can connect to any specified MCP server.

Configuration

The MCP Inspector supports the following configuration settings. To change them, click on the Configuration button in the MCP Inspector UI:

Setting	Description	Default
MCP_SERVER_REQUEST_TIMEOUT	Timeout for requests to the MCP server (ms)	10000
MCP_REQUEST_TIMEOUT_RESET_ON_PROGRESS	Reset timeout on progress notifications	true
MCP_REQUEST_MAX_TOTAL_TIMEOUT	Maximum total timeout for requests sent to the MCP server (ms) (Use with progress notifications)	60000
MCP_PROXY_FULL_ADDRESS	Set this if you are running the MCP Inspector Proxy on a non-default address. Example: http://10.1.1.22:5577	""

These settings can be adjusted in real-time through the UI and will persist across sessions.

The inspector also supports configuration files to store settings for different MCP servers. This is useful when working with multiple servers or complex configurations:

npx @modelcontextprotocol/inspector --config path/to/config.json --server everything

Example server configuration file:

{
  "mcpServers": {
    "everything": {
      "command": "npx",
      "args": ["@modelcontextprotocol/server-everything"],
      "env": {
        "hello": "Hello MCP!"
      }
    },
    "my-server": {
      "command": "node",
      "args": ["build/index.js", "arg1", "arg2"],
      "env": {
        "key": "value",
        "key2": "value2"
      }
    }
  }
}

From this repository

If you're working on the inspector itself:

Development mode:

npm run dev

Note for Windows users: On Windows, use the following command instead:

npm run dev:windows

Production mode:

npm run build
npm start

CLI Mode

CLI mode enables programmatic interaction with MCP servers from the command line, ideal for scripting, automation, and integration with coding assistants. This creates an efficient feedback loop for MCP server development.

npx @modelcontextprotocol/inspector --cli node build/index.js

The CLI mode supports most operations across tools, resources, and prompts. A few examples:

# Basic usage
npx @modelcontextprotocol/inspector --cli node build/index.js

# With config file
npx @modelcontextprotocol/inspector --cli --config path/to/config.json --server myserver

# List available tools
npx @modelcontextprotocol/inspector --cli node build/index.js --method tools/list

# Call a specific tool
npx @modelcontextprotocol/inspector --cli node build/index.js --method tools/call --tool-name mytool --tool-arg key=value --tool-arg another=value2

# List available resources
npx @modelcontextprotocol/inspector --cli node build/index.js --method resources/list

# List available prompts
npx @modelcontextprotocol/inspector --cli node build/index.js --method prompts/list

# Connect to a remote MCP server
npx @modelcontextprotocol/inspector --cli https://my-mcp-server.example.com

# Call a tool on a remote server
npx @modelcontextprotocol/inspector --cli https://my-mcp-server.example.com --method tools/call --tool-name remotetool --tool-arg param=value

# List resources from a remote server
npx @modelcontextprotocol/inspector --cli https://my-mcp-server.example.com --method resources/list

UI Mode vs CLI Mode: When to Use Each

Use Case	UI Mode	CLI Mode
Server Development	Visual interface for interactive testing and debugging during development	Scriptable commands for quick testing and continuous integration; creates feedback loops with AI coding assistants like Cursor for rapid development
Resource Exploration	Interactive browser with hierarchical navigation and JSON visualization	Programmatic listing and reading for automation and scripting
Tool Testing	Form-based parameter input with real-time response visualization	Command-line tool execution with JSON output for scripting
Prompt Engineering	Interactive sampling with streaming responses and visual comparison	Batch processing of prompts with machine-readable output
Debugging	Request history, visualized errors, and real-time notifications	Direct JSON output for log analysis and integration with other tools
Automation	N/A	Ideal for CI/CD pipelines, batch processing, and integration with coding assistants
Learning MCP	Rich visual interface helps new users understand server capabilities	Simplified commands for focused learning of specific endpoints

License

This project is licensed under the MIT License—see the LICENSE file for details.