kargnas's Better MCP Inspector


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

What's New

- Enhanced the JsonView with JSON node size calculation and display capabilities for clearer insights into payloads.
- Refined the JsonView size formatting and byte-length detection to improve accuracy across environments.

!MCP Inspector Screenshot

Architecture Overview

The MCP Inspector consists of two main components that work together:

- MCP Inspector Client (MCPI): A React-based web UI that provides an interactive interface for testing and debugging MCP servers
- MCP Proxy (MCPP): A Node.js server that acts as a protocol bridge, connecting the web UI to MCP servers via various transport methods (stdio, SSE, streamable-http)

Note that the proxy is not a network proxy for intercepting traffic. Instead, it functions as both an MCP client (connecting to your MCP server) and an HTTP server (serving the web UI), enabling browser-based interaction with MCP servers that use different transport protocols.

Running the Inspector

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- Node.js: ^22.7.5

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To get up and running right away with the UI, just execute the following:

``bash
npx kargnas-mcp-inspector@latest
`

The server will start up and the UI will be accessible at http://localhost:6274.

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You can also start it in a Docker container with the following command:

`bash
docker run --rm --network host -p 6274:6274 -p 6277:6277 ghcr.io/modelcontextprotocol/inspector:latest
`

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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:

`bash
npx kargnas-mcp-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:

`bash


Pass arguments only

npx kargnas-mcp-inspector node build/index.js arg1 arg2

Pass environment variables only

npx kargnas-mcp-inspector -e key=value -e key2=$VALUE2 node build/index.js

Pass both environment variables and arguments

npx kargnas-mcp-inspector -e key=value -e key2=$VALUE2 node build/index.js arg1 arg2

Use -- to separate inspector flags from server arguments

npx kargnas-mcp-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:

`bash
CLIENT_PORT=8080 SERVER_PORT=9000 npx kargnas-mcp-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.

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The MCP Inspector provides convenient buttons to export server launch configurations for use in clients such as Cursor, Claude Code, or the Inspector's CLI. The file is usually called mcp.json.

- Server Entry - Copies a single server configuration entry to your clipboard. This can be added to your mcp.json file inside the mcpServers object with your preferred server name.

STDIO transport example:

`json
{
"command": "node",
"args": ["build/index.js", "--debug"],
"env": {
"API_KEY": "your-api-key",
"DEBUG": "true"
}
}
`

SSE transport example:

`json
{
"type": "sse",
"url": "http://localhost:3000/events",
"note": "For SSE connections, add this URL directly in Client"
}
`

Streamable HTTP transport example:

`json
{
"type": "streamable-http",
"url": "http://localhost:3000/mcp",
"note": "For Streamable HTTP connections, add this URL directly in your MCP Client"
}
`

- Servers File - Copies a complete MCP configuration file structure to your clipboard, with your current server configuration added as default-server. This can be saved directly as mcp.json.

STDIO transport example:

`json
{
"mcpServers": {
"default-server": {
"command": "node",
"args": ["build/index.js", "--debug"],
"env": {
"API_KEY": "your-api-key",
"DEBUG": "true"
}
}
}
}
`

SSE transport example:

`json
{
"mcpServers": {
"default-server": {
"type": "sse",
"url": "http://localhost:3000/events",
"note": "For SSE connections, add this URL directly in Client"
}
}
}
`

Streamable HTTP transport example:

`json
{
"mcpServers": {
"default-server": {
"type": "streamable-http",
"url": "http://localhost:3000/mcp",
"note": "For Streamable HTTP connections, add this URL directly in your MCP Client"
}
}
}
`

These buttons appear in the Inspector UI after you've configured your server settings, making it easy to save and reuse your configurations.

For SSE and Streamable HTTP transport connections, the Inspector provides similar functionality for both buttons. The "Server Entry" button copies the configuration that can be added to your existing configuration file, while the "Servers File" button creates a complete configuration file containing the URL for direct use in clients.

You can paste the Server Entry into your existing mcp.json file under your chosen server name, or use the complete Servers File payload to create a new configuration file.

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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.

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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.

#### Authentication

The MCP Inspector proxy server requires authentication by default. When starting the server, a random session token is generated and printed to the console:

`
🔑 Session token: 3a1c267fad21f7150b7d624c160b7f09b0b8c4f623c7107bbf13378f051538d4

🔗 Open inspector with token pre-filled:
http://localhost:6274/?MCP_PROXY_AUTH_TOKEN=3a1c267fad21f7150b7d624c160b7f09b0b8c4f623c7107bbf13378f051538d4
`

This token must be included as a Bearer token in the Authorization header for all requests to the server. The inspector will automatically open your browser with the token pre-filled in the URL.

Automatic browser opening - The inspector now automatically opens your browser with the token pre-filled in the URL when authentication is enabled.

Alternative: Manual configuration - If you already have the inspector open:

1. Click the "Configuration" button in the sidebar
2. Find "Proxy Session Token" and enter the token displayed in the proxy console
3. Click "Save" to apply the configuration

The token will be saved in your browser's local storage for future use.

If you need to disable authentication (NOT RECOMMENDED), you can set the DANGEROUSLY_OMIT_AUTH environment variable:

`bash
DANGEROUSLY_OMIT_AUTH=true npm start
`

---

🚨 WARNING 🚨

Disabling authentication with DANGEROUSLY_OMIT_AUTH is incredibly dangerous! Disabling auth leaves your machine open to attack not just when exposed to the public internet, but also via your web browser. Meaning, visiting a malicious website OR viewing a malicious advertizement could allow an attacker to remotely compromise your computer. Do not disable this feature unless you truly understand the risks.

Read more about the risks of this vulnerability on Oligo's blog: Critical RCE Vulnerability in Anthropic MCP Inspector - CVE-2025-49596

---

You can also set the token via the MCP_PROXY_AUTH_TOKEN environment variable when starting the server:

`bash
MCP_PROXY_AUTH_TOKEN=$(openssl rand -hex 32) npm start
`

#### Local-only Binding

By default, both the MCP Inspector proxy server and client bind only to localhost to prevent network access. This ensures they are not accessible from other devices on the network. If you need to bind to all interfaces for development purposes, you can override this with the HOST environment variable:

`bash
HOST=0.0.0.0 npm start
`

Warning: Only bind to all interfaces in trusted network environments, as this exposes the proxy server's ability to execute local processes and both services to network access.

#### DNS Rebinding Protection

To prevent DNS rebinding attacks, the MCP Inspector validates the Origin header on incoming requests. By default, only requests from the client origin are allowed (respects CLIENT_PORT if set, defaulting to port 6274). You can configure additional allowed origins by setting the ALLOWED_ORIGINS environment variable (comma-separated list):

`bash
ALLOWED_ORIGINS=http://localhost:6274,http://localhost:8000 npm start
`

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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 | "" |
| MCP_AUTO_OPEN_ENABLED | Enable automatic browser opening when inspector starts (works with authentication enabled). Only as environment var, not configurable in browser. | true |

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:

`bash
npx kargnas-mcp-inspector --config path/to/config.json --server everything
`

Example server configuration file:

`json
{
"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"
}
}
}
}
`

#### Transport Types in Config Files

The inspector automatically detects the transport type from your config file. You can specify different transport types:

STDIO (default):

`json
{
"mcpServers": {
"my-stdio-server": {
"type": "stdio",
"command": "npx",
"args": ["@modelcontextprotocol/server-everything"]
}
}
}
`

SSE (Server-Sent Events):

`json
{
"mcpServers": {
"my-sse-server": {
"type": "sse",
"url": "http://localhost:3000/sse"
}
}
}
`

Streamable HTTP:

`json
{
"mcpServers": {
"my-http-server": {
"type": "streamable-http",
"url": "http://localhost:3000/mcp"
}
}
}
`

#### Default Server Selection

You can launch the inspector without specifying a server name if your config has:

1. A single server - automatically selected:

`bash


Automatically uses "my-server" if it's the only one


npx kargnas-mcp-inspector --config mcp.json
`

2. A server named "default-server" - automatically selected:

`json
{
"mcpServers": {
"default-server": {
"command": "npx",
"args": ["@modelcontextprotocol/server-everything"]
},
"other-server": {
"command": "node",
"args": ["other.js"]
}
}
}
`

> Tip: You can easily generate this configuration format using the Server Entry and Servers File buttons in the Inspector UI, as described in the Servers File Export section above.

You can also set the initial transport type, serverUrl, serverCommand, and serverArgs via query params, for example:

`
http://localhost:6274/?transport=sse&serverUrl=http://localhost:8787/sse
http://localhost:6274/?transport=streamable-http&serverUrl=http://localhost:8787/mcp
http://localhost:6274/?transport=stdio&serverCommand=npx&serverArgs=arg1%20arg2
`

You can also set initial config settings via query params, for example:

`
http://localhost:6274/?MCP_SERVER_REQUEST_TIMEOUT=10000&MCP_REQUEST_TIMEOUT_RESET_ON_PROGRESS=false&MCP_PROXY_FULL_ADDRESS=http://10.1.1.22:5577
`

Note that if both the query param and the corresponding localStorage item are set, the query param will take precedence.

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If you're working on the inspector itself:

Development mode:

`bash
npm run dev

To co-develop with the typescript-sdk package (assuming it's cloned in ../typescript-sdk; set MCP_SDK otherwise):

npm run dev:sdk "cd sdk && npm run examples:simple-server:w"

then open http://localhost:3000/mcp as SHTTP in the inspector.

To go back to the deployed SDK version:

npm run unlink:sdk && npm i

`

> Note for Windows users:
> On Windows, use the following command instead:
>
> `bash
> npm run dev:windows
> `

Production mode:

`bash
npm run build
npm start
`

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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.

`bash
npx kargnas-mcp-inspector --cli node build/index.js
`

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

`bash


Basic usage

npx kargnas-mcp-inspector --cli node build/index.js

With config file

npx kargnas-mcp-inspector --cli --config path/to/config.json --server myserver

List available tools

npx kargnas-mcp-inspector --cli node build/index.js --method tools/list

Call a specific tool

npx kargnas-mcp-inspector --cli node build/index.js --method tools/call --tool-name mytool --tool-arg key=value --tool-arg another=value2

Call a tool with JSON arguments

npx kargnas-mcp-inspector --cli node build/index.js --method tools/call --tool-name mytool --tool-arg 'options={"format": "json", "max_tokens": 100}'

List available resources

npx kargnas-mcp-inspector --cli node build/index.js --method resources/list

List available prompts

npx kargnas-mcp-inspector --cli node build/index.js --method prompts/list

Connect to a remote MCP server (default is SSE transport)

npx kargnas-mcp-inspector --cli https://my-mcp-server.example.com

Connect to a remote MCP server (with Streamable HTTP transport)

npx kargnas-mcp-inspector --cli https://my-mcp-server.example.com --transport http --method tools/list

Connect to a remote MCP server (with custom headers)

npx kargnas-mcp-inspector --cli https://my-mcp-server.example.com --transport http --method tools/list --header "X-API-Key: your-api-key"

Call a tool on a remote server

npx kargnas-mcp-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 kargnas-mcp-inspector --cli https://my-mcp-server.example.com --method resources/list
``

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| 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.