How MCP Inspector Works: A Simple Look at Its Architecture and Setup

MCP Inspector is a browser-based debugging tool designed to help developers inspect and test MCP-compatible servers during local development or remote debugging. It offers a live interface to view your tool definitions, prompt schemas, resources, and full interaction history—all in one place. This guide explains how the Inspector is built, how it connects to your server, and how to set it up in seconds using a single command¹.

The Two Components of MCP Inspector

MCP Inspector runs as two co-operating parts:

1. Inspector Client (MCPI): A React-based web app that loads in your browser and acts as the user interface. This is where you browse tools, send test inputs, and view outputs².
2. Inspector Proxy (MCPP): A Node.js-based process that handles all communication with your actual MCP server. It spawns or connects to the server, sends requests, receives responses, and feeds them into the UI.

The proxy acts as a local reverse proxy for both launching the server and hosting the UI. When you start Inspector, MCPP creates an HTTP server on a local port (default: 6274) and bridges the data between your browser and your MCP agent³.

How the Communication Works Behind the Scenes

When you run the Inspector with a command like:

the proxy process performs two tasks simultaneously: it starts your MCP server (via subprocess using stdio) and launches the Inspector web UI¹. Depending on your configuration, the server can be connected via:

• stdio: the proxy opens your server as a child process and uses stdin/stdout pipes to communicate.
• streamable-http: the proxy acts as a client to a long-lived remote HTTP connection, suitable for cloud environments².
• sse: an older Server-Sent Events transport mode, now largely deprecated³.

Once the server is running, the proxy sends a JSON-RPC initialization message that asks the server to describe its tools, resources, and prompts⁴. This data is structured as capability definitions and sent back to the proxy, which forwards them to the client UI.

From this point onward, every interaction with a tool—such as sending input to a summarize_text function—follows the JSON-RPC protocol. You type into the UI, the proxy wraps the input as a JSON-RPC call, sends it to the MCP server, waits for a response, and returns it to the browser for display. If an error occurs, such as a validation failure or timeout, it appears in the "History" tab of the interface⁵.

Launching MCP Inspector with One Command

Inspector is designed to work out of the box with minimal setup. If your server is written in Python or TypeScript and follows the MCP specification, you can start inspecting it using:

or

The npx command downloads the Inspector package and launches the proxy. You can add CLI flags to specify transport modes (--transport=streamable-http), set a different port (--port=8000), or provide environment variables. Once running, open your browser and go to http://localhost:6274 to begin inspecting.

Inspector also works with remote servers by specifying HTTP endpoints directly. In that case, it acts purely as a proxy and UI without spawning the server locally. You can export connection configurations from Inspector for use in other tools like Claude, Cursor, or Windsurf¹.

My Thoughts

The real value, in my view, lies in the transparency MCP Inspector offers. Having live insight into tool definitions, prompts, and the entire JSON‑RPC exchange through a clean UI removes a lot of friction from debugging. It prevents guesswork and reduces the need for log file digging or crafting ad-hoc test scripts. Overall, MCP Inspector feels like a simple solution that enhances the development experience without introducing unnecessary complexity.

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