MCP Inspector

What the Inspector does

When you build an MCP server, the hardest bugs are the ones you cannot see: a tool that does not appear, a schema a client rejects, a result in the wrong shape. The MCP Inspector lets you connect to your server directly, without any model in the way, and try everything by hand.

It ships as one package, @modelcontextprotocol/inspector, with three ways in:

• Web UI (the default): npx @modelcontextprotocol/inspector. Browse tools, resources and prompts, fill in arguments in a form and read the full responses.
• CLI: npx @modelcontextprotocol/inspector --cli, scriptable for automation, CI and fast feedback loops.
• TUI: npx @modelcontextprotocol/inspector --tui, an interactive terminal interface.

The Python SDK also opens a server in the Inspector with uv run mcp dev server.py.

Why it is worth using on every server

Testing a server only through a chat app mixes two questions: does the server work, and does the model choose to call it? The Inspector answers the first one on its own. You can check that tool names and descriptions read well, that invalid arguments return helpful errors, that resources resolve and that authentication behaves, before any agent touches the server. The CLI mode means the same checks can run in CI.

Things to know

• Version 2 is the current line; version 1 receives security fixes only and is published under the v1-latest tag. A migration guide explains changed CLI flags and configuration.
• Building from source needs Node.js 22.19 or newer.
• The README warns that on machines without an operating system keychain, such as containers, headless servers and some Linux setups, secrets like OAuth client secrets are saved to a plaintext file by default. It explains how to encrypt the file or keep secrets in memory instead.

Prerequisites and time

Node.js with npx, and an MCP server to test. You can be inspecting a server within minutes.

Free or paid?

Free and open source under the MCP project's licences.

What you can do after it

Verify every tool, resource and prompt of your server by hand, reproduce client bugs without a model, script regression checks in CI and debug authentication flows for remote servers.

Common questions

Do I need it if my SDK has tests? Unit tests check your handlers; the Inspector checks what a real MCP client sees over the wire. Use both.

Can it connect to remote servers? Yes, over Streamable HTTP, including servers that need OAuth.