Inspecting With MCP Inspector

A debugger for servers, before you write a client

Every lesson so far tested a server by writing a Python client for it. That's the point eventually, but it's slow while you're still building a server: right now you just want to see what a client would see. The MCP Inspector is a standalone dev tool, maintained by the MCP project itself, that connects to any server and lets you browse its tools, resources, and prompts, and call them, without writing any client code.

Running it

The Inspector is a Node package, run through npx (no separate install needed, it downloads and runs on demand):

npx @modelcontextprotocol/inspector uv run python lessons/mcp/01_beginner/09_inspecting_with_mcp_inspector/server.py

This launches a local web UI (it prints a URL to open) that connects to server.py over stdio, exactly the way a real host would, and lets you click through its tools, call them with arguments, read its resources, and fill in its prompts interactively.

The --cli mode: scriptable, no browser

The Inspector also has a non-interactive CLI mode, useful when you want a quick one-off check without opening a browser, or when scripting a check into something like a test suite.

npx @modelcontextprotocol/inspector --cli \
    uv run python lessons/mcp/01_beginner/09_inspecting_with_mcp_inspector/server.py \
    --method tools/list

npx @modelcontextprotocol/inspector --cli \
    uv run python lessons/mcp/01_beginner/09_inspecting_with_mcp_inspector/server.py \
    --method tools/call --tool-name add --tool-arg a=2 --tool-arg b=3

The first prints the same tool metadata mcp.list_tools() showed you back in Lesson 3, but now from the outside, over the real protocol. The second actually calls add, returning {"content": [...], "isError": false}, identical in shape to what session.call_tool(...) gave you in Lesson 7.

Why bother, if you're about to write a client anyway

Two reasons. First, speed: checking a schema or a return value with the Inspector is one command, no client script to write or debug. Second, isolation: if something's wrong, the Inspector lets you rule the server out (or in) before you start suspecting bugs in your own client code, useful every time something in Lessons 11 onward doesn't behave the way you expect.