How do you use the MCP Inspector to test a server during development?
Quick Answer
The MCP Inspector is an official interactive testing tool that launches your server (or connects to a running remote one), performs the initialize handshake for you, and gives you a browser-based UI to browse and directly invoke its tools, resources, and prompts — inspecting exact request/response JSON for each call — without needing a full LLM-driven host in the loop at all. Run it via npx @modelcontextprotocol/inspector <command to launch your server> for a stdio server, or point it at a URL for an HTTP server; it's the standard first checkpoint before wiring a server into Claude Desktop or another host, since it isolates "does my server actually work correctly" from "is the host configured correctly."
Detailed Answer
Before ever pointing a real host at a new server, it's worth confirming the server itself behaves correctly in isolation — that's exactly the gap the Inspector fills.
Launching it against a local server
npx @modelcontextprotocol/inspector python server.py
This starts your server as a subprocess, connects to it, runs the handshake, and opens a local web UI where you can:
- Browse the tool/resource/prompt catalogs returned by
tools/list,resources/list,prompts/list. - Fill in a form for a tool's arguments and invoke
tools/calldirectly, seeing the exact result (includingisErrorand anystructuredContent). - Read a specific resource by URI and inspect its returned content.
- Watch the raw JSON-RPC traffic in both directions, which is invaluable for spotting a malformed response or an unexpected error code.
Why this beats testing through a full host first
A full host — Claude Desktop, an IDE — adds an LLM in the loop deciding whether to call your tool at all. That introduces a second source of uncertainty: is the failure in your server, or in the model's tool selection and argument construction? The Inspector removes that variable entirely. You invoke exactly the call you intend, with exactly the arguments you choose, and see exactly what comes back.
A typical development loop
- Write/modify a tool.
- Run the Inspector, invoke it directly with a few representative and edge-case inputs.
- Confirm the schema validates correctly and the result shape (content, isError, structuredContent) is what you expect.
- Only then connect the server to a real host and test the end-to-end, model-driven experience.
Skipping straight to host-based testing for every iteration is slower. It also makes it much harder to tell whether a bug is in your server's logic, or just in how the model happened to phrase its call that particular time.