Lesson 14 / 25

Testing with the MCP Inspector

Use the Inspector to list and call your tools interactively before connecting a real host.

Try it without a model

The MCP Inspector is a developer tool with a web UI. It connects to your server, lists tools, resources and prompts, and lets you call them with sample inputs. Testing here separates server bugs from model behaviour: if it works in the Inspector, any remaining problem is in how the model uses it.

Launch the Inspector

Either command opens the UI against your script. Requires Node.js for the npx form.

mcp dev server.py
# or
npx @modelcontextprotocol/inspector python server.py

Quick check: Why test a server in the Inspector first?

  • To train the model
  • To separate server bugs from model behaviour
  • It is required by Python
  • To avoid writing tools
Answer

To separate server bugs from model behaviour — If a tool works when called directly, model-related problems are easier to isolate.