Debug with MCP Inspector

The MCP Inspector is the official debugging UI for any MCP server. It connects over stdio, lists the tools your server exposes, and lets you call them with arbitrary arguments — no LLM required.

Launch

npx @modelcontextprotocol/inspector native-mcp time

A web UI opens in your browser. The first run will fetch the package via npx; subsequent runs are cached.

What to look for

• Tools tab — every tool registered by your server should appear with its description and JSON schema. Missing tools usually mean the server crashed before the tools/list response was sent.
• Schema preview — the form on the right is generated from your inputSchema. If a field is missing from the form, your schema is wrong.
• Call — invoke a tool and inspect the raw ToolResult. isError: true indicates a tool-level failure (the protocol didn't fail; your handler did).

Faster iteration

Rebuild while the inspector is open and click Reconnect to re-spawn the binary. No need to restart npx.

Use it before wiring a new server into Claude

The Inspector is the fastest feedback loop for protocol-level work. Once it's green, registering the server in Claude Code or Claude Desktop is just plumbing.