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MCP Inspector

Official interactive debugger for MCP servers — list tools, call them, and inspect JSON-RPC frames.

MCP Serversofficialmcpdebugui
Visit projectmodelcontextprotocol/inspector

MCP Inspector is the official reference debugging tool for MCP servers. It launches a local web UI that lets you list tools, call them with arbitrary inputs, inspect JSON-RPC frames, and watch protocol-level exchanges in real time. Essential for any server author. The npm package was renamed to @modelcontextprotocol/inspector; the slug 'mcp-cli' is kept for backward compatibility.

What it does

  • Interactive web UI for tools, resources, and prompts — no need to write test scripts
  • Raw JSON-RPC frame inspector — see the exact bytes the Host sends the server
  • Multiple transport support: stdio (default), Streamable HTTP via proxy
  • Per-call history with timing — catch slow handlers and missing notifications
  • Schema auto-discovery from initialize handshake — no manual config

When to use it

Best for any MCP server author iterating on tools, any Host author debugging transport negotiation, and anyone investigating a client-reported 'tool doesn't work' bug. Less suited for load testing or production traffic monitoring.

Getting started

npx @modelcontextprotocol/inspector@latest

Then point it at your server: select stdio transport, paste your server command (e.g. node ./server.js or python -m my_server).

Things to know

Inspector runs locally — it doesn't validate how a remote Host behaves. For Host-side debugging, capture the Host's --debug log and compare against what Inspector sees.

Repository

modelcontextprotocol/inspector

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