How to debug why your MCP tool isn't being called
언제 쓸까: You wrote a tool, Claude doesn't use it, and you don't know why.
사전 조건
- Node — nodejs.org or nvm
흐름
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Launch Inspectornpx @mcpjam/inspector@latest and open the printed URL.✓ 복사됨→ UI opens in browser
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Connect your serverIn the UI, add your server (stdio or HTTP) and connect.✓ 복사됨→ Tools list populates
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Simulate a promptIn the Chat tab, ask a question that should trigger your tool. Watch the JSON-RPC trace.✓ 복사됨→ Either tool fires (good) or you see why the model rejected it (bad description, missing input schema)
결과: Concrete reason (wrong tool description, schema issue, model choice) with a visible fix.
함정
- Testing with a weaker model that never uses tools — Test across multiple models in the Evals tab — Haiku, Sonnet, GPT-4o — behavior varies