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RAG & MCP

4.4 — How the LLM Discovers and Selects MCP Tools

This is the most common interview question about MCP: "How does the model know which tool to use?" The answer involves three stages.

Stage 1: Discovery — tools/list at Connection Time

Tool Discovery at Connection Time

Stage 2: Schema Injection — Tools Become Part of the Prompt

The MCP client (Claude Desktop, your app) takes every discovered tool schema and injects them into the API call to the LLM. Here's what the LLM actually sees in its system prompt:

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