Why are MCP tools called model-controlled and when should you use them?

Tools are controlled entirely by Claude. The AI model decides when to call these functions, and the results are used directly by Claude to accomplish tasks. Tools are perfect for giving Claude additional capabilities it can use autonomously. When you ask Claude to calculate the square root of three using JavaScript, it is Claude that decides to use a JavaScript execution tool to run the calculation. You expose the capability through a tool, and Claude figures out the rest. So whenever you find yourself thinking the model should be able to do this on its own when it makes sense, that is a strong signal you should expose the capability as a tool.