A tool is something outside the model that an agent can use to read, write, search, calculate, or act: reading a file, running tests, opening a browser, calling QuickBooks through an API, or reaching Gmail through an MCP server. The model can think and talk; tools are how it reaches outside its own text box to do real work. The moment the agent uses one, that’s a tool call.
Arms and legs for the brain capture the idea. Tools give the model reach without giving it more brains. A calculator makes a person reliable at math, and a file reader lets an agent read the files you gave it.
Why you care
Tools have boundaries, and that’s the point. A tool might only read, or it might write, delete, or send messages, scoped to one folder or one channel. Once an AI has tools, your risk changes: it can change files, expose private data, or spend money through another service. That makes tools serious rather than bad. The practical question is what the agent should be allowed to reach for: enough arms and legs for the job, and no more keys than it needs.