Tool

What is a Tool

A tool is a discrete, named action the model can invoke: read this file, run this command, search this index. Each tool has a defined shape for its inputs, so the model's request can be checked and executed reliably rather than parsed loosely from free text. The tool runs outside the model, in real code, and its result is fed back into context.

Tools are what separate an agent from a model that only writes text. Every meaningful action an agent takes, editing a file, calling an API, querying a database, happens through a tool. Where that tool comes from varies: the harness ships some built in, an MCP server can add more, a plugin can bundle its own.

A tool is not the same as a skill. A skill is instructions that teach the model how and when to use tools it already has. A tool is the capability itself. Skills often exist specifically to explain the right way to combine several tools for a recurring job.