MCP tool
A single named function a server exposes, with a written description and typed parameters, that a model can choose to call.
In detail.
A tool is the unit of capability. It carries a name, a description in prose, and a schema for its arguments. The model reads the description to decide whether the tool answers the question in front of it, then fills the schema.
The description is not documentation, it is the interface. A tool whose description says what it returns, over what period, and what it costs will be chosen correctly. A tool named after an API endpoint, with the endpoint's own wording, will be chosen wrongly or not at all.
Through an agent.
What changes when a model asks instead of a person.
Tool descriptions are the only thing the model sees before calling. Everything a person would learn from trying, the shape of the response, the units, the fact that money arrives in micros, has to be in the description or it is not known.