The core idea
A model does not directly send UPI, edit your database, or call an API. It emits a structured tool request, then your app or the provider executes that tool and sends the result back.
Tools are how an agent acts. Without tools, the model can only answer from text it already has.
What counts as a tool
A tool can fetch data, run code, call an external API, use a computer, or call another agent. Current SDKs group tools in different ways, such as hosted tools, built-in execution tools, function tools, and agents-as-tools.