Definition
With tool use, also called function calling, developers describe the tools available to a model (their names, purpose and input format). The model decides when a tool is needed and returns a structured request; the application runs the tool and passes the result back to the model.
Tool use is what turns a model from a text generator into something that can look things up, update records and act in other systems. It is the foundation of agents, and the place where most security and reliability work in an AI project concentrates.