Function Calling

Function Calling (also called Tool Use) is a capability that allows language models to invoke external functions, APIs, or tools in a structured, reliable manner. Instead of generating free-form text that requires parsing, the model outputs structured data (typically JSON) specifying which function to call and with what arguments.

This capability enables building AI agents that can: search the web, query databases, execute code, control external systems, retrieve real-time information, and perform actions in the world. The typical flow involves: providing function definitions to the model, receiving structured function call requests, executing the functions, and returning results to the model for continued reasoning.

Function calling is fundamental to building practical AI applications beyond simple chat. Engineers should understand: how to define clear function schemas, handling parallel function calls, managing function execution safely, dealing with model errors in function selection, and designing function sets that give models appropriate capabilities without excessive complexity.