Term
Function calling
The specific implementation of tool use exposed by OpenAI in 2023 and now standard across providers. The model returns a structured JSON object specifying which function to call and with what arguments; the runtime handles execution and result injection.
Background
Function calling refers to the API-level mechanism: send the model a list of function schemas, receive back a function-call response. Anthropic calls the equivalent feature "tool use". The mechanics are nearly identical across providers in 2026, with minor differences in parallel-call support and error semantics. Function calling is a building block for agents but doesn't define them — an agent is the loop that uses function calls repeatedly toward a goal.