Function calling is OpenAI's name for what most people now call tool use: you describe a set of functions to the model with JSON Schema, and the model decides which to invoke and with what arguments. Your code runs the function and feeds the result back.
It's the foundation of nearly every agent and assistant in production. Best practices: keep schemas small and unambiguous, use enum for fixed choices, validate the model's output before executing, and return errors as readable strings the model can recover from. See also our System Prompt Builder for prompt-side patterns.
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