What is the difference between ReAct and function calling?

ReAct is a prompting pattern that any model can implement through structured text. Function calling is a model API feature where the model produces structured tool invocations with validated arguments. Modern LLM APIs from OpenAI, Anthropic, and Google support function calling natively, which is more reliable than text-based ReAct parsing. Many production systems use function calling as the underlying transport for ReAct-style logic.