ReAct Inside — From Message to State, Understanding How AI Agents Really Work

When people first encounter ReAct (Reason + Act), they often think it's just adding three fields—Thought / Action / Observation—to the prompt.

But in reality, the core of ReAct isn't the prompt format. It's the Agent's State Machine.

This article explains, from an engineering perspective, how ReAct actually works inside an LLM, and how it relates to modern Function Calling and Tool Calling.

1. What Is ReAct?

ReAct (Reason + Act) comes from the 2022 paper ReAct: Synergizing Reasoning and Acting in Language Models, authored by Shunyu Yao et al., a collaboration between Princeton University and Google Research.