Reason plus act
ReAct stands for Reason and Act. Instead of planning everything upfront or acting blindly, the agent alternates a short thought with a single action, then reads the observation before thinking again. This grounds reasoning in real feedback.
The cycle
- Thought the model explains what it wants to do and why
- Action the model calls a tool with concrete arguments
- Observation the environment returns a result
- The loop repeats, each thought informed by the latest observation
Why interleaving helps
Pure reasoning can hallucinate facts. Pure acting cannot adapt. ReAct mixes both so each action is justified and each thought is corrected by evidence.
Practical notes
Keep thoughts short so context stays small. Log the full thought action observation trace, because it is the clearest record of why the agent did what it did. When a tool fails, the next thought should acknowledge the error and adjust.
Key idea
ReAct interleaves a brief thought, one action, and its observation, so reasoning stays grounded in real feedback instead of drifting into hallucination.