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2026-06-08

Why ReAct is Still the Foundation

Understanding the Thought-Action-Observation loop.

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Reading time: 5 min

The ReAct pattern (2022) remains the most fundamental building block of AI agent design, and understanding why is the fastest way to reason about every newer framework. ReAct stands for Reasoning + Acting: the model interleaves a Thought step (what should I do next?), an Action step (call a tool or emit an output), and an Observation step (read the result). This Thought-Action-Observation loop runs until the task is complete. The idea seems simple, but the consequences are deep. The loop forces the model to separate reasoning from action. When the reasoning is written down, you can see whether the model is chasing a sensible hypothesis. When the action result comes back, the observation gives the model real feedback from the environment instead of asking it to improvise in a vacuum. That feedback loop is what turns a chat model into an agent: it can try, observe, and course-correct. Modern frameworks like LangGraph and the OpenAI Agents SDK still use ReAct as their default execution loop. They add orchestration, state, and tool management on top, but the core rhythm is the same: think, act, observe, repeat. When you read a framework's agent implementation, you are almost always reading a polished ReAct loop. The practical lesson for builders: start with ReAct and only add complexity when a simpler pattern is demonstrably insufficient. Teams often jump straight to sophisticated graph structures, multi-agent topologies, and long-term memory before they have verified that a plain ReAct loop fails on a concrete example. In most production agents, the bottleneck is tool quality and observation handling, not the loop topology. Iterate on the tools, make observations structured, and keep the loop legible. LoopCraft's visual builder makes the ReAct loop explicit by default, so you can see and test each Thought-Action-Observation step before adding orchestration patterns on top. Start there. Available at https://getloopcraft.com.