What is the difference between planning and reacting in agent design?

A reactive agent (like ReAct) decides its next action one step at a time from the current state, no explicit plan, just choose, observe, repeat. A planning agent first produces a multi-step plan (e.g. Plan-and-Execute or a task decomposition) and then executes the steps, optionally re-planning when reality diverges. Reacting is flexible and adapts to surprises but can wander or lose the thread on long tasks. Planning gives structure, parallelism, and better long-horizon coherence, but a plan built on wrong assumptions can fail wholesale. In practice many production agents blend both: decompose the goal into subtasks, execute each reactively, and re-plan at checkpoints. The choice hinges on task length and predictability: short, dynamic tasks favor reacting; long, structured workflows favor explicit planning.