AI agent
A system where a model plans a sequence of steps and calls tools to accomplish a goal, rather than just answering once.
The useful mental model is a loop: perceive, plan, act with tools, then check the result and repeat. Anything that only produces text in one shot is an assistant, not an agent.
Agents earn their complexity when the task is repetitive, multi-step and tolerant of a review step. Irreversible actions should always keep a human in the loop.