What actually makes something an 'agent' — one practitioner's definition

There are a thousand different definitions of an agent and it's far from a solved science. This practitioner's definition cuts through the noise: given a task, can the system solve it with sufficient detail and high enough accuracy, without requiring a perfectly crafted prompt every time? A true agent runs largely in the background with a repetitive, reliable motion — 'I can prompt like an idiot and it will still happen.' That's the bar. It's distinct from simply asking Claude to do something in a chat window. The agent must be robust to imperfect inputs and operate with consistent, repeatable behavior.