“An agent is anything that can be viewed as perceiving its environment through sensors and acting upon that environment through actuators.”Stuart Russell, Peter Norvig · Artificial Intelligence: A Modern Approach · 1995 · Artificial Intelligence: A Modern Approach, ed. a 4-a (2020), cap. 2 «Intelligent Agents», §2.1 — definiția agentului
An agent is a perceive–decide–act loop, and the guardrails sit on every turn.
The textbook definition maps exactly onto what runs today: the sensors are the context (the user's message, files, tool results), the actuators are tool calls (search, run code, send an email). The loop closes when a tool result comes back into the context and the model chooses the next step. That is the difference from a chat call: a call never sees the consequence of its own action. Autonomy has a plain measure — how many turns the loop makes without a human. An agent is defined by its tools and its stopping rule, not by its model.
Why it matters Whoever sees the agent as a loop knows where the guardrails go: on every turn, not just at the entrance.