Agent vs Assistant
Something that acts and loops toward a goal, versus something that mainly replies
Agent
A model wired to tools and a task loop: it can act, observe the real result of that action, and decide the next step itself, repeating until a goal is met. Verification against reality is built into how it works.
Assistant
A model that primarily produces a single helpful reply per input, with little or no ability to take real actions or verify its own output against the real world beyond that reply.
Which one do you actually need
The line is the loop and the tools: an assistant answers, an agent acts, checks the result, and adjusts. A question answered correctly in one shot is assistant behavior even from a very capable model. A task completed by editing files, running commands, and confirming the result actually worked is agent behavior, regardless of how the underlying model is branded. Many products blend both, replying conversationally most of the time and switching into agent behavior when a real task is handed to them.