The word "agent" is so overloaded in the AI space.
Sometimes it means a sophisticated interaction system, but other times it just means API.
I think it's partly a sign of how new the space is. We don't have consensus on the best way to use these systems yet.
A fun computer concept video from Sun in 1993:
https://www.youtube.com/watch?v=NKJNxgZyVo0
It's interesting to see their screen form factor: both vertical and horizontal! They also expect a ton of voice command interaction.
CLIs are reified UIs:
https://www.expressionsofchange.org/reification-of-interaction/
Interesting exploration of what properties a text interface provides, and what it means for computer interaction.