Nothing was added to the model. The same next-word machine was wired into a circle, and allowed to keep going until it decides it is finished.
Find the cheapest supplier for 500 units.
Press Step. Watch which stage repeats — that repetition is the entire difference.
The problem it explains
How it works
What it costs
So what
An agent is not a smarter model. It is the same model, given a tool, a loop, and permission to decide when to stop.