Simon Willison · Independent · 2025-09-18
After eighteen months of refusing to use the word, Willison concedes — and his concession is the cleanest one-line definition the field now has: an agent is an LLM running tools in a loop to achieve a goal. Short, narrow, falsifiable. Everything broader is sales.
This is a useful piece for learners because it shows what good terminological hygiene looks like in a hype cycle. He documents his earlier objections, lists the competing definitions he is now overruling, and is explicit about what the new consensus excludes. Whole consulting decks have been rendered uninteresting by this post.
> An LLM agent runs tools in a loop to achieve a goal.