I am not sure if I agree with Holden that there's a meaningful distinction between tools an agents. However, one definition I could think of is this:

"A tool, unlike an agent, includes blocking human input in its perceive/decide/act loop."

Thus, an agent may work entirely autonomously, whereas a tool would wait for a human to make a decision before performing an action.

Of course, under this definition, Google's webcrawler would be an agent, not a tool -- which is one of the reasons I might disagree with Holden.