Please note that AIXI with outputs connected only to a monitor seems like an instance of the Tool AI.

As I read Holden, and on my proposed way of making "agent" precise, this would be an agent rather than a tool. The crucial thing is that this version of AIXI selects actions on the basis of how well they serve certain goals without user approval. If you had a variation on AIXI that identified the action that would maximize a utility function and displayed the action to a user (where the method of display was not done in an open-ended goal-directed way), that would count as a tool.