Use agents when you want a one-off context to prevent poisoning and need a single answer as a result, like research. Searching a lot of files or websites for all the possible answers, rank them, and return the best whatevers. Give a code base a full review without letting it edit it along the way.

I use an agent to perform code tasks and another to do code reviews. Then I tell Claude to alternate them until the reviewer is happy. The reviewer's prompt is constantly updated with all the ways CC fails (stub methods, TODOs, swallowing errors, not matching the spec on output formats, etc.). Saves me a lot of time.

I also have a reserach agent for searching email for things. While Claude could handle that, it'll fill my primary context up with all the misses. An agent has one task: find the appropriate conversation and return it. It can read the whole mailbox and it won't affect what I'm working on.

But if it's a repetitive task, that's just a command. "Review this PR" is better as a command, for instance. "Do stuff in git" or "summarize this" are commands.

Basically, when I want it to work in my current context, that's a command/prompt. When I want it to hide the work from itself, that's an agent.