"Message me" is through a bot in my main messenger app, so the agent is just a chat window for me. "Later" happens per my requests, and agent has an ability to do something later because I just automatically invoke it on a schedule periodically, and ask it "check notes and chat history — do you need to do something, like message the user to remind them of something", and agent has the message_user tool to do that (that goes to the messenger bot API). Or it can decide to do nothing.