To to set your statusline, the statusline command creates prompt text that TELLS THE MAIN LOOP to SPAWN AN AGENT with a given prompt. note that this does ***not*** directly spawn an agent, it is merely a suggestion. so right off the bat it is POSSIBLE TO FAIL EVEN INVOKING the command.
But before we get there, we have to pass through what happens after a slash command is issued, and one of the steps along the way is "if something looks like a slash command but we don't know about that command, then throw it back up to the main loop with some special Caveat Message that says "ok just ignore this please"