Without a harness, all an LLM can do is output text tokens. Nothing else. It's the harness that allows it to read a file, write a file, run commands, start subagents. But even a simple chat application would need a harness, because something has to take the input from the user, mark it up that it's a user message, then parse the LLM's generated tokens and display the message content, or diplay a referenced image inline, or whatnot. All of this is independent of any kind of safety or filtering of naughtiness.