Simple example: a while back LLMs would trip over questions like "how many Rs are in strawberry". Now, the system prompts have a line like "when a user asks for a count, actually count the value by calling a tool if needed". The LLMs cannot get smarter in this regard, next token predictors will hallucinate here.

A harness is that covering every blind spot or sub-optimal but probable output people have hit in the wild, and a lot of problems just have better solutions if you say "break problem A into subproblem B and subproblem C, then solve".