Can LLMs improve their own harness? Self-improving harnesses are a hot topic. And one interesting question is whether LLMs can generate and improve them on their own. They find that two full rewrites of the harness raise held-out scores by only 2.80 points, and the 95% interval crosses zero. Each frontier model writes a complete harness from a fixed coding bootstrap, plays five development games through it, and then rewrites its whole codebase twice. Every version is scored on 20 games that…