AutoHarness: improving LLM agents by automatically synthesizing a code harness

Researchers developed AutoHarness, a technique where smaller LLMs like Gemini-2.5-Flash can automatically generate code harnesses to prevent illegal moves in games, outperforming larger models like Gemini-2.5-Pro and GPT-5.2-High. The method eliminates 78% of failures attributed to illegal moves in chess competitions and demonstrates superior performance across 145 different games.