Frontier models were asked to build and then rewrite their own agent harness on ARC-AGI-3. Two revisions bought them almost nothing
AspireAI community · arXiv
This paper isolates the part of agent scaffolding that usually goes unwritten: the harness, the symbolic program that orchestrates perception, memory, tool use and recovery on the model's behalf. Using the interactive ARC-AGI-3 benchmark, each model built a complete harness through a fixed coding bootstrap, acted through it on five development games, and then revised the whole codebase twice, with every version scored on twenty games that never influenced adaptation. Across 27 lineages, performance ranged from zero to near ceiling. The two revisions raised mean held-out Relative Human Action Efficiency by only 2.80 points, with a 95% interval from -1.44 to 7.35 that includes no improvement. Development gains overstated held-out improvement, and inference-cost reductions sometimes cost task performance. The finding is a gap between autonomous harness construction and reliable adaptation, not evidence that harness design does not matter.
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