Co-Evolving Harnesses and Models: On-Policy Correction Helps Weaker Models Catch Up Where Imitation Fails

TL;DR - This paper finds that fine-tuning weaker agent models to imitate complete expert trajectories can undermine their compatibility with an evolved agent harness. Correcting only failed turns in the weaker model’s own rollouts preserves its planning style and combines harness-evolution gains with model adaptation.

• Across seven enterprise agent tasks, full-trajectory imitation reduced performance by 4–30 points for Qwen3-Coder and Gemma 4 under evolved harnesses.
• Expert imitation transferred knowledge and increased scaffold usage but induced planning strategies the weaker models could not execute reliably.
• The proposed on-policy pipeline uses a meta-level MLE agent to locate a failing turn, then asks an expert model to rewrite only that turn.
• Localized correction maintained model–harness fit while helping economical, domain-specific agents narrow the performance gap.