CoTrace matches training trajectories to the runtime, lifting Qwen3.5-9B on Tmax from 78 to 88 solved tasks (RL variant reaches 90)
The work introduces CoTrace, a harness-aware data recipe for alternating model–harness co-evolution that explicitly governs trajectory routing, provenance matching, and curriculum refresh: execution failures are routed to harness synthesis while policy supervision uses only verified successes matched to the adopted runtime. On the 102-task Tmax promotion split, Qwen3.5-9B advances from 78 to 88 solved tasks under supervised fine-tuning and reaches 90 with an online reinforcement variant, whereas much larger corpora pooled across sibling harnesses yield no accepted model update.