harness-updating is flat in base capability: models across capability tiers produce updates that yield similar gains, and even the Qwen3.5-9B evolver induces gains comparable to Claude Opus 4.6

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• Harness-updating capability is flat in base capability: models from different capability tiers produce harness updates that lead to surprisingly similar gainsclaim0.932
  First major claim of the paper, supported by narrow spread across evolvers and case study
• Qwen3.5-9B evolver achieves highest harness-updating gain on SkillsBench (3.8 pp), exceeding Claude Opus 4.6 (2.3 pp) and Qwen3-235B (1.5 pp)finding0.851
  Case demonstrating that model scale does not predict harness-updating quality
• Capability budget should be allocated to the task-solving agent rather than the evolver, since harness-updating varies by at most 3.1 pp across evolversclaim0.834
  Primary design recommendation derived from harness-updating flatness finding
• Harness-updating gain spread is at most 3.1 percentage points across all evolvers on any single benchmarkfinding0.826
  Core finding that harness-updating capability does not scale with model base capability
• Harness-Updating Capabilityconcept0.814
  The capability of an evolver model to produce useful persistent harness updates from execution evidence
• Harness-benefit is non-monotonic in base capability: weak-tier models benefit little, mid-tier models benefit most, and strong-tier models benefit less than mid-tierclaim0.804
  Second major claim of the paper, supported by Δbenefit measurements across six models on three benchmarks
• End-to-end evaluation scores conflate three sources of improvement: base capability, harness-updating quality, and harness-benefit, leaving it unclear which models produce useful updates or benefit most from themclaim0.801
  Motivating claim for the paper's controlled analysis approach
• Qwen3-235B leads as evolver on SWE-bench with 8.2 pp harness-updating gain but ranks last on MCP with 0.6 ppfinding0.798
  Illustrates benchmark-dependent reshuffling of evolver rankings, no evolver dominates across all substrates