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arXiv · 2605.24539

DemoEvolve: Overcoming Sparse Feedback in Agentic Harness Evolution with Demonstrations

Lirong Che·Yuzhe yang·Peiwen lin·Chuang wang·Xueqian wang·Jian su
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Abstract

Agent harness evolution improves frozen language-model agents by modifying the executable structures around them. We study this paradigm as a form of sample-efficient fast adaptation: instead of updating model weights, an agent can acquire task-specific competence by changing its external harness, while leaving the base model's general capabilities intact. Prior work shows that self-generated rollouts can support harness search, suggesting that agents may acquire new task competence through practice. Yet in long-horizon stochastic environments, self-practice becomes fragile: rewards are sparse, outcomes are high-variance, and failures are hard to attribute to concrete harness mechanisms. We introduce DemoEvolve, a demonstration-bootstrapped approach to harness evolution. When reward-only search is too broad and noisy, competent human trajectories serve as expert reference experience for the coding proposer, guiding harness-level diagnosis and editing. Experiments on Liar's Dice show that self-rollout evolution can work when episodes are short and failures are attributable. In contrast, Balatro exposes a harder long-horizon stochastic regime, where self-rollout evolution is misled by sparse feedback and candidate-selection noise, while tutorial-like textual knowledge alone does not yield stable improvement. Under the same limited budget, DemoEvolve produces more effective and auditable harness edits and achieves better performance. Overall, demonstrations make sparse-feedback harness evolution more diagnosable, localizable, and stable.

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Lirong Che, Yuzhe yang, Peiwen lin, Chuang wang, Xueqian wang, Jian su. 2026-05-23. DemoEvolve: Overcoming Sparse Feedback in Agentic Harness Evolution with Demonstrations. https://arxiv.org/abs/2605.24539

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