想比较哪些大模型最会自己改提示词和工具编排？这个新基准用4个任务111次运行测了5个模型，结果挺反直觉。
论文提出 HarnessOpt-Bench，专门评测 LLM 在自动优化 agent harness（提示词、工具、控制流、记忆与编排代码）时的端到端表现。优化器需要在固定评估预算下修改种子 harness，并在搜索过程不可访问的测试分区上按归一化增益打分。实验覆盖 5 个前沿 LLM、4 个下游任务，共 111 次评分运行。结果显示，优化器模型之间的能力差异大于其所用的编码 harness 差异，原生 harness 并不总是更优，增益在不同任务和种子设定下波动明显。
HarnessOpt-Bench: Evaluating LLMs at Harness Optimization
As LLMs are increasingly deployed within agentic systems, their capabilities depend not only on the model weights but also on the harness: the prompts, tools, control flow, memory, and orchestration code surrounding them. This makes automated harness optimization -- the iterative and evaluation-guided improvement of a harness by an AI system -- both an important route to improving AI systems and a demanding capability for AI systems themselves. Yet the community lacks a common protocol for measuring how well frontier LLMs perform at this task. We introduce HarnessOpt-Bench, a benchmark for end-to-end harness optimization under expensive and stochastic evaluation. An optimizer, an LLM paired with a coding harness, receives a target agent's seed harness, graded evaluation feedback, and a fixed target-evaluation budget. It edits the harness and nominates a final candidate, which is scored by its normalized gain over the seed on a held-out test partition that remains inaccessible throughout search. A trusted execution environment enforces the evaluation boundary, meters target-agent resource use, and preserves candidate versions for audit. We evaluate 5 frontier LLMs as optimizers both under a shared coding harness and under their native harnesses across 4 downstream tasks, over 111 scored runs. Experiment results show that optimizer models separate more than the coding harnesses they act through, native harnesses are not consistently superior, and gains vary substantially across tasks and seed regimes. These results establish harness optimization as a measurable and discriminative capability with large space for improvement.