REVIEW 4 major objections 6 minor 2 cited by
ArenaRL: Scaling RL for Open-Ended Agents via Tournament-based Relative Ranking
T0 review · 4 major / 6 minor · reviewed 2026-08-03 · deepseek-v4-flash
Pith's one-line read Open-ended agent RL fails because pointwise judge scores collapse within high-quality trajectory groups; ArenaRL replaces them with seeded single-elimination rankings, reporting near-round-robin accuracy at linear cost and large task gains.
desk verdict Seeded single-elimination is a clever, well-motivated tournament design, but the headline O(N) accuracy equivalence is asserted rather than directly measured; worth refereeing, not accepting on these experiments alone. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The central object is the seeded single-elimination tournament over a group of N trajectories sampled from the current policy. A greedy-decoding trajectory acts as a 'quality anchor': each exploratory trajectory is first compared with it, producing a seed ordering at low bias. The bracket then pairs the strongest seed against the weakest seed in a binary elimination tree, and each match is decided by the process-aware pairwise judge, which scores both trajectories under a rubric covering chain-of-thought coherence, tool-call effectiveness, and answer reliability in both presentation orders to cancel positional bias. Final rank comes from survival depth, with ties within a round broken by acc
What would settle it
Take held-out trajectory groups, compute the seeded single-elimination ranking and the exhaustive round-robin ranking, and measure their agreement (e.g., Kendall tau). Then corrupt or replace the greedy anchor—random seeds, worst-quality seeds, or seeds from a different decoding temperature—and repeat. The central claim is falsified if anchor-based seeding does not robustly beat random seeding in agreement with round-robin across trajectory distributions, or if there is any natural distribution where the O(N) ranking drops sharply below round-robin accuracy.
Extended reading notes
Core claim
The core claim is that the reward format, not the absence of a reward model, is what stalls RL in open-ended domains. ArenaRL formalises discrimination collapse—within-group pointwise scores compress toward a narrow band whose width is comparable to judge noise—and argues that normalisation tricks inherit that noise. The fix is intra-group relative ranking: a bidirectional, rubric-guided judge compares trajectories on reasoning, tool use, and final answer, and a tournament converts the comparisons into a stable ordering. The paper's key empirical assertion is that a seeded single-elimination design—using a greedy-decoding trajectory as an anchor to seed the bracket—gives advantage estimates
Load-bearing premise
The entire O(N)-with-round-robin-accuracy claim rests on the greedy-decoding trajectory being a reliable, low-bias quality anchor; if it is degenerate or unrepresentative, the seed ordering is wrong, strong trajectories can eliminate each other early, and the tournament ranking stops approximating the full pairwise ranking.
Editorial extensions
If this is right
- If discrimination collapse is the binding constraint, pointwise-reward RL will plateau once judge noise covers within-group quality differences; the reported gains are direct evidence for that failure mode.
- Pairwise comparison becomes affordable in online training: O(N) advantage estimation removes the quadratic bottleneck that made full preference ranking impractical for long-context agents.
- The mechanism transfers across domains—tool-using travel and research agents and ordinary open-ended writing—so relative ranking is a general fix for open-ended reward acquisition, not a benchmark-specific patch.
- ArenaRL's reported ability to train directly from a generic base model without SFT cold-start implies tournament ranking can bootstrap tool-use behavior where annotated trajectories are scarce.
- Open-Travel and Open-DeepResearch provide a full SFT-Training-Evaluation pipeline, letting later work compare complete agent training loops instead of static test sets.
Reading between the lines
- Because the greedy anchor is never refreshed, a natural extension is to make the anchor adaptive—re-seeding from the current policy's best trajectory each training round—so the bracket stays calibrated as the policy outgrows greedy decoding.
- The tournament idea transfers beyond agent RL: any expensive pairwise-preference setting (reward-model training, human evaluation, model selection) could use a cheap prior-seeded tournament to approximate full round-robin rankings.
- The reported 73.9% human-judge agreement suggests a calibration protocol: if tournament rankings from two independent judge families disagree, the ranking signal itself is suspect; this could be monitored as a training-time diagnostic.
- The paper's group-size ablation shows monotonic improvement up to N=16 but does not identify a saturation point; testing larger N would reveal whether the exploration benefit continues or whether judge and compute costs eventually dominate.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This paper proposes ArenaRL, a reinforcement-learning framework for open-ended LLM agents that replaces pointwise scalar rewards with an intra-group relative ranking derived from pairwise comparisons. The authors argue that pointwise LLM-judge scores suffer from a 'discrimination collapse' (low signal-to-noise ratio) and introduce a process-aware pairwise judge with bidirectional scoring, plus several tournament topologies to aggregate comparisons. The main algorithmic contribution is a 'seeded single-elimination' (SSE) scheme that uses a greedy-decoding trajectory as an anchor for seeding, claiming O(N) cost with advantage-estimation accuracy close to the O(N^2) round-robin benchmark. The paper also introduces two benchmarks, Open-Travel and Open-DeepResearch, with SFT/RL pipelines, and reports experiments showing ArenaRL outperforming GRPO, GSPO, and several closed-source models on travel planning, deep research, and open-ended writing.
Significance. If the SSE claim were established, ArenaRL would provide a practical answer to a real bottleneck in preference-based RL for open-ended tasks: extracting stable intra-group rankings at linear comparison cost. The paper also contributes two full-cycle open-ended agent benchmarks, a detailed rubric-based pairwise evaluation protocol, and a human-consistency check, all of which are useful community assets. These strengths are real and should be credited. However, the central claim of 'nearly equivalent advantage estimation accuracy' is currently supported only by end-to-end win rates under a partly circular judge protocol; direct ranking-fidelity evidence is missing. The idea is promising, but the manuscript does not yet demonstrate the headline result as rigorously as claimed.
major comments (4)
- [Abstract; §6.2, Table 2] The headline claim that seeded single-elimination (SSE) achieves 'nearly equivalent advantage estimation accuracy' to round-robin is never tested directly. Table 2 reports end-to-end RL win rates after full training; this conflates ranking fidelity with exploration, group size, judge noise, and the policy update. There is no correlation measure between SSE rankings and round-robin rankings on the same trajectory groups, no ranking-error metric, and no ablation isolating the tournament topology. As written, the O(N) equivalence is a plausible conjecture, not an established result.
- [§4.3, Phase 1] The 'quality anchor' assumption is load-bearing. The paper asserts that the greedy-decoding trajectory provides a 'low-biased initial estimate' for seeding, but offers no empirical support: no distribution of anchor scores, no correlation between anchor-based seed scores and round-robin win rates, and no ablation with random or simple-alternative seeding (e.g., median score, second greedy run). If greedy decoding produces a degenerate or stylistically atypical trajectory, seeds are misordered and the SSE ranking can diverge from the round-robin ranking. This needs to be measured or, at minimum, ablated.
- [Appendix A; §5.3, §6.4] The evaluation protocol is partly circular. Qwen3-Max is used as the arena judge during RL training (Appendix A) and is also one of the two final evaluation judges (§5.3). Reported win rates may therefore reward alignment to Qwen3-Max's preferences rather than general output quality. The 73.9% human-agreement check is encouraging, but the manuscript gives no sample size, rater details, or per-method agreement, and it does not rule out a judge-specific advantage. Please report results with a judge family not seen during training, or provide a human evaluation on a representative, documented sample.
- [§6.2 vs. Appendix A] Tournament topology selection is performed at group size N=8 (§6.2), but the final Open-Travel and open-ended-writing runs use N=16, and Open-DeepResearch uses N=8 with K=4 (Appendix A). Table 2's conclusion that SSE 'achieves' round-robin performance is therefore established only at N=8; no evidence shows that the equivalence transfers to the deployed N=16. Moreover, no results carry error bars or significance tests, so the 32.5 vs. 32.9 difference may be within noise. At minimum, the paper should state the actual training configuration used for each table and justify extrapolating the topology choice from N=8.
minor comments (6)
- [§6.2] 'Swiss Round' should be 'Swiss-System' to match §4.5 and Figure 2.
- [Introduction] The text refers to 'Figure 1(c)', but the displayed figure has only panels (a) and (b). Please correct the citation or add the missing panel.
- [§5.1] The phrase 'denoted as (defined as the 1-Day subtask)' contains a duplicated parenthetical; please fix.
- [Appendix A] The evaluation judges are described as 'Qwen3-Max and Claude-4-Sonnet Anthropic (2023)', but the reference list only contains a generic Claude (2023) entry. A specific citation for Claude-4-Sonnet is needed.
- [Algorithm 1] The bracket-array construction (lines 11–21) is not explained in the main text. A sentence explaining why seeds are assigned alternately from front and back would improve readability and reproducibility.
- [Code availability] The repository link is given as https://github.com/Alibaba-NLP/qqr, which appears unrelated to ArenaRL. Please verify that the URL points to the correct code release.
Circularity Check
No significant circularity: the ArenaRL derivation is algorithmic and its headline O(N) claim is empirical, though the greedy-anchor assumption is untested and the training judge also appears in final evaluation.
full rationale
The claimed derivation chain — pointwise discrimination collapse, pairwise process-aware evaluation, tournament-based ranking, rank-to-advantage mapping, and RL update — contains no equation or fitted parameter that reduces by construction to its own inputs. The seeded single-elimination scheme is an algorithm whose O(N) accuracy is asserted from downstream win rates in Table 2 rather than derived; the 'greedy decoding as quality anchor' premise in Sec. 4.3 is an untested assumption about anchor quality, but an unsupported assumption is not circularity. The evaluation protocol is partially self-referential because Qwen3-Max serves as the arena judge during RL training and is also one of the two final evaluation judges (Appendix A), so reported win rates on Open-Travel/Open-DeepResearch could partly reflect alignment to that judge's preferences; however, this is an external-validity confound rather than a definitional equivalence, and the paper mitigates it by averaging with Claude-4-Sonnet, reporting a 73.9% human-agreement check, and including independent public writing benchmarks with established protocols. No load-bearing self-citation, imported uniqueness theorem, ansatz smuggled in via citation, or renaming of a known result was found.
Assumptions & free parameters
free parameters (4)
- Group size N (and number of groups K) =
N=16/K=8 for Open-Travel and writing; N=8/K=4 for Open-DeepResearch
- Exploration sampling temperature =
0.8 (exploration), 0 (greedy anchor)
- Advantage standardization epsilon =
not reported (small constant in Eq. 7)
- KL penalty coefficient β =
not reported
assumptions (4)
- domain assumption LLM pairwise judge scores (after bidirectional averaging, Eq. 3) are stable, low-bias estimates of trajectory quality and are transitive enough for tournament ranking.
- ad hoc to paper The greedy-decoding trajectory is a low-bias quality anchor for seeding.
- domain assumption Accumulated pairwise scores from different matches are comparable across opponents, so scores can rank trajectories eliminated in the same tier.
- domain assumption The seven (deep research) and four/three (travel/writing) rubric dimensions capture the true quality of open-ended agent outputs.
Cite this review
Pith. "Pith review of ArenaRL: Scaling RL for Open-Ended Agents via Tournament-based Relative Ranking." pith.science (2026). https://pith.science/paper/HHVYB4GX
@misc{pith2026260106487,
author = {Pith},
title = {Pith review of: ArenaRL: Scaling RL for Open-Ended Agents via Tournament-based Relative Ranking},
year = {2026},
howpublished = {\url{https://pith.science/paper/HHVYB4GX}},
note = {Machine review of arXiv:2601.06487}
}
read the original abstract
Reinforcement learning has substantially improved the performance of LLM agents on tasks with verifiable outcomes, but it still struggles on open-ended agent tasks with vast solution spaces (e.g., complex travel planning). Due to the absence of objective ground-truth for these tasks, current RL algorithms largely rely on reward models that assign scalar scores to individual responses. We contend that such pointwise scoring suffers from an inherent discrimination collapse: the reward model struggles to distinguish subtle advantages among different trajectories, resulting in scores within a group being compressed into a narrow range. Consequently, the effective reward signal becomes dominated by noise from the reward model, leading to optimization stagnation. To address this, we propose ArenaRL, a reinforcement learning paradigm that shifts from pointwise scalar scoring to intra-group relative ranking. ArenaRL introduces a process-aware pairwise evaluation mechanism, employing multi-level rubrics to assign fine-grained relative scores to trajectories. Additionally, we construct an intra-group adversarial arena and devise a tournament-based ranking scheme to obtain stable advantage signals. Empirical results confirm that the built seeded single-elimination scheme achieves nearly equivalent advantage estimation accuracy to full pairwise comparisons with O(N^2) complexity, while operating with only O(N) complexity, striking an optimal balance between efficiency and precision. Furthermore, to address the lack of full-cycle benchmarks for open-ended agents, we build Open-Travel and Open-DeepResearch, two high-quality benchmarks featuring a comprehensive pipeline covering SFT, RL training, and multi-dimensional evaluation. Extensive experiments show that ArenaRL substantially outperforms standard RL baselines, enabling LLM agents to generate more robust solutions for complex real-world tasks.
Figures
Figures from the paper (7 more)
Forward citations
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Reference graph
Works this paper leans on
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[1]
WritingBenchWu et al. (2025b) is designed to assess models’ domain-specific writing proficiency and controllability across six professional and creative domains: A (Academic & Engineering), B (Finance & Business), C (Politics & Law), D (Literature & Arts), E (Education), and F (Advertising & Marketing). It focuses in particular on the ability to comply wi...
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in-the-wild
HelloBenchQue et al. (2024) evaluates model performance on a diverse set of “in-the-wild” tasks derived from real user queries. Our analysis concentrates on three key subsets: HelloBench-QA (open-ended QA), which tests the generation of detailed and nuanced responses; HelloBench- Summ. (text summarization), which evaluates the ability to summarize long-fo...
2024
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No. 10 Fuyong Street, Wangjing, Chaoyang District, Beijing
LongBench-writeBai et al. (2024) is designed to evaluate a model’s ability to produce coherent ultra-long texts (e.g., on the order of 10,000 words), enabling us to assess the fundamental capacity to maintain topical consistency and global coherence as output length scales. C Benchmarking Tool Annotation In this section, we will outline the tools employed...
arXiv 2024
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Take a high-speed train from Chengdu to Qingchengshan Station, then transfer to a scenic area shuttle bus or taxi to get to the Back Mountain area
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name": "get_navigation
Drive directly to Qingcheng Back Mountain. I will use the get_navigation tool to plan the transportation route from Chengdu to Qingcheng Back Mountain.</think> <tool_call> {"name": "get_navigation", "arguments": {"start_lon": 104.066301, "start_lat": 30.572961, "end_lon": 103.487136, "end_lat": 30.93071}} </tool_call> <tool_response> ... </tool_response> ...
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推理广度(Breadth):是否全面覆盖用户需求,同时避免出现冗余或重复步骤。
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需求匹配度(Relevance):各步骤与用户核心需求契合程度。
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完全缺失”,10 表示“极为出色
细节信息丰富度(Detail):引用的事实、数据、时间点、费用、预约规则等细节是否充分、准确且有用。 【评分规则】 • 推理路径评测时要求只关注推理路径中的实际工具调用,不用关注推理内容对信息的深入分析。 • 每个维度 0-10 分;0 表示“完全缺失”,10 表示“极为出色”。 • 推理路径综合得分(Overall_P)=三个维度均值后四舍五入取整。 —————————— 三、回答结果评测(Answer Evaluation) —————————— 【评估维度说明】
Show all 44 references
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匹配度(Relevance):完整响应所有子需求/限制?顺序与场景贴合?
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可行性(Feasibility):安排逻辑自洽、切实可行,避免明显冲突?
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细节丰富度(Details):时间表、票价、交通耗时、Tips 等信息是否丰富且实用?
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完全缺失”,10 表示“极为出色
清晰度(Clarity):结构清晰、排版友好、可读性高? 【评分规则】 • 回答结果评测时需参考对应推理路径中的参考知识。 • 每个维度 0-10 分;0 表示“完全缺失”,10 表示“极为出色”。 • 回答结果综合得分(Overall_A)=四个维度均值后四舍五入取整。 —————————— 四、综合得分与胜负判定 —————————— 综合得分 combined_scores = 0.6 * Overall_P(路径总体分) + 0.4 * Overall_A(答案总体分),四舍五入保留 1 位小数。 若 Combined 相同,则胜负判定结果...
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研究框架完整性(Framework):是否在首轮给出条理清晰、递进合理、覆盖全面的研究步骤。
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工具调用策略(Tool Usage):search_web 等工具调用是否针对性强、查询多样且无冗余,调用顺序与研究步骤匹配度高。
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完全缺失”,10=“极为出色
信息覆盖完整性(Coverage):所检索信息能否充分覆盖用户需求,是否为后续回答奠定扎实依据,且没有重复冗余行动步骤。 【评分规则】 • 仅评估研究过程设计与工具使用本身,不评价其对资料的解读结果。 • 每维度 0-10 分;0=“完全缺失”,10=“极为出色”。 • 研究路径综合得分(overall_p)= 三个维度均值,四舍五入取整。 —————————— 三、回答结果评测(Answer Evaluation) —————————— 【评估维度说明】
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契合度(Relevance):是否完整、准确地回应了用户所有问题与限制条件。
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事实准确性(Accuracy):关键数据、定义、结论是否充分,无明显错误或自相矛盾。
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论证深度(Depth):是否进行深入分析、比较与推理,展示批判性思考与清晰逻辑链。
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完全缺失”,10=“极为出色
表达清晰度(Clarity):结构排版、用词与逻辑是否清晰易读,可直接为用户所用。 【评分规则】 • 评估时须参考对应研究路径所呈现的已检索信息;不得依据外部记忆。 • 每维度 0-10 分;0=“完全缺失”,10=“极为出色”。 • 回答结果综合得分(overall_a)= 四个维度均值,四舍五入取整。 —————————— 四、综合得分与胜负判定 —————————— combined_scores = 0.5 × overall_p + 0.5 × overall_a,四舍五入保留 1 位小数。 若两者 combined_scores 相同,...
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理解与拆解 (Understanding):是否全面抓住用户所有写作要点、受众、限制。
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逻辑严谨性 (Logic):思考步骤是否井然、论据与结论衔接顺畅,无跳步或自相矛盾。
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完全缺失”,10=“极为出色
丰富度与创造性 (Richness):是否从主题、结构、素材等多角度提出多种可选思路或素材。 【评分规则】 • 每维度 0-10 分;0=“完全缺失”,10=“极为出色”。 • 路径综合得分 overall_p =三维度均值,四舍五入取整。 三、写作结果评测 Answer Evaluation(结合对应 PATH 参考知识) 【维度说明】
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需求契合度 (Relevance):是否完整覆盖用户列出的全部要点,并严格满足用户设定的所有限制。
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内容质量与说服力 (Content Quality):主题深度、论点论据充分性、吸引力与原创性。
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语言与文采 (Language & Style):专业性、亲和力、流畅度,遣词造句精准多样。
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完全缺失”,10=“极为出色
结构与可读性 (Clarity):逻辑清晰、层次分明、排版友好。 【评分规则】 • 每维度 0-10 分;0=“完全缺失”,10=“极为出色”。 • 答案综合得分 overall_a =四维度均值,四舍五入取整。 四、综合得分与胜负判定 1.combined_score = 0.4 × overall_p + 0.6 × overall_a,四舍五入保留 1 位小数。 2.若两个LLM combined_score 相同,则判定为 Tie。 五、输出格式(严格遵循,勿增删字段或更换顺序) { "analysis": { "path_A": "<80...
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只依据提供文本,不引入外部信息或个人偏好。
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若写作结果中反复出现重复内容,或包含以<think>字符开头的推理路径,则应大幅降低答案得分。
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评述需具体可溯源,可引用“第 X 段”或原句关键词。
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Comprehensive LLM Writing Evaluator
必须输出合法 JSON,确保后续程序可解析。 Figure 7: Chinese prompt of open-ended writing task. 35 Technical Report Tongyi DeepResearch You are a rigorously minded “Comprehensive LLM Writing Evaluator” versed in writing-assessment methodology. Your task is to perform a multi-dimensional quantitat...
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Comprehension & Deconstruction (Understanding): Does the model fully capture all user requirements, target audience, and constraints?
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Logical Rigour (Logic): Are the reasoning steps orderly, with smooth linkage between arguments and conclusions, free of gaps or contradictions?
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entirely missing
Richness & Creativity (Richness): Does the model propose multiple viewpoints, structures, or materials from diverse angles? [Scoring Rules] • Each dimension: 0-10 points (0 = “entirely missing”, 10 = “outstanding”). • Overall path score (overall_p) = arithmetic mean of the thr...
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Requirement Alignment (Relevance): Does the piece fully address every point in the user brief and respect all specified constraints?
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Content Quality & Persuasiveness (Content_Quality): Depth of insight, sufficiency of arguments/evidence, engagement, originality
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Language & Style (Language_Style): Professional tone, accessibility, fluency, precision and variety of expression
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entirely missing
Clarity & Readability (Clarity): Clear logic, well-structured sections, reader-friendly formatting. [Scoring Rules] • Each dimension: 0-10 points (0 = “entirely missing”, 10 = “outstanding”). • Overall answer score (overall_a) = arithmetic mean of the four dimensions, rounded ...
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combined_score = 0.4 × overall_p + 0.6 × overall_a, rounded to one decimal place
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Tie”. V. Output Format (strictly follow; do not add, remove or reorder fields) {
If both models obtain the same combined_score, declare “Tie”. V. Output Format (strictly follow; do not add, remove or reorder fields) { "analysis": { "path_A": "<80-120 Chinese characters: highlight strengths and weaknesses of A’s path>", "path_B": "<80-120 Chinese characters...
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Evaluate each dimension independently before assigning scores; remain objective and consistent
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Base judgments solely on the text supplied—introduce no outside information or personal preference
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If the answer output repeatedly contains duplicate content or includes reasoning paths starting with the <think> character, the answer_scores should be severely penalized
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paragraph X
Analytic comments must be traceable and specific; you may cite “paragraph X” or key phrases from the source
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航班查询结果模拟专家
Output must be valid JSON so that downstream programs can parse it. Figure 8: English prompt of open-ended writing task. 36 Technical Report Tongyi DeepResearch 角色设定 你是一名“航班查询结果模拟专家 ”,能够根据用户给出的日期 、出发城市与到达城市 ,生成覆盖全天主要时段的机 票信息(6-14 条)。所有信息均为模拟数据,但必须符合以下“真实性规则”。 输入格式 用户将以 JSON 形式...
2025
Reviewed August 3, 2026 · model on record in the stance chip above.
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