REVIEW 4 major objections 7 minor 26 references
The Synergy Dilemma of Long-CoT SFT and RL: Investigating Post-Training Techniques for Reasoning VLMs
T0 review · 4 major / 7 minor · reviewed 2026-08-06 · deepseek-v4-flash
Pith's one-line read Long-CoT SFT and RL do not combine synergistically in vision-language reasoning models: all five tested fusion methods underperform standalone RL.
desk verdict Useful difficulty-level analysis of SFT vs RL for reasoning VLMs, but the 'synergy dilemma' headline is not supported by the reported statistics. 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 load-bearing mechanism is difficulty-stratified analysis: every benchmark question is placed into one of five difficulty levels using the baseline model's pass rate over 16 runs, so gains can be separated by question type rather than averaged away. On top of that, the paper uses token-level KL divergence before and after fine-tuning to expose "reasoning pivotal tokens"—words like "first", "then", "check", and "maybe"—that SFT injects at sentence beginnings, and it tracks response lengths and RL training dynamics under the GRPO objective with a KL regularization term. These tools let the authors attribute SFT's gains to hard questions and verbosity, RL's gains to concise generalization, and the failure of hybrids to unresolved trade-offs.
What would settle it
Replace Eureka-Distill with long-CoT traces distilled from the target model itself, keep every other setting identical, and rerun the five combination strategies on the same five benchmarks; if any variant averages above 55.1 percent across the five, the dilemma is a property of the tested recipe, not of reasoning VLMs generally.
Extended reading notes
Core claim
On the paper's own terms, the central discovery is that long-CoT SFT and RL play distinct, difficulty-dependent roles in reasoning VLMs, and naive ways of stacking them do not combine those roles. SFT raises accuracy on the hardest questions (levels 4-5) by injecting structured, step-by-step reasoning traces, but makes responses roughly an order of magnitude longer than baseline and lowers accuracy on easy questions. RL produces concise responses with stable token distributions and improves accuracy across all five difficulty levels, yet its gains on the hardest questions are smaller than SFT's. Five integration strategies—two-stage, interleaved, and progressive training, data mixing, and model merging—all land between SFT and RL or below RL on average accuracy (51.4, 53.6, 54.9, 54.0, and 54.3 percent versus RL's 55.1), showing trade-offs in accuracy, reasoning style, and response length.
Load-bearing premise
The generalization of the dilemma rests on one base model, one distilled long-CoT training set, one RL algorithm with one KL setting, and the authors' specific choices for each combination; if those choices are not representative, the trade-off could be an artifact of the recipe rather than a property of reasoning VLMs.
Editorial extensions
If this is right
- For average accuracy on current multimodal reasoning benchmarks, standalone RL with KL regularization is the strongest recipe tested; stacking SFT first, interleaving it, mixing data, or merging models does not add value.
- SFT has a measurable strength on the hardest questions, so any future combination should target hard questions rather than apply SFT uniformly.
- Retaining easy questions in RL training, even though their advantages normalize to zero, preserves accuracy on easy problems through the KL term.
- High-quality reasoning traces matter more than volume or length: 1k textual traces outperformed a 34k multimodal set, and longer traces of lower quality hurt performance.
Reading between the lines
- A testable extension: apply SFT loss only to difficulty levels 4-5 and RL loss elsewhere, instead of the paper's zero-pass-rate threshold; the paper's own difficulty curves suggest this could recover parts of both gains.
- The authors' data-mixing recipe collects RL-generated correct responses and then runs SFT on them; the resulting 10x response-length increase hints that SFT imitates format and verbosity from the distilled traces, which a format-reward ablation could confirm.
- If the dilemma is a general property, inference-time routing—send hard questions to the SFT model and easy ones to the RL model—would sidestep training incompatibility entirely and could beat either model alone, a consequence the paper does not test.
- Language-model synergy reported elsewhere may depend on SFT data being self-distilled or model-compatible; reproducing this study with self-distilled traces would clarify whether the dilemma is multimodal or a data-mismatch artifact.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This manuscript studies post-training of Qwen2.5-VL-7B with long-CoT SFT and GRPO-based RL. It reports that SFT improves the most difficult questions but hurts easy ones and produces verbose outputs, whereas RL produces concise, broad gains. It then evaluates five combination strategies—two-stage, interleaved, progressive training, data mixing, and model merging—and concludes on the basis of Table 6 that none surpasses standalone RL, terming this a 'synergy dilemma.' The difficulty-level analyses in Figures 1 and 6 are used to support the complementary-but-incompatible characterization of SFT and RL.
Significance. If the negative result were robust, the paper would be a useful counterpoint to the prevailing assumption that SFT and RL automatically complement each other in reasoning VLMs. The study is systematic in its coverage of combination strategies and benchmarks, and it reports training/evaluation details and a code repository link. However, the central quantitative claim currently rests on point estimates without measures of uncertainty, and the evaluation protocol's best-checkpoint selection compounds the issue. The headline 'synergy dilemma' is therefore not yet supported at the strength claimed.
major comments (4)
- [Sec. 2.1, Table 6] Table 6 is the load-bearing evidence for the 'synergy dilemma,' but no variance or significance testing is reported despite the statement in Sec. 2.1 that results are averaged over 4 runs. The closest combination, Progressive SFT & RL, averages 54.9 versus RL's 55.1, and per-benchmark deltas such as MathVerse -1.1 and MMStar -0.6 are within one standard error of a binomial proportion on benchmarks of only hundreds to about a thousand items. The conclusion that all five combination strategies 'fail to produce additive benefits' is statistically indistinguishable from a null result under sampling noise. Please report standard deviations or confidence intervals and run paired or bootstrap significance tests across the four runs, and soften the 'fundamental' framing until these analyses support the point estimates.
- [Sec. 2.1, Evaluation settings] The evaluation protocol states 'For fine-tuned models, we report their best checkpoint results.' Because each method has a different training curve and a different number of saved checkpoints, selecting the best checkpoint on the evaluation benchmarks can differentially inflate each method's score and makes the comparisons in Table 6 non-comparable. This is especially problematic given the small margins (e.g., 54.9 versus 55.1). Please either evaluate a fixed checkpoint per training budget, report full training curves or the sensitivity of rankings to checkpoint selection, or perform selection on a held-out split and then report the resulting test numbers.
- [Sec. 2.3, Fig. 6] The difficulty-level accuracy gains in Fig. 6 (and the corresponding gains in Fig. 1) are shown without error bars, despite the four-run protocol. The claim that 'SFT degrades easier questions and is strongest on the hardest questions' depends on these per-level deltas, which can be small and are based on small per-level item counts. Please provide per-level intervals or raw per-level counts; without them, the qualitative difficulty-dependent claims are not quantitatively supported.
- [Sec. 3.1, Sec. 3.3] The paper presents the 'synergy dilemma' as a general property of reasoning VLMs ('fundamental,' 'often behave more like a trade-off'), but the study uses one base model, one training set (Eureka-Distill, distilled from the authors' own SFT model), one RL algorithm with one KL value, and method-specific choices that are known to be consequential—for example, interleaved SFT applied only to pass-rate-zero questions and progressive prefix weight 0.2 in Sec. 3.1. These choices may be representative, but the current evidence cannot rule out that the dilemma is an artifact of this particular recipe. Please add supporting variation (e.g., a second base model or data source, or a sensitivity analysis over the method-specific hyperparameters) or explicitly restrict the claim to 'under the configurations tested here.'
minor comments (7)
- [Throughout] There are numerous typos and misspellings, for example 'contorling' in Sec. 2.2, 'Abalation' in the Table 2 caption, 'Acuracy' in the Table 6 caption, 'varing' in Sec. 2.1, and 'thourgh' in Sec. 3.1; these should be corrected.
- [Sec. 2.1, Data] The data section refers to 's1.1-R1' while the training settings refer to 's1.1'; please clarify whether these are the same dataset and use one consistent name.
- [Sec. 2.3, Fig. 7] The token-level KL divergence used to identify 'reasoning pivotal tokens' is not defined in the text; please specify how tokens are aligned and what reference distribution is used.
- [Sec. 2.3, Difficulty levels] The difficulty levels are defined using 16 independent runs of the baseline model, while the evaluation protocol in Sec. 2.1 says results are averaged over 4 runs; please clarify the relation between the two settings.
- [Sec. 3.1, Two-stage SFT & RL] The statement 'We have also attempted reducing the number of preliminary SFT epochs to 1' is not accompanied by any result or table; please add a sentence summarizing the outcome or remove the claim.
- [References] The reference list contains duplicates: Chen et al. 2025a and 2025b are the same arXiv paper, and Yang et al. 2025a and 2025b are also the same; please merge or distinguish them correctly.
- [Table 5] The SFT count of 249,091 for the word 'wait' appears implausibly large for a single benchmark unless counting across many generated responses; please clarify the counting procedure and verify the reported numbers.
Circularity Check
No significant circularity: the 'synergy dilemma' is an empirical measurement directly reported in Table 6, not a quantity fitted from or defined by its own inputs.
full rationale
This paper is an empirical measurement study. The central claim, that combining long-CoT SFT and RL produces trade-offs rather than additive benefits, is a direct experimental comparison in Table 6: each listed method is trained and evaluated under the same protocol, and the reported averages are measurements, not predictions derived from fitted parameters. No equation in the paper defines a target quantity in terms of the inputs, and no fitted parameter is renamed as a prediction. The difficulty levels are defined from the baseline model's pass rate (Sec. 2.3), independently of the SFT/RL gains, so the difficulty-stratified analysis is a measurement convention rather than a self-definitional result. The Eureka-Distill dataset is distilled from the authors' own SFT model, but both the SFT and RL arms use the same data, making the comparison fair rather than forced by construction. The absence of error bars and the use of best-checkpoint selection are statistical robustness concerns, not circularity. None of the load-bearing conclusions reduces to a self-citation or to an imported uniqueness theorem. Therefore, no specific circular step can be exhibited, and the appropriate score is 0.
Assumptions & free parameters
free parameters (5)
- KL coefficient beta =
0.005
- SFT prefix loss weight =
0.2
- Training epochs =
15 for s1, 5 for Eureka-Distill SFT, 2 for RL/data mixing
- Max new generation length N =
4k for RL, 16k for two-stage
- Difficulty thresholds =
12/16, 8/16, 5/16, 2/16
assumptions (5)
- domain assumption Eureka-Distill queries and answers are a representative training distribution for multimodal reasoning
- domain assumption Baseline pass rate over 16 runs is a stable measure of question difficulty
- domain assumption Qwen2.5-VL-7B is representative of current reasoning VLMs
- domain assumption The evaluation benchmarks are valid measures of multimodal reasoning
- standard math GRPO is an appropriate RL algorithm
Cite this review
Pith. "Pith review of The Synergy Dilemma of Long-CoT SFT and RL: Investigating Post-Training Techniques for Reasoning VLMs." pith.science (2026). https://pith.science/paper/VSNBATHH
@misc{pith2026250707562,
author = {Pith},
title = {Pith review of: The Synergy Dilemma of Long-CoT SFT and RL: Investigating Post-Training Techniques for Reasoning VLMs},
year = {2026},
howpublished = {\url{https://pith.science/paper/VSNBATHH}},
note = {Machine review of arXiv:2507.07562}
}
read the original abstract
Large vision-language models (VLMs) increasingly adopt post-training techniques such as long chain-of-thought (CoT) supervised fine-tuning (SFT) and reinforcement learning (RL) to elicit sophisticated reasoning. While these methods exhibit synergy in language-only models, their joint effectiveness in VLMs remains uncertain. We present a systematic investigation into the distinct roles and interplay of long-CoT SFT and RL across multiple multimodal reasoning benchmarks. We find that SFT improves performance on difficult questions by in-depth, structured reasoning, but introduces verbosity and degrades performance on simpler ones. In contrast, RL promotes generalization and brevity, yielding consistent improvements across all difficulty levels, though the improvements on the hardest questions are less prominent compared to SFT. Surprisingly, combining them through two-staged, interleaved, or progressive training strategies, as well as data mixing and model merging, all fails to produce additive benefits, instead leading to trade-offs in accuracy, reasoning style, and response length. This ``synergy dilemma'' highlights the need for more seamless and adaptive approaches to unlock the full potential of combined post-training techniques for reasoning VLMs.
Figures
Figures from the paper (10 more)
Reference graph
Works this paper leans on
-
[1]
Shuai Bai, Keqin Chen, Xuejing Liu, Jialin Wang, Wenbin Ge, Sibo Song, Kai Dang, Peng Wang, Shijie Wang, Jun Tang, Humen Zhong, Yuanzhi Zhu, Mingkun Yang, Zhaohai Li, Jianqiang Wan, Pengfei Wang, Wei Ding, Zheren Fu, Yiheng Xu, Jiabo Ye, Xi Zhang, Tianbao Xie, Zesen Cheng, Hang Zhang, Zhibo Yang, Haiyang Xu, and Junyang Lin. Qwen2.5-vl technical report. a...
-
[4]
Kanishk Gandhi, Ayush Chakravarthy, Anikait Singh, Nathan Lile, and Noah D Goodman. Cogni- tive behaviors that enable self-improving reasoners, or, four habits of highly effective stars.arXiv preprint arXiv:2503.01307,
-
[5]
Arcee’s mergekit: A toolkit for merging large language models
Charles Goddard, Shamane Siriwardhana, Malikeh Ehghaghi, Luke Meyers, Vladimir Karpukhin, Brian Benedict, Mark McQuade, and Jacob Solawetz. Arcee’s mergekit: A toolkit for merging large language models. In Proceedings of the 2024 Conference on Empirical Methods in Natural Language Processing: Industry Track, pp. 477–485,
work page 2024
-
[6]
URL https://cloud.google.com/vertex-ai/generative-ai/docs/ thinking-mode. Daya Guo, Dejian Yang, Haowei Zhang, Junxiao Song, Ruoyu Zhang, Runxin Xu, Qihao Zhu, Shirong Ma, Peiyi Wang, Xiao Bi, et al. Deepseek-r1: Incentivizing reasoning capability in llms via reinforcement learning. arXiv preprint arXiv:2501.12948,
-
[7]
Mammoth-vl: Eliciting multimodal reasoning with instruction tuning at scale
Jarvis Guo, Tuney Zheng, Yuelin Bai, Bo Li, Yubo Wang, King Zhu, Yizhi Li, Graham Neubig, Wenhu Chen, and Xiang Yue. Mammoth-vl: Eliciting multimodal reasoning with instruction tuning at scale. arXiv preprint arXiv:2412.05237,
-
[8]
Measuring mathematical problem solving with the math dataset
Dan Hendrycks, Collin Burns, Saurav Kadavath, Akul Arora, Steven Basart, Eric Tang, Dawn Song, and Jacob Steinhardt. Measuring mathematical problem solving with the math dataset. arXiv preprint arXiv:2103.03874,
-
[10]
11 Technical Report on Reasoning VLMs Aaron Jaech, Adam Kalai, Adam Lerer, Adam Richardson, Ahmed El-Kishky, Aiden Low, Alec Helyar, Aleksander Madry, Alex Beutel, Alex Carney, et al. Openai o1 system card. arXiv preprint arXiv:2412.16720,
-
[11]
Acereason-nemotron 1.1: Advancing math and code reasoning through sft and rl synergy
Zihan Liu, Zhuolin Yang, Yang Chen, Chankyu Lee, Mohammad Shoeybi, Bryan Catanzaro, and Wei Ping. Acereason-nemotron 1.1: Advancing math and code reasoning through sft and rl synergy. arXiv preprint arXiv:2506.13284,
Show all 26 references
-
[13]
Yi Peng, Xiaokun Wang, Yichen Wei, Jiangbo Pei, Weijie Qiu, Ai Jian, Yunzhuo Hao, Jiachun Pan, Tianyidan Xie, Li Ge, et al
URL https://arxiv.org/abs/2501.19393. Yi Peng, Xiaokun Wang, Yichen Wei, Jiangbo Pei, Weijie Qiu, Ai Jian, Yunzhuo Hao, Jiachun Pan, Tianyidan Xie, Li Ge, et al. Skywork r1v: Pioneering multimodal reasoning with chain-of- thought. arXiv preprint arXiv:2504.05599,
-
[14]
Deepseekmath: Pushing the limits of mathematical reasoning in open language models
Zhihong Shao, Peiyi Wang, Qihao Zhu, Runxin Xu, Junxiao Song, Xiao Bi, Haowei Zhang, Mingchuan Zhang, YK Li, Y Wu, et al. Deepseekmath: Pushing the limits of mathematical reasoning in open language models. arXiv preprint arXiv:2402.03300,
-
[15]
Hybridflow: A flexible and efficient rlhf framework
Guangming Sheng, Chi Zhang, Zilingfeng Ye, Xibin Wu, Wang Zhang, Ru Zhang, Yanghua Peng, Haibin Lin, and Chuan Wu. Hybridflow: A flexible and efficient rlhf framework. arXiv preprint arXiv: 2409.19256,
-
[17]
Kimi Team, Angang Du, Bofei Gao, Bowei Xing, Changjiu Jiang, Cheng Chen, Cheng Li, Chenjun Xiao, Chenzhuang Du, Chonghua Liao, et al. Kimi k1. 5: Scaling reinforcement learning with llms. arXiv preprint arXiv:2501.12599,
-
[18]
Vl- rethinker: Incentivizing self-reflection of vision-language models with reinforcement learning
Haozhe Wang, Chao Qu, Zuming Huang, Wei Chu, Fangzhen Lin, and Wenhu Chen. Vl- rethinker: Incentivizing self-reflection of vision-language models with reinforcement learning. arXiv preprint arXiv:2504.08837, 2025a. Ke Wang, Junting Pan, Weikang Shi, Zimu Lu, Houxing Ren, Aojun...
-
[19]
Skywork r1v2: Multimodal hybrid reinforcement learn- ing for reasoning
Peiyu Wang, Yichen Wei, Yi Peng, Xiaokun Wang, Weijie Qiu, Wei Shen, Tianyidan Xie, Jiangbo Pei, Jianhao Zhang, Yunzhuo Hao, et al. Skywork r1v2: Multimodal hybrid reinforcement learn- ing for reasoning. arXiv preprint arXiv:2504.16656, 2025b. 12 Technical Report on Reasoning ...
-
[21]
R1-onevision: Advancing general- ized multimodal reasoning through cross-modal formalization
Yi Yang, Xiaoxuan He, Hongkun Pan, Xiyan Jiang, Yan Deng, Xingtao Yang, Haoyu Lu, Dacheng Yin, Fengyun Rao, Minfeng Zhu, Bo Zhang, and Wei Chen. R1-onevision: Advancing general- ized multimodal reasoning through cross-modal formalization. arXiv preprint arXiv:2503.10615, 2025a...
-
[22]
Demystifying long chain- of-thought reasoning in llms
Edward Yeo, Yuxuan Tong, Morry Niu, Graham Neubig, and Xiang Yue. Demystifying long chain- of-thought reasoning in llms. arXiv preprint arXiv:2502.03373,
-
[23]
Dapo: An open-source llm reinforcement learning system at scale
Qiying Yu, Zheng Zhang, Ruofei Zhu, Yufeng Yuan, Xiaochen Zuo, Yu Yue, Weinan Dai, Tiantian Fan, Gaohong Liu, Lingjun Liu, et al. Dapo: An open-source llm reinforcement learning system at scale. arXiv preprint arXiv:2503.14476,
-
[24]
R1-vl: Learning to reason with multimodal large language models via step-wise group relative policy optimization
Jingyi Zhang, Jiaxing Huang, Huanjin Yao, Shunyu Liu, Xikun Zhang, Shijian Lu, and Dacheng Tao. R1-vl: Learning to reason with multimodal large language models via step-wise group relative policy optimization. arXiv preprint arXiv:2503.12937,
-
[25]
Mathverse: Does your multi-modal llm truly see the diagrams in visual math problems? In European Conference on Computer Vision , pp
Renrui Zhang, Dongzhi Jiang, Yichi Zhang, Haokun Lin, Ziyu Guo, Pengshuo Qiu, Aojun Zhou, Pan Lu, Kai-Wei Chang, Yu Qiao, et al. Mathverse: Does your multi-modal llm truly see the diagrams in visual math problems? In European Conference on Computer Vision , pp. 169–186. Spring...
-
[26]
URL http://arxiv.org/abs/2403.13372
Association for Computational Linguis- tics. URL http://arxiv.org/abs/2403.13372. Hengguang Zhou, Xirui Li, Ruochen Wang, Minhao Cheng, Tianyi Zhou, and Cho-Jui Hsieh. R1- zero’s” aha moment” in visual reasoning on a 2b non-sft model.arXiv preprint arXiv:2503.05132,
-
[1985]
Reason-rft: Reinforcement fine-tuning for visual reasoning
Huajie Tan, Yuheng Ji, Xiaoshuai Hao, Minglan Lin, Pengwei Wang, Zhongyuan Wang, and Shanghang Zhang. Reason-rft: Reinforcement fine-tuning for visual reasoning. arXiv preprint arXiv:2503.20752,
-
[2021]
Vision-r1: Incentivizing reasoning capability in multimodal large language models
Wenxuan Huang, Bohan Jia, Zijie Zhai, Shaosheng Cao, Zheyu Ye, Fei Zhao, Zhe Xu, Yao Hu, and Shaohui Lin. Vision-r1: Incentivizing reasoning capability in multimodal large language models. arXiv preprint arXiv:2503.06749,
-
[2022]
Llava-o1: Let vision language models reason step-by-step
Guowei Xu, Peng Jin, Li Hao, Yibing Song, Lichao Sun, and Li Yuan. Llava-o1: Let vision language models reason step-by-step. arXiv preprint arXiv:2411.10440,
-
[2023]
Mm-eureka: Exploring the frontiers of multi- modal reasoning with rule-based reinforcement learning
Fanqing Meng, Lingxiao Du, Zongkai Liu, Zhixiang Zhou, Quanfeng Lu, Daocheng Fu, Tiancheng Han, Botian Shi, Wenhai Wang, Junjun He, et al. Mm-eureka: Exploring the frontiers of multi- modal reasoning with rule-based reinforcement learning. arXiv preprint arXiv:2503.07365,
-
[2024]
Unlocking the potential of difficulty prior in rl-based multimodal reasoning
Mingrui Chen, Haogeng Liu, Hao Liang, Huaibo Huang, Wentao Zhang, and Ran He. Unlocking the potential of difficulty prior in rl-based multimodal reasoning. arXiv preprint arXiv:2505.13261 , 2025c. Yifan Du, Zikang Liu, Yifan Li, Wayne Xin Zhao, Yuqi Huo, Bingning Wang, Weipeng...
-
[2025]
Sft or rl? an early investigation into training r1-like reasoning large vision-language models
Hardy Chen, Haoqin Tu, Fali Wang, Hui Liu, Xianfeng Tang, Xinya Du, Yuyin Zhou, and Cihang Xie. Sft or rl? an early investigation into training r1-like reasoning large vision-language models. arXiv preprint arXiv:2504.11468, 2025a. Hardy Chen, Haoqin Tu, Fali Wang, Hui Liu, Xi...
Reviewed August 6, 2026 · model on record in the stance chip above.
Discussion (0). Sign in to comment.