REVIEW 5 major objections 5 minor 3 cited by
Enhancing Reasoning through Process Supervision with Monte Carlo Tree Search
T0 review · 5 major / 5 minor · reviewed 2026-08-10 · deepseek-v4-flash
Pith's one-line read Tree-search step scores lift LLM accuracy on MATH and GSM8K
desk verdict Plausible dense-scored MCTS self-training method, but test-set-based early stopping and one flat transfer cell weaken the reported gains. 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 object doing the work is the 'relative correctness' score in Eq. (1): for each candidate next step, the model's MCTS rolls out many completions, propagates a binary success reward back through the tree, and then scores the step as $\alpha$ times its visit count times the difference between its average reward and the average reward of its sibling steps. This turns whole-path success or failure into dense per-step supervision without human labels or a separate reward model. Training then minimizes a weighted negative log-likelihood of the step token sequence, weighted by that score, plus a KL-divergence penalty toward the previous iteration's model, which the paper frames as offline reinforcement learning with a distribution-shift regularizer.
What would settle it
Train two matched models: one with the Eq. (1) relative-correctness weights and one with the same steps weighted only by whether the sampled path ended correct; if the score-weighted model does not beat the whole-path-weighted control, the sibling-relative term is not doing the claimed work. A second check is to take a held-out set of traced solutions and ask whether the steps ranked highest by Eq. (1) match human or process-reward-model judgments; a large mismatch would indicate the supervision signal is miscalibrated.
Extended reading notes
Core claim
The paper's central claim is that a continuous step-level score generated by MCTS—the visit-weighted average success of a candidate next step minus the sibling-average success—can serve as process supervision for training an LLM, and that training with this score improves mathematical reasoning more than outcome-level or binary preference supervision. On Llama-3.1-8B-Instruct, MATH accuracy rises from 47.07 with zero-shot CoT to 51.92 after four iterations, and GSM8K rises from 80.77 to 85.80. On DeepSeek-Math-7B-Instruct, MATH rises from 41.20 to 44.57 and GSM8K from 78.79 to 82.02. Models trained on one dataset also outperform the zero-shot baseline on the other, suggesting the learned ability transfers rather than memorized answers. The paper also reports that binary Step-level DPO yields only marginal MATH improvements, which it interprets as evidence that continuous relative-correctness labels carry more information than chosen/rejected pairs.
Load-bearing premise
The results rest on the assumption that the Eq. (1) score measures a single step's quality, even though the score is built from whole-path success or failure, so a step can look good or bad mainly because of what later steps happened to do.
Editorial extensions
If this is right
- MATH accuracy for Llama-3.1-8B-Instruct improves from 47.07 to 51.92 and GSM8K from 80.77 to 85.80 over four self-training iterations.
- DeepSeek-Math-7B-Instruct improves from 41.20 to 44.57 on MATH and from 78.79 to 82.02 on GSM8K.
- Continuous relative-correctness labels outperform binary best/worst step preferences, so denser process feedback is worth generating.
- Models trained on one dataset beat the zero-shot baseline on the other, indicating transferable reasoning gains.
- Process supervision via MCTS needs no human step labels, no reward model, and no stronger teacher, so it applies to the strongest available LLM.
Reading between the lines
- The paper never checks its step scores against human labels or an external process reward model; if the score mostly reflects luck from later steps, a matched control that weights steps by whole-path success would reveal whether the sibling-relative term is the real driver.
- The quick saturation and later degradation suggest a self-generated-data distribution collapse, and a temperature or confidence filter on which problems enter the training set might extend the improvement regime.
- Because MATH and GSM8K are both math word problems, the reported transfer is a weak test of general reasoning; testing on planning, code, or non-math logic benchmarks would more sharply separate genuine reasoning gains from math-specific pattern learning.
- Since training is done with LoRA, the paper's own limitation is that full fine-tuning might show larger or more stable gains than reported.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes an iterative self-training method that uses MCTS to assign per-step 'relative correctness' scores to reasoning steps generated by the target LLM, then fine-tunes the model with a weighted negative log-likelihood plus a KL penalty (Eq. (2)). Experiments on Llama-3.1-8B-Instruct and deepseek-math-7b-instruct report consistent gains on MATH and GSM8K over Zero-shot-CoT, RFT, and a Step-level DPO baseline, plus transfer between the two datasets. The core claim is that automatically generated process supervision from MCTS improves mathematical reasoning and transfers across benchmarks.
Significance. If the reported gains survive a properly controlled evaluation, the method is a useful addition to the process-supervision toolbox: it removes the need for human step labels or a trained PRM, uses graded step scores rather than binary preferences, and is a relatively simple generate-then-fine-tune loop. The paper also honestly reports convergence and degradation with more data, a useful negative result. However, the current manuscript does not yet establish the central empirical claim because the iteration-selection protocol is test-set-based and key hyperparameters are omitted, and the transfer claim is not supported in one of the four directions.
major comments (5)
- [Main Results/Setup, Tables 1–2] The evaluation protocol is the most load-bearing threat to the headline numbers. The paper states, 'We report the results of iterative training until the accuracies do not increase anymore,' and no held-out validation split is described in Setup. Because the same test sets are used both to decide when to stop and to produce the reported accuracy, the reported gains (e.g., Llama-3.1-8B MATH 47.07 to 51.92, GSM8K 80.77 to 85.80) may be the maximum over the iteration curve rather than the accuracy of a predetermined training schedule. Please stop on a validation split, report all iterations with standard errors, and pre-specify the comparison.
- [Eq. (1) and Limitations] The step score in Eq. (1) is an aggregated credit-assignment heuristic derived from binary whole-path success; the paper does not validate it against human step labels (e.g., PRM800K) or an external process reward model. A step that is itself wrong can receive a high score if later sampled continuations happen to lead to a correct answer, so the supervision signal may be noisy. The limitations section's observation that training for more iterations or using more problems 'actually degrades' performance is consistent with unstable supervision. Please add an external validation of the step scores, an ablation that replaces them with oracle step labels, and an analysis of the degradation.
- [Related Work and Table 1] The closest MCTS-based self-training baselines are cited but not compared. MCTS-DPO and ReST-MCTS* are discussed in Related Work, but Table 1 only includes Zero-shot-CoT, RFT, and a Step-level DPO baseline that forms best/worst-step pairs. To support the claim that graded relative-correctness scores are superior to pairwise preference and to other MCTS self-training methods, the paper needs direct comparisons under matched compute and data conditions.
- [Table 2 and Abstract] The transfer claim is not supported in all four directions. For deepseek-math-7b-instruct, GSM8K-to-MATH after Iteration 1 is 41.15 ± 0.31, which is no better than Zero-shot-CoT's 41.20 ± 0.27, and no significance test is reported for any transfer cell. The abstract's 'consistently outperform' therefore overstates the results. Please report significance tests and either weaken the claim or analyze why this direction fails.
- [Setup] Several hyperparameters that control the data-generation and training loop are not reported: α in Eq. (1), the number of MCTS simulations, the sampling temperature, tree width, rollout length, LoRA rank, and learning rate. Without these values and a sensitivity analysis, the results are not reproducible and it is unclear whether the gains depend on a narrow set of settings.
minor comments (5)
- [Experiments, Setup] The 'Rejective Sampling Fine-Tuning' baseline should be named 'Rejection Sampling Fine-Tuning'.
- [Proposed Methods] The sentence 'we are not accessible to LLMs stronger than the target LLM' should read 'we do not have access to LLMs stronger than the target LLM'.
- [Eq. (2)] In the loss in Eq. (2), the score r from Eq. (1) can be negative when a step's average reward is below the sibling average; please state explicitly whether negative weights are kept and how this interacts with the KL penalty.
- [Tables 1–2] Table 1 and Table 2 would benefit from consistent column spacing and from a caption stating the number of seeds used for the mean±standard error estimates.
- [Abstract] The sentence 'This generate-then-train process is repeated iteratively until convergence' in the abstract is not precise because the paper stops when test accuracy ceases to increase; please rephrase to match the actual stopping rule.
Circularity Check
No significant circularity: the step-score formula is a proposed MCTS heuristic, and the reported gains are empirical benchmark results rather than outputs derived from the method's own inputs by construction.
full rationale
The paper's claimed chain is an empirical training method: MCTS rollouts from the current LLM produce step scores via Eq. (1), and Eq. (2) trains the same LLM with weighted log-likelihood plus a KL penalty. Neither equation defines a target quantity in terms of the quantity it is used to predict; the score is a hand-designed Monte Carlo estimate of relative step quality, not a fitted parameter relabeled as a prediction. The central claim (improved MATH/GSM8K accuracy and transfer to the other dataset) is evaluated on held-out test sets and compared with Zero-shot-CoT, RFT, and Step-level DPO, so the derivation is not equivalent to its inputs by construction. The self-training nature of the loop (the model labels its own rollouts) is a bootstrapping strategy rather than a definitional circularity, and no load-bearing self-citations are present. The limitations section's observation that more iterations or more problems degrade performance is a substantive robustness concern, and the stopping criterion 'until the accuracies do not increase anymore' is described without an explicit validation split; these are correctness and evaluation risks, not circularity.
Assumptions & free parameters
free parameters (3)
- alpha (α) in score formula =
not reported
- MCTS hyperparameters (simulation count, temperature, tree width) =
not reported
- Number of training iterations selected =
varies: Llama MATH 4, deepseek MATH 3, GSM8K 2
assumptions (4)
- domain assumption Ground-truth final answers are available and reliably detectable.
- domain assumption The binary success of a simulated continuation is a valid signal for the quality of every preceding step.
- ad hoc to paper Steps separated by newlines are atomic, commensurable units of reasoning.
- domain assumption The KL penalty against the previous iteration prevents distribution shift and training collapse.
Cite this review
Pith. "Pith review of Enhancing Reasoning through Process Supervision with Monte Carlo Tree Search." pith.science (2026). https://pith.science/paper/3NNJMMT7
@misc{pith2026250101478,
author = {Pith},
title = {Pith review of: Enhancing Reasoning through Process Supervision with Monte Carlo Tree Search},
year = {2026},
howpublished = {\url{https://pith.science/paper/3NNJMMT7}},
note = {Machine review of arXiv:2501.01478}
}
read the original abstract
Large language models (LLMs) have demonstrated their remarkable capacity across a variety of tasks. However, reasoning remains a challenge for LLMs. To improve LLMs' reasoning ability, process supervision has proven to be better than outcome supervision. In this work, we study using Monte Carlo Tree Search (MCTS) to generate process supervision data with LLMs themselves for training them. We sample reasoning steps with an LLM and assign each step a score that captures its "relative correctness," and the LLM is then trained by minimizing weighted log-likelihood of generating the reasoning steps. This generate-then-train process is repeated iteratively until convergence.Our experimental results demonstrate that the proposed methods considerably improve the performance of LLMs on two mathematical reasoning datasets. Furthermore, models trained on one dataset also exhibit improved performance on the other, showing the transferability of the enhanced reasoning ability.
Figures
Forward citations
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Reference graph
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Reviewed August 10, 2026 · model on record in the stance chip above.
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