REVIEW 2 major objections 6 minor 20 references
Instability of LLM Pre-Pretraining: It Doesn't Always Help. An Investigation on Multiple Languages
T0 review · 2 major / 6 minor · reviewed 2026-08-14 · deepseek-v4-flash
Pith's one-line read Pre-pretraining on artificial languages does not reliably improve token efficiency across languages: benefits hinge on tokenizer, model size, and random seed, with stable gains only for small Llama-tokenizer models pretrained on 128-Dyck.
desk verdict A useful cautionary replication study on pre-pretraining, but the token-efficiency metric's asymmetric negative branch should temper how you read the 'loss' numbers. 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 token efficiency gain metric: the percentage of training tokens saved when the pretrained model reaches the baseline model's final validation loss, with the pretraining tokens counted and negative values assigned when it never catches up. The transfer recipe consists of a fixed 16,000-sequence, 2,048-token pretraining corpus of a formal bracket language (k-Dyck or k-Shuffle Dyck, with bracket types encoded as integers), followed by 10,000 steps on natural-language text. The argument is carried by comparing the loss curves of baseline and pretrained models and seeing whether the pretrained model's curve crosses the baseline's final loss before the training budget ends; the instability conclusion rests on how often that crossing point moves from before the end to after the end solely because the seed changed.
What would settle it
Take an unstable configuration, such as the 154M model with the Gemma tokenizer pretrained on 64-Dyck and then trained on Czech, and extend training to 20,000 or 40,000 steps across at least five seeds; compare how many tokens each model needs to reach the baseline's loss at the later checkpoint. If the pretrained model consistently reaches that loss sooner, the paper's instability conclusion would be falsified for that regime; if the ordering still flips across seeds, the conclusion stands.
Extended reading notes
Core claim
The paper's central discovery is that pre-pretraining on artificial languages is not a generally beneficial transfer technique; its effect on token efficiency is controlled by configuration details and by random seed. The 33% token-efficiency gain reported in prior work is reproduced only in a narrow corner of the experiment space: small models with the Llama tokenizer and 128-Dyck pretraining, where gains are stable across runs for most languages, and English also benefits with most artificial languages at small scale. Outside that corner, the same setup can yield a gain for one seed and a loss for another, and larger models frequently lose tokens outright. The authors also find no stable, statistically significant relationship between gains and twenty-plus linguistic properties including crossing dependencies, contrary to the structural-match hypothesis.
Load-bearing premise
The paper assumes that a model's relative standing at step 10,000, roughly 655M tokens, predicts its relative standing with more training, so the token-efficiency gain computed at that endpoint is the true gain; if longer training would reorder the baseline and pretrained models, the reported sign of the gain could reverse.
Editorial extensions
If this is right
- For practitioners, pre-pretraining should be treated as a hyperparameter rather than a guaranteed saving; a configuration that helps in one seed can hurt in another, so budgets should include multiple seeds before adopting the method.
- The stable recipe (small model, Llama tokenizer, 128-Dyck) is a concrete candidate for low-resource-language settings and for further scaling studies, but its limits outside this configuration remain unknown.
- Because larger models in this study show repeated losses, the benefit of pre-pretraining appears to shrink as the model's token-to-parameter ratio decreases, which matters for compute-optimal training planning.
- The near-absence of correlations with linguistic structure suggests that the mechanism of transfer is not the abstract syntax of the artificial language matching the natural language, so future work should look elsewhere, such as at tokenization interactions or optimization dynamics.
Reading between the lines
- If the seed variance is as large as reported, the common practice of reporting a single run overstates confidence in transfer results; one implication not drawn by the paper is that paired seed analysis, using the same seed for baseline and pretrained models, would isolate the pretraining effect from initialization noise.
- The tokenizer dependence hints that the gain may come from how integer symbols are segmented, since the Llama tokenizer keeps multidigit numbers intact while the Gemma tokenizer splits them into digits, rather than from bracket syntax itself; a direct test would vary the symbol vocabulary while keeping the formal language fixed.
- The paper's own token-to-parameter ratios are far below Chinchilla-optimal, and if longer training reverses the ordering of baseline and pretrained models, the instability could be an artifact of undertrained models, a possibility the authors flag but do not settle.
- A cheap extension would be to test non-default pretraining budgets across languages: the paper's single Czech experiment suggests 1,000 pretraining steps may be better than 500 or 2,000, so the pretraining amount itself could be a tunable knob that escapes the unstable window.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper revisits Hu et al. (2025)'s claim that pre-pretraining on Dyck languages improves token efficiency for LLMs. It replicates the setup across six European languages, two tokenizers (Llama and Gemma), three model sizes (154M, 308M, 481M effective parameters), four formal languages (64/128-Dyck and 64/128-Shuffle Dyck), and three seeds per configuration, reporting per-run token-efficiency gains. The central finding is that gains are highly sensitive to the exact setup and seed; consistent positive gains appear only for the smallest Llama-tokenized model pretrained on 128-Dyck for most languages. The paper also correlates gains with linguistic features and finds little stable relationship, leading to the methodological recommendation that multiple training runs be performed before drawing conclusions.
Significance. If the conclusions hold, the paper provides a valuable cautionary counterpoint to the enthusiasm around pre-pretraining: the reported 33% token-efficiency gain does not generalize broadly, and the sign of the effect can flip with the random seed. The paper's strengths include explicit per-run results (Table 6), a multi-seed protocol, and a thorough ablation over tokenizer and model size. The paper also makes a fair methodological point about the danger of single-run evaluations. However, the main claim is currently weakened by a potentially biased token-efficiency metric: the negative branch of Algorithm 1 can manufacture large negative gains that are not a direct measure of the pretrained model's disadvantage. Because the instability conclusion relies on these values, the paper requires substantial revision before the central claim can be accepted.
major comments (2)
- [Appendix A.3, Algorithm 1; Table 6; Table 3] The negative branch of Algorithm 1 is asymmetric and can manufacture large negative 'losses'. When the pretrained model never reaches the baseline's final validation loss, the algorithm recursively computes the gain of the baseline relative to the pretrained model's final loss and negates it. This measures how quickly the baseline reaches the pretrained model's higher final loss, not how many tokens the pretrained model would need to reach the baseline's final loss. A small final-loss gap can therefore yield a large negative gain even if the pretrained model would catch up with only a short extension of training. Many of the negative values in Table 6 (e.g., -32.98%) and the '−' classifications in Table 3 are computed by this branch. Since the paper's central claim that pre-pretraining 'doesn't always help' depends on these values, the metric must be corrected (e.g., by interpolating the number of tokens at which the pretrained curve crosses the baseline's final loss, or by comparing area under the loss curves), or the authors must demonstrate that the negative branch does not affect the sign of the conclusions.
- [Section 4.1 and Table 2] The models are trained for only 655M tokens with token-to-parameter ratios as low as 0.74, far below the Chinchilla-optimal range of roughly 20–25. The assertion 'we see little to no chance that the setups (baseline vs. pretrained) would change order with more training' is not supported: the loss curves are shown to be stable only over the last 3,000 steps, and a small final-loss gap could plausibly reverse under continued training, especially given the documented seed variance. Since the token-efficiency gain is computed at the endpoint, an undetected reordering would change the sign of the gain and could undermine the instability conclusion. Please provide direct evidence (e.g., training a subset of setups for substantially longer, or an extrapolation analysis) or soften the conclusion.
minor comments (6)
- [Figure 1] The average token efficiency gain is shown without any measure of spread. Because the paper's main argument is that seed dependence is large, the figure should display per-run values or error bars; otherwise the reader cannot see the instability that the text emphasizes.
- [Section 4.1, Table 2] The comparison with Hu et al. (2025)'s token-to-parameter ratios is not exact: our ratios range from 0.74 to 2.6, while Hu et al. report 4.0 (160M) and 1.63 (1B). Please clarify that the models are generally more undertrained than the reference and discuss the implications for the comparability of the results.
- [Appendix A.3, Algorithm 1] The condition 'ifi≥T argetLoss∀i∈P' is ambiguous; it should read 'if for all i, P[i] ≥ TargetLoss'.
- [Section 5 and Limitations] The linguistic correlation analysis is limited to 154M models and six languages; the significant correlations for 154M+GT should be presented as exploratory, not confirmatory, and the abstract/conclusion should avoid overstating the relation. The Limitations section already acknowledges some of this, but the wording in Section 5 could be more cautious.
- [Table 6] A brief note explaining that the gain is defined as a percentage of token savings relative to the baseline (and that a negative value under the current metric indicates the baseline reaches the pretrained model's final loss earlier) would help readers interpret the entries.
- [Appendix A.3, Algorithm 1] There is a typo in 'P reP retrainedT okens' (the repeated 'P re' and missing space); the pseudocode would also benefit from a comment explaining why the negative branch is chosen and what it measures.
Circularity Check
No circularity found: the paper is an empirical multi-run replication whose gains are measured against an external baseline; Algorithm 1's negative branch is a metric-validity concern, not a circular step.
full rationale
The paper is an empirical replication study, not a derivation chain. Its central quantity, token efficiency gain, is computed by Algorithm 1 against the baseline model's final validation loss, which is an external reference, and no parameter is fitted to the instability conclusion. The reported variance across seeds, model sizes, tokenizers, and pretraining languages is a direct empirical measurement, and the few self-citations are not load-bearing: Kocmi and Bojar (2018) is cited only as background motivation for transfer learning, and Bojar (2015) only supplies the morphological-richness metric. The dependency on Hu et al. (2025) for Dyck sequence generation and for the general gain protocol is an external benchmark, and the paper re-implements the setup and reports its own three-run results, so there is no imported uniqueness theorem or forced ansatz. The skeptic concern about Algorithm 1's negative branch is legitimate as a measurement-validity issue: the branch recursively computes the baseline's gain relative to the pretrained model's final loss and negates it, so a negative value may not represent the number of tokens the pretrained model would need to reach the baseline's final loss. However, that is a critique of the metric's semantics, not circularity: the paper does not derive a prediction from that metric by construction, and the instability conclusion is not equivalent to the metric's definition. No step reduces an output to an input by construction.
Assumptions & free parameters
assumptions (3)
- domain assumption Validation loss on mC4 is an adequate metric for language modeling performance and token efficiency.
- domain assumption Training and evaluation loss curves are stable in the last 3,000 steps and will not reorder with more training.
- domain assumption The generated k-Dyck and Shuffle-Dyck sequences correctly implement the intended formal languages.
Cite this review
Pith. "Pith review of Instability of LLM Pre-Pretraining: It Doesn't Always Help. An Investigation on Multiple Languages." pith.science (2026). https://pith.science/paper/4HI7PQU2
@misc{pith2026260808800,
author = {Pith},
title = {Pith review of: Instability of LLM Pre-Pretraining: It Doesn't Always Help. An Investigation on Multiple Languages},
year = {2026},
howpublished = {\url{https://pith.science/paper/4HI7PQU2}},
note = {Machine review of arXiv:2608.08800}
}
read the original abstract
Pretraining LLMs on artificial languages ("pre-pretraining") is a technique that could reportedly increase token efficiency by 33%, i.e., save up to 33% of training tokens needed to reach a certain performance. We validate this prior result for English on a larger set of natural languages across four language families, using two different tokenizers and varying model sizes. We also relate the observed gains (or losses) in token efficiency to quantified linguistic properties of the languages, such as sentence length, morphological richness, and features of dependency syntactic trees (tree depth, number of children, number of crossing dependencies). Our empirical results indicate that the reported gains depend heavily on the experiment setup and the choice of random seed, although we can confirm the trend of stable gains with 128-Dyck pretraining of small models with the Llama tokenizer for most of the examined languages. On a general note, we argue that multiple training runs should be carried out at least for a subset of experiments to avoid the community adopting unstable approaches.
Figures
Figures from the paper (17 more)
Reference graph
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Reviewed August 14, 2026 · model on record in the stance chip above.
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