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arxiv: 2605.11416 · v2 · pith:XK5AKJGWnew · submitted 2026-05-12 · 💻 cs.CL

Freeze Deep, Train Shallow: Interpretable Layer Allocation for Continued Pre-Training

Pith reviewed 2026-05-25 06:38 UTC · model grok-4.3

classification 💻 cs.CL
keywords continued pre-traininglayer-wise updatesLLM interpretabilityLayerTracerfreeze-train strategiesdeep layer stabilityC-EvalCMMLU
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The pith

Deep layers execute tasks stably in LLMs, so freezing them while training shallow layers improves continued pre-training.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper introduces LayerTracer to diagnose which layers handle task execution and how stable they remain under updates. Analysis finds deep layers both critical for performance and resistant to disruptive changes. Experiments then compare freeze-train allocations during continued pre-training and show that training only shallow layers while freezing deep ones beats full-parameter updates and the reverse strategy on C-Eval and CMMLU. The approach supplies an interpretable, lower-cost method for adapting large models without erasing core capabilities.

Core claim

LayerTracer reveals that deep layers act as critical regions for task execution and maintain high stability against disruptive updates. Controlled continued pre-training trials demonstrate that training shallow layers while freezing deep layers consistently outperforms full-parameter fine-tuning and the opposite allocation on both C-Eval and CMMLU benchmarks. A hybrid model case study validates that placing high-quality pre-trained modules in deep layers preserves the model's inherent knowledge.

What carries the argument

LayerTracer, an architecture-agnostic diagnostic framework that locates task execution positions and quantifies layer sensitivity by tracking representation evolution and stability.

If this is right

  • Training shallow layers while freezing deep layers yields higher scores than full-parameter fine-tuning on knowledge benchmarks.
  • The opposite allocation of freezing shallow layers and training deep layers underperforms both full fine-tuning and the shallow-train strategy.
  • Placing high-quality pre-trained modules in deep layers preserves inherent model knowledge during hybrid construction.
  • Selective layer updates guided by stability analysis reduce compute cost while retaining task performance.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The observed stability gradient may reflect a broader division where shallow layers adapt features and deep layers store stable abstractions.
  • Teams with limited resources could apply the same diagnostic to decide layer allocation without exhaustive search.
  • The pattern could guide decisions when merging or distilling models by protecting deep-layer knowledge.
  • If the pattern holds across languages, the same allocation rule might apply to English-centric continued pre-training.

Load-bearing premise

The layer sensitivity and stability patterns identified on the analyzed models and tasks are assumed to generalize to other models, scales, and tasks.

What would settle it

A controlled continued pre-training run on a different LLM scale or non-Chinese task where the freeze-shallow train-deep allocation matches or exceeds the performance of the freeze-deep train-shallow strategy.

Figures

Figures reproduced from arXiv: 2605.11416 by Bo Jiang, Jiang-Feng Yang, Qing-Wei Cong, Qin-Yuan Liu, Qiu-Yang Zhao, Yu-Hang Wu.

Figure 1
Figure 1. Figure 1: Comparison of three layer-wise pre-training [PITH_FULL_IMAGE:figures/full_fig_p001_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: The architectures of Qwen3 model and Qwen3.5 model. Qwen3 adopts a single architecture, while Qwen3.5 is a hybrid architecture with a 3:1 ratio of Full Attention to Linear Attention. tion: the lack of interpretable guidance for layer￾wise freeze–train allocation during continued pre￾training. Thus, small and medium-sized teams are forced to rely on empirical heuristics when deter￾mining which layers to fre… view at source ↗
Figure 3
Figure 3. Figure 3: Overview of the LayerTracer framework. (a) Baseline projection: hidden states at each layer are projected [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Distribution details in the AntSynNET dataset, where 500 samples are evenly divided into 10 groups of 50 [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗
Figure 6
Figure 6. Figure 6: Illustration of the proposed zone-strategy [PITH_FULL_IMAGE:figures/full_fig_p006_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: The architectures of Nemotron-Qwen model [PITH_FULL_IMAGE:figures/full_fig_p008_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: Training loss curves across three layer-wise allocation strategies on the CCI3.0-HQ corpus. (a) Train [PITH_FULL_IMAGE:figures/full_fig_p013_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: Robustness analysis under Top-50 probability support on the AntSynNET dataset, where 500 samples [PITH_FULL_IMAGE:figures/full_fig_p014_9.png] view at source ↗
Figure 9
Figure 9. Figure 9: Robustness analysis under Top-50 probability support on the AntSynNET dataset, where 500 samples [PITH_FULL_IMAGE:figures/full_fig_p015_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: Layer-wise TP and LS profiles of Qwen3- 710M-Base. The light green region denotes the shallow layer used in the midpoint split, where LS is relatively stronger and layers are more sensitive to perturbation. The light red region denotes the deep layers, where TP becomes more prominent and task evidence is mainly consolidated. The dashed line marks the 50% split used in the main experiments. The inset repor… view at source ↗
read the original abstract

Selective layer-wise updates are essential for low-cost continued pre-training of Large Language Models (LLMs), yet determining which layers to freeze or train remains an empirical black-box problem due to the lack of interpretable guidance. To address this issue, we propose LayerTracer, an architecture-agnostic diagnostic framework that reveals the evolution patterns of layer-wise representations and stability by locating task execution positions and quantifying layer sensitivity. Analysis results reveal that deep layers act as critical regions for task execution and maintain high stability against disruptive updates. Guided by this finding, we conduct three controlled continued pre-training trials to compare diverse freeze-train strategies, demonstrating that training shallow layers while freezing deep layers consistently outperforms full-parameter fine-tuning and the opposite allocation on both C-Eval and CMMLU benchmarks. We further present a hybrid model case study, which validates that placing high-quality pre-trained modules in deep layers effectively preserves inherent knowledge of the model. This work delivers a low-cost and interpretable solution for resource-constrained teams, offering actionable guidance for layer-wise parameter allocation in continued pre-training and hybrid model construction.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 1 minor

Summary. The paper introduces LayerTracer, an architecture-agnostic diagnostic framework that analyzes the evolution of layer-wise representations and stability in LLMs to locate task-execution positions and quantify sensitivity. Analysis of the studied models reveals that deep layers serve as critical, high-stability regions for task execution. Guided by this, the authors run three controlled continued pre-training trials comparing freeze-train allocations and report that training shallow layers while freezing deep layers outperforms both full-parameter fine-tuning and the reverse allocation on C-Eval and CMMLU; a hybrid-model case study is also presented to show preservation of pre-trained knowledge when high-quality modules are placed in deep layers.

Significance. If the deep-layer stability pattern generalizes, the work supplies an interpretable, low-cost heuristic for layer allocation during continued pre-training that could reduce compute for resource-constrained teams while preserving model capability. The empirical comparison on two standard Chinese benchmarks and the hybrid-model validation constitute concrete, actionable guidance; explicit credit is due for the controlled-trial design that directly contrasts allocation strategies rather than relying on post-hoc analysis alone.

major comments (2)
  1. [Abstract] Abstract and the description of the three controlled continued pre-training trials: the claim of consistent outperformance on C-Eval and CMMLU is presented without any report of the number of random seeds, run-to-run variance, statistical significance tests, model sizes, or architectures used. These omissions are load-bearing because the central allocation recommendation rests on the reliability and generality of the observed superiority.
  2. [Abstract / Experiments] The transition from LayerTracer analysis to the recommended shallow-train/deep-freeze strategy assumes that the identified stability ranking of deep layers will hold at other scales and on other tasks; no cross-scale or cross-benchmark validation of this assumption is described, leaving the justification for the allocation strategy dependent on the specific regime examined in the LayerTracer runs.
minor comments (1)
  1. [Abstract] The phrase 'architecture-agnostic' in the abstract would benefit from an explicit statement of the model families and layer counts to which LayerTracer was applied.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive feedback and for recognizing the potential utility of the LayerTracer framework and controlled allocation trials. We respond to each major comment below.

read point-by-point responses
  1. Referee: [Abstract] Abstract and the description of the three controlled continued pre-training trials: the claim of consistent outperformance on C-Eval and CMMLU is presented without any report of the number of random seeds, run-to-run variance, statistical significance tests, model sizes, or architectures used. These omissions are load-bearing because the central allocation recommendation rests on the reliability and generality of the observed superiority.

    Authors: We agree that the abstract omits these details and that they are important for assessing the reliability of the central claims. Model sizes and architectures are described in the experimental setup of the full manuscript, but the number of random seeds, run-to-run variance, and statistical significance tests are not reported. In the revised manuscript we will add these elements to both the abstract and the experimental sections, including means and standard deviations across multiple seeds together with the results of significance testing. revision: yes

  2. Referee: [Abstract / Experiments] The transition from LayerTracer analysis to the recommended shallow-train/deep-freeze strategy assumes that the identified stability ranking of deep layers will hold at other scales and on other tasks; no cross-scale or cross-benchmark validation of this assumption is described, leaving the justification for the allocation strategy dependent on the specific regime examined in the LayerTracer runs.

    Authors: The LayerTracer analysis and the three controlled trials were performed on the specific models and benchmarks reported in the paper; the allocation recommendation follows directly from the stability patterns and performance results observed in that regime. We acknowledge that the manuscript contains no cross-scale or additional cross-benchmark experiments that would support broader generalization. In the revision we will explicitly delimit the scope of the findings to the examined models and tasks and will add a statement that validation at other scales remains future work. revision: partial

Circularity Check

0 steps flagged

No circularity; empirical analysis and trials are self-contained

full rationale

The paper's core contribution is an empirical diagnostic (LayerTracer) that measures layer representations and stability on specific models/tasks, followed by three controlled continued pre-training experiments comparing freeze/train allocations on C-Eval and CMMLU. No equations, fitted parameters, or self-citations are invoked to derive the allocation recommendation; the superiority claim rests on direct experimental comparison rather than any reduction to inputs by construction. Generalization beyond the tested regime is an external-validity question, not a circularity issue.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Abstract-only review; no explicit free parameters, axioms, or invented entities are stated. The central claim rests on the empirical generalization of LayerTracer patterns rather than on any derived constants or new postulated objects.

pith-pipeline@v0.9.0 · 5735 in / 1145 out tokens · 25576 ms · 2026-05-25T06:38:39.646333+00:00 · methodology

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