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arxiv: 2601.10348 · v2 · pith:AECZLY3Knew · submitted 2026-01-15 · 💻 cs.CL · cs.AI· cs.LG

Training-Trajectory-Aware Token Selection

Pith reviewed 2026-05-22 11:36 UTC · model grok-4.3

classification 💻 cs.CL cs.AIcs.LG
keywords continual distillationtoken selectionreasoning modelstraining trajectoryimitation anchor tokensautoregressive modelsdiffusion LLMsmodel compression
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The pith

Training-trajectory-aware token selection clears the optimization path for suppressed tokens and fixes continual distillation of reasoning ability.

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

The paper shows that continual distillation of strong reasoning models often fails with a sharp performance drop even while loss keeps falling. This drop stems from a token-level split where Imitation-Anchor Tokens quickly gain high confidence and anchor the model, while other tokens stay suppressed and cannot coexist with them. The authors introduce Training-Trajectory-Aware Token Selection (T3S) that reconstructs the per-token training objective to remove the anchors and let the suppressed tokens learn. With this change, small models reach or exceed much larger ones using only hundreds of examples in both standard and diffusion-based language model settings.

Core claim

Even as loss decreases, performance drops at a bottleneck because token confidence splits into steadily rising Imitation-Anchor Tokens that lock in the optimization and yet-to-learn tokens whose confidence remains suppressed. These two token types cannot coexist, which is the root cause of failure in continual distillation. T3S identifies tokens by their training trajectories and rebuilds the objective to clear the path for the suppressed tokens.

What carries the argument

Training-Trajectory-Aware Token Selection (T3S) that distinguishes Imitation-Anchor Tokens from yet-to-learn tokens along the training trajectory and adjusts the loss to favor the latter.

Load-bearing premise

The non-coexistence of imitation-anchor tokens and suppressed yet-to-learn tokens is the root cause of the observed distillation failure.

What would settle it

Run the same distillation with and without T3S and check whether the sharp performance drop still appears exactly when the token-confidence bifurcation occurs.

Figures

Figures reproduced from arXiv: 2601.10348 by Guoshan Lu, Hao Chen, Jiaqi Hu, Junbo Zhao, Junlin Zhou, Wentao Ye, Yihong Zhuang, Zenan Huang, Zeyu Qin, Zhanming Shen.

Figure 1
Figure 1. Figure 1: Imitation Shock under DeepSeek-R1 distillation on BOBA-200. Although training loss decreases monotonically (a), training-set answer accuracy and multiple benchmarks (AIME24/25, MMLU-Pro) drop sharply to a shared minimum stage and then recover, revealing an Imitation Shock. 0 10 20 30 40 50 Checkpoint 10 20 30 40 50 60 70 Score AIME25 Performance (a) Different teacher (QWQ) 0 10 20 30 40 50 Checkpoint 30 35… view at source ↗
Figure 2
Figure 2. Figure 2: Imitation Shock is universal across settings. We observe the same “crash then recover” trajectory across (a) different teachers, (b) different datasets, (c) larger-scale datasets, (d) different student backbones, and (e) different training domains. See Appendix B for detailed setups and metrics. code information that is neither necessary nor beneficial. Crucially, it motivates us to attribute the distillat… view at source ↗
Figure 3
Figure 3. Figure 3: Word clouds at the Imitation Bottleneck (identified by training-set accuracy). Token sizes are proportional to the magni￾tude of confidence change relative to the base model. Dataset Tokens with Confidence Drop (%) BOBA-200 68.51 S1K-200 53.03 [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figure 5
Figure 5. Figure 5: Checkpoint-wise one-step intervention on Imitation￾Anchor Tokens. Panel (a) reports ∆Lother after one gradient step that optimizes only anchor tokens at each checkpoint. Panel (b) shows the corresponding anchor-token loss Lanchor at that check￾point. points. As shown in [PITH_FULL_IMAGE:figures/full_fig_p005_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Workflow of our method. Let D = {(x, y)} be a dataset of inputs x (e.g., prompts) and target sequences y = (y1, . . . , yT ) of length T. Autoregressive (AR) models. An AR model factorizes pθ(y | x) = Y T t=1 pθ(yt | y<t, x). (1) 5 [PITH_FULL_IMAGE:figures/full_fig_p005_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Training dynamics comparison between T3S and SFT on BOBA-200. gains rather than diluting them. For completeness, we also report the base performance of the teacher models them￾selves. Results and analysis. Several observations emerge from [PITH_FULL_IMAGE:figures/full_fig_p008_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: Imitation Shock under DeepSeek-R1 distillation on S1K-200. The same sharp performance drop and recovery pattern observed on BOBA-200 also appears on S1K-200. (5) Different domain. Beyond math reasoning, we sample a code subset of comparable scale from RL-Code-Math-v5 (typhoon ai, 2025) and distill Qwen3-8B from QWQ. Using LiveCodeBench as the metric, we again observe the same crash-and-recovery dynamics ( … view at source ↗
Figure 9
Figure 9. Figure 9: Imitation Shock persists under an in-distribution teacher (QWQ) on BOBA-200. D. OOD Reasoning Gains and Forgetting Mitigation To assess whether T3S improves general reasoning beyond AIME-style evaluation and whether it mitigates catastrophic forgetting, we evaluate distilled students on two task families: Reasoning-related tasks. We report performance on PhyBench (Meng et al., 2024), GPQA-Diamond (GPQA-D) … view at source ↗
Figure 10
Figure 10. Figure 10: Token-gradient sketch visualization does not cleanly separate anchors. We project token-level LoRA gradient sketches into 2D (t-SNE/PCA-style embedding) and color tokens by sign(∆ct) (anchor vs. non-anchor). The two groups heavily overlap, indicating that local gradient geometry alone is insufficient for reliably identifying imitation anchors. baselines perform far worse than T3S ( [PITH_FULL_IMAGE:figur… view at source ↗
Figure 11
Figure 11. Figure 11: Base-model confidence is insufficient to separate anchors. We plot token groups using their base-model confidence and color by the sign of trajectory-based confidence change ∆ct (anchor if ∆ct > 0, otherwise non-anchor). The two groups overlap substantially, showing that the anchor/non-anchor partition cannot be recovered by initial confidence alone. equal-sized halves: Anchor-Easy, Anchor-Hard, Other-Eas… view at source ↗
Figure 12
Figure 12. Figure 12: Imitation Shock persists under initial-confidence masking. Even when masking the same token fraction as T3S, selecting tokens by base-model confidence (highest or lowest) still exhibits the characteristic crash-then-recover pattern in AIME25 across checkpoints. T . We report the percentage loss change: ∆S→T = 100 × ℓT (θS ) − ℓT (θ0) ℓT (θ0) , where θS is the checkpoint obtained by training only on S. Neg… view at source ↗
read the original abstract

Efficient distillation is a key pathway for converting expensive reasoning capability into deployable efficiency, yet in the frontier regime where the student already has strong reasoning ability, naive continual distillation often yields limited gains or even degradation. We observe a characteristic training phenomenon: even as loss decreases monotonically, all performance metrics can drop sharply at almost the same bottleneck, before gradually recovering. We further uncover a token-level mechanism: confidence bifurcates into steadily increasing Imitation-Anchor Tokens that quickly anchor optimization and other yet-to-learn tokens whose confidence is suppressed until after the bottleneck. And the characteristic that these two types of tokens cannot coexist is the root cause of the failure in continual distillation. To this end, we propose Training-Trajectory-Aware Token Selection (T3S) to reconstruct the training objective at the token level, clearing the optimization path for yet-to-learn tokens. T3S yields consistent gains in both AR and dLLM settings: with only hundreds of examples, Qwen3-8B surpasses DeepSeek-R1 on competitive reasoning benchmarks, Qwen3-32B approaches Qwen3-235B, and T3-trained LLaDA-2.0-Mini exceeds its AR baseline, achieving state-of-the-art performance among all of 16B-scale no-think models.

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 / 2 minor

Summary. The manuscript proposes Training-Trajectory-Aware Token Selection (T3S) to address failures in continual distillation of reasoning capabilities from large teacher models to smaller students. It reports an empirical observation that loss decreases monotonically while performance metrics drop sharply at a bottleneck before recovering, attributes this to a bifurcation where some tokens become Imitation-Anchor Tokens with steadily increasing confidence while others (yet-to-learn tokens) have suppressed confidence, and claims that the inability of these two token types to coexist is the root cause. T3S reconstructs the token-level training objective to clear the optimization path for yet-to-learn tokens. Experiments in both autoregressive and diffusion LLM settings report large gains, including Qwen3-8B surpassing DeepSeek-R1 on reasoning benchmarks with only hundreds of examples, Qwen3-32B approaching Qwen3-235B, and T3-trained LLaDA-2.0-Mini exceeding its AR baseline to achieve SOTA among 16B-scale no-think models.

Significance. If the causal mechanism is confirmed and the gains prove robust across settings, the work would be significant for efficient reasoning distillation, offering a trajectory-aware alternative to standard imitation that could reduce data and compute requirements for deploying strong reasoning models. The empirical identification of the bifurcation phenomenon during training is a useful observation that merits further study. The application to both AR and dLLM architectures is a strength, as is the focus on the frontier regime where the student already possesses strong reasoning ability.

major comments (2)
  1. [Abstract and the section describing the token-level mechanism] The central claim that 'the characteristic that these two types of tokens cannot coexist is the root cause of the failure in continual distillation' (abstract) is not supported by a direct causal test. The reported correlation between token bifurcation and the performance bottleneck does not isolate coexistence incompatibility from alternative explanations such as optimization dynamics, data ordering, or capacity limits. A controlled intervention (e.g., a modified loss or sampling procedure that forces coexistence while holding other factors fixed) is needed to establish that clearing this specific path, rather than generic token reweighting, produces the observed recovery and T3S gains.
  2. [Experimental results section] The performance claims (e.g., Qwen3-8B surpassing DeepSeek-R1 and T3-trained LLaDA-2.0-Mini achieving SOTA among 16B-scale no-think models) require fuller experimental details on baselines, number of runs, variance, and exact evaluation protocols to be load-bearing. Without these, it is difficult to assess whether the gains are attributable to T3S specifically addressing the bifurcation or to other factors in the token selection.
minor comments (2)
  1. [Method section] Clarify the precise definition and detection criteria for Imitation-Anchor Tokens versus yet-to-learn tokens, ideally with a formal equation or algorithm box.
  2. [Related work] Add a brief discussion of how T3S relates to prior token-level importance or curriculum learning methods in the related work section.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their thoughtful and constructive review of our manuscript. Their comments on establishing stronger causal evidence for the token bifurcation mechanism and on improving experimental reporting are well taken. We address each major comment below and indicate the revisions we will make.

read point-by-point responses
  1. Referee: [Abstract and the section describing the token-level mechanism] The central claim that 'the characteristic that these two types of tokens cannot coexist is the root cause of the failure in continual distillation' (abstract) is not supported by a direct causal test. The reported correlation between token bifurcation and the performance bottleneck does not isolate coexistence incompatibility from alternative explanations such as optimization dynamics, data ordering, or capacity limits. A controlled intervention (e.g., a modified loss or sampling procedure that forces coexistence while holding other factors fixed) is needed to establish that clearing this specific path, rather than generic token reweighting, produces the observed recovery and T3S gains.

    Authors: We appreciate the referee's emphasis on the need for a direct causal test. T3S was explicitly developed as the controlled intervention that reconstructs the token-level training objective to clear the optimization path for yet-to-learn tokens, thereby mitigating the suppression caused by Imitation-Anchor Tokens. This change is applied while holding the training data, model architecture, and optimization hyperparameters fixed, allowing us to attribute the observed recovery and downstream gains to addressing the specific incompatibility rather than generic reweighting. To further isolate this effect, we will add a targeted comparison in the revised manuscript between T3S and standard token reweighting baselines. revision: partial

  2. Referee: [Experimental results section] The performance claims (e.g., Qwen3-8B surpassing DeepSeek-R1 and T3-trained LLaDA-2.0-Mini achieving SOTA among 16B-scale no-think models) require fuller experimental details on baselines, number of runs, variance, and exact evaluation protocols to be load-bearing. Without these, it is difficult to assess whether the gains are attributable to T3S specifically addressing the bifurcation or to other factors in the token selection.

    Authors: We agree that fuller experimental details are essential for reproducibility and for confirming that gains stem from T3S. In the revised manuscript we will expand the experimental results section to include complete descriptions of all baselines, the number of independent runs, variance or standard deviation measures, and the precise evaluation protocols for each benchmark. revision: yes

Circularity Check

0 steps flagged

No significant circularity; derivation grounded in empirical observations

full rationale

The paper identifies a training bottleneck and token-level confidence bifurcation through direct observation of loss curves and per-token dynamics during continual distillation. The statement that non-coexistence of Imitation-Anchor Tokens and yet-to-learn tokens is the root cause is presented as an empirical finding from these trajectories rather than a definitional equivalence or fitted parameter. T3S is then introduced as a token-level objective modification motivated by this observation, with gains demonstrated on external benchmarks (Qwen3-8B vs DeepSeek-R1, LLaDA-2.0-Mini). No equation reduces a claimed result to its inputs by construction, no self-citation chain carries the uniqueness or derivation load, and the work remains self-contained against the reported empirical evaluations.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on the empirical observation of a training bottleneck and the assumption that the described token mechanism is causal; no free parameters or invented entities are mentioned in the abstract.

axioms (1)
  • domain assumption The characteristic that these two types of tokens cannot coexist is the root cause of the failure in continual distillation.
    Directly stated in the abstract as the explanation for distillation failure.

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    and AIME25 (Math-AI, 2025), with each score reported as the average over 16 runs. 17 Training-Trajectory-Aware Token Selection F. Why Static Signals are Insufficient for Targeting This appendix section studies whether the anchor/non-anchor partition discovered by T3S can be recovered fromstatic signals available at a single checkpoint, without using train...