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arxiv: 2605.14589 · v2 · pith:X5R6XHVJnew · submitted 2026-05-14 · 💻 cs.CL

EndPrompt: Efficient Long-Context Extension via Terminal Anchoring

Han Tian , Luxuan Chen , Xinran Chen , Rui Kong , Fang Wang , Jiamin Chen , Jinman Zhao , Yuchen Li
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Jiashu Zhao Shuaiqiang Wang Haoyi Xiong Linghe Kong Dawei Yin
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Pith reviewed 2026-05-15 01:26 UTC · model grok-4.3

classification 💻 cs.CL
keywords long-context extensionposition embeddingsRotary Position Embeddingefficient fine-tuninglanguage model adaptationsparse positional supervisioncontext window extensionterminal anchoring
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The pith

EndPrompt extends LLM context windows to 64K by training only on short sequences with a terminal prompt anchored at target positions.

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

The paper shows that long-context generalization does not require training on full-length sequences. Instead, EndPrompt keeps the original short text intact as the first segment and appends a brief terminal prompt whose positions are set near the desired target length. This creates both local and long-range relative distances inside short physical inputs while preserving semantic continuity of the training data. A theoretical argument based on Rotary embeddings and the Bernstein inequality establishes that position interpolation imposes smoothness on the attention function, and shared parameters limit unstable extrapolation. Experiments extending LLaMA models from 8K to 64K yield higher average scores on RULER and LongBench than full-length fine-tuning or prior methods, at far lower compute cost.

Core claim

By preserving the short context as an intact first segment and assigning the terminal prompt positional indices near the target length, the construction introduces long-range relative distances within short sequences; combined with the smoothness constraint from interpolation and parameter sharing, this suffices for reliable long-context generalization without dense long-sequence training.

What carries the argument

Two-segment terminal anchoring: the original short context remains the first segment while a brief prompt receives positional indices near the target context length, generating long-range relative distances inside short physical inputs.

If this is right

  • Context extension from 8K to 64K is achievable with short sequences, delivering 76.03 average RULER score versus 69.23 for full-length fine-tuning.
  • Memory and compute costs drop substantially because training sequences remain short.
  • Semantic continuity is maintained better than in chunk-based splitting methods.
  • Shared Transformer parameters suppress unstable extrapolation to unobserved intermediate distances.

Where Pith is reading between the lines

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

  • The same anchoring principle may transfer to position-embedding families other than RoPE if the smoothness constraint generalizes.
  • Researchers with limited hardware could now adapt large models to long contexts that were previously inaccessible.
  • Similar sparse-supervision tricks might improve data efficiency in other sequence modeling domains such as time-series or protein sequences.

Load-bearing premise

Assigning target-length positional indices to a brief terminal prompt appended to short sequences preserves the necessary relative distances and semantic continuity without creating artifacts from the artificial split.

What would settle it

A controlled test on recall tasks that place critical information at intermediate positions between the short context and the terminal prompt, where EndPrompt-trained models show sharply lower accuracy than full-length fine-tuned models.

Figures

Figures reproduced from arXiv: 2605.14589 by Dawei Yin, Fang Wang, Han Tian, Haoyi Xiong, Jiamin Chen, Jiashu Zhao, Jinman Zhao, Linghe Kong, Luxuan Chen, Rui Kong, Shuaiqiang Wang, Xinran Chen, Yuchen Li.

Figure 1
Figure 1. Figure 1: Overview of the proposed method. 3.2 Positional Index Manipulation Let L denote the target context length. Given a short context sequence x = (x0, x1, . . . , xa−1) of length a, an end prompt e = (e0, e1, . . . , eb−1) of length b is appended to form the physical training sequence: y = (x0, x1, . . . , xa−1, e0, e1, . . . , eb−1), |y| = a + b. (4) While the physical length is a + b, the assigned positional… view at source ↗
Figure 3
Figure 3. Figure 3: Performance comparison among the standard ET, Positional Skip-Embedding, and ET(PoSE) across the benchmarks of LongBench and RULER. 4.4 Structural Analysis To evaluate compatibility, the proposed approach is compared against Positional Skip-Embedding, a method that extends the context window by partitioning inputs into chunks and manipulating 8 [PITH_FULL_IMAGE:figures/full_fig_p008_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Comparison of memory footprint and time consumption across different methods. [PITH_FULL_IMAGE:figures/full_fig_p013_4.png] view at source ↗
read the original abstract

Extending the context window of large language models typically requires training on sequences at the target length, incurring quadratic memory and computational costs that make long-context adaptation expensive and difficult to reproduce. We propose EndPrompt, a method that achieves effective context extension using only short training sequences. The core insight is that exposing a model to long-range relative positional distances does not require constructing full-length inputs: we preserve the original short context as an intact first segment and append a brief terminal prompt as a second segment, assigning it positional indices near the target context length. This two-segment construction introduces both local and long-range relative distances within a short physical sequence while maintaining the semantic continuity of the training text--a property absent in chunk-based simulation approaches that split contiguous context. We provide a theoretical analysis grounded in Rotary Position Embedding and the Bernstein inequality, showing that position interpolation induces a rigorous smoothness constraint over the attention function, with shared Transformer parameters further suppressing unstable extrapolation to unobserved intermediate distances. Applied to LLaMA-family models extending the context window from 8K to 64K, EndPrompt achieves an average RULER score of 76.03 and the highest average on LongBench, surpassing LCEG (72.24), LongLoRA (72.95), and full-length fine-tuning (69.23) while requiring substantially less computation. These results demonstrate that long-context generalization can be induced from sparse positional supervision, challenging the prevailing assumption that dense long-sequence training is necessary for reliable context-window extension. The code is available at https://github.com/clx1415926/EndPrompt.

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

3 major / 2 minor

Summary. The paper proposes EndPrompt for extending LLM context windows from 8K to 64K using only short sequences: the original short context is preserved as the first segment and a brief terminal prompt is appended as the second segment with positional indices assigned near the target length. This introduces long-range relative distances within short physical inputs while claiming to maintain semantic continuity. A theoretical analysis based on RoPE and the Bernstein inequality is used to argue that position interpolation imposes a smoothness constraint on attention, with shared parameters suppressing unstable extrapolation. Empirically, on LLaMA-family models the method reports an average RULER score of 76.03 and the highest LongBench average, outperforming LCEG (72.24), LongLoRA (72.95), and full-length fine-tuning (69.23) at lower compute cost. Code is released.

Significance. If the central claim holds, the work would be significant for demonstrating that sparse positional supervision can induce reliable long-context generalization, lowering the barrier to context extension. The benchmark improvements and code availability are concrete strengths that support reproducibility. The result would challenge the assumption that dense long-sequence training is required, provided the theoretical analysis can be shown to cover the actual extrapolation performed.

major comments (3)
  1. [Theoretical analysis] Theoretical analysis section: the smoothness constraint is derived under position interpolation via the Bernstein inequality, but the EndPrompt construction assigns terminal-prompt indices near 64K on short sequences, producing direct extrapolation over large unobserved relative-position deltas (approximately 8K–56K). The bound therefore does not automatically transfer to the precise distances the model encounters at inference.
  2. [Experiments] Experiments section (RULER and LongBench tables): the reported gains (76.03 on RULER, highest on LongBench) are given without error bars, number of runs, or statistical significance tests, making it impossible to determine whether the improvements over full-length fine-tuning (69.23) are robust rather than within noise.
  3. [Method] Method section: the two-segment construction is described at a high level, but the exact length of the terminal prompt, the precise rule for choosing its positional indices, and the data-sampling procedure that produces the short sequences are not specified, which are load-bearing for verifying that semantic continuity is preserved and that the sparse-supervision claim can be reproduced.
minor comments (2)
  1. [Abstract] Abstract and method: training hyperparameters (learning rate, batch size, number of steps) and the exact composition of the training data are omitted, which should be added for reproducibility even if the central claim is sound.
  2. [Notation] Notation: the paper uses “target context length” without consistently defining whether it refers to 64K tokens or a different value in the equations; a single clarifying sentence would remove ambiguity.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for the thoughtful and constructive comments on our paper. We address each major point below and indicate the revisions we will make to strengthen the manuscript.

read point-by-point responses

  1. Referee: [Theoretical analysis] Theoretical analysis section: the smoothness constraint is derived under position interpolation via the Bernstein inequality, but the EndPrompt construction assigns terminal-prompt indices near 64K on short sequences, producing direct extrapolation over large unobserved relative-position deltas (approximately 8K–56K). The bound therefore does not automatically transfer to the precise distances the model encounters at inference.

    Authors: We appreciate this observation regarding the distinction between interpolation and extrapolation. Our analysis uses the Bernstein inequality to establish a smoothness constraint under position interpolation as a foundation, and we claim that the shared parameters help control extrapolation. However, we recognize that a direct application to the large relative deltas in EndPrompt requires further elaboration. In the revised manuscript, we will expand the theoretical section to discuss the specific extrapolation distances and provide additional reasoning on how the smoothness property extends to the terminal anchoring setup. revision: partial

  2. Referee: [Experiments] Experiments section (RULER and LongBench tables): the reported gains (76.03 on RULER, highest on LongBench) are given without error bars, number of runs, or statistical significance tests, making it impossible to determine whether the improvements over full-length fine-tuning (69.23) are robust rather than within noise.

    Authors: We agree that the absence of error bars and statistical analysis makes it difficult to assess the reliability of the reported improvements. We will revise the experiments section to include results from multiple independent runs (at least three seeds) with standard deviations, and we will add statistical significance tests (e.g., paired t-tests) comparing EndPrompt to the baselines to confirm that the gains are robust. revision: yes

  3. Referee: [Method] Method section: the two-segment construction is described at a high level, but the exact length of the terminal prompt, the precise rule for choosing its positional indices, and the data-sampling procedure that produces the short sequences are not specified, which are load-bearing for verifying that semantic continuity is preserved and that the sparse-supervision claim can be reproduced.

    Authors: We concur that these details are essential for reproducibility. In the updated method section, we will specify that the terminal prompt consists of 128 tokens, its positional indices are assigned consecutively starting from position (target_length - 128), and the short sequences are sampled by selecting contiguous segments from the original training data for the first segment while appending a fixed terminal prompt template. This preserves the semantic integrity of the primary context. revision: yes

Circularity Check

0 steps flagged

No significant circularity; derivation is self-contained

full rationale

The paper's central claim rests on the two-segment construction (short context plus terminal prompt with target-length indices) plus empirical results on RULER (76.03) and LongBench (highest average), which are independent of any internal fitted quantities. The theoretical analysis invokes standard RoPE properties and the Bernstein inequality to argue for smoothness under position interpolation; this is an application of external mathematics rather than a self-definition or a fitted parameter renamed as a prediction. No load-bearing step reduces the claimed long-context generalization to quantities defined inside the paper, and benchmark comparisons supply external validation.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The approach relies on standard rotary embedding mathematics and benchmark evaluation rather than new free parameters, axioms, or invented entities.

axioms (1)
  • standard math Rotary Position Embedding induces a smoothness constraint on the attention function under position interpolation, bounded by the Bernstein inequality.
    Invoked in the theoretical analysis to justify stability of the two-segment construction.

pith-pipeline@v0.9.0 · 5620 in / 1168 out tokens · 59858 ms · 2026-05-15T01:26:59.577618+00:00 · methodology

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