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REVIEW 3 major objections 6 minor 4 cited by

Lost in the Middle is a real effect, but only up to half a model's context window.

Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →

The lost-in-the-middle effect in LLMs appears mainly when inputs fill up to half the model's context window; beyond that, accuracy favors information closest to the end.

T0 review reviewed 2026-08-05 challenge →

load-bearing objection The relative-length framing cleanly resolves the LiM contradiction; the retrieval-prerequisite claim is correlation dressed as causation and should be softened. the 3 major comments →

arxiv 2508.07479 v1 pith:CGHB5RZ5 submitted 2025-08-10 cs.CL

Positional Biases Shift as Inputs Approach Context Window Limits

classification cs.CL
keywords positional biasLost in the Middleprimacy biasrecency biasrelative input lengthcontext windowlong-context language modelsretrieval-reasoning
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved

The pith

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

The reading

The paper tries to establish that the conflicting reports about the Lost-in-the-Middle effect come from a measurement choice: studies used absolute token counts instead of input length normalized by each model's context window. Measured relative to the window, the effect appears consistently in every tested model when inputs occupy up to 50% of available context; beyond that, the bias for information at the start collapses while the bias for the end survives, leaving a distance-based preference for information near the end. The paper further argues that successful retrieval is a prerequisite for reasoning on long inputs, and that the positional biases seen in reasoning are mostly inherited from retrieval. If correct, this gives researchers a simple normalization rule—compare positions at comparable fractions of the context window—and redirects long-context reasoning failures toward retrieval failures.

Core claim

The paper's central claim is that the inconsistent record on positional biases is an artifact of measuring input length in absolute token counts rather than relative to each model's context window. Across six instruction-tuned open-source LLMs with context windows from 8K to 128K tokens, on four controlled reasoning datasets, the Lost-in-the-Middle effect appears consistently whenever the input fills at most half the model's context window; beyond that threshold, primacy bias falls sharply, even dropping below middle-position accuracy for retrieval in some cases, while recency bias persists. The result is that the V-shaped LiM curve flattens and is replaced by a distance-based bias: the clos

What carries the argument

The central object is relative input length, $L_{\mathrm{rel}} = L_{\mathrm{input}} / L_{\mathrm{max}}$, the ratio of an input's token count to the model's declared context-window size, evaluated at seven levels from 6% to 100%. Around it the paper defines three intensity metrics: primacy bias intensity (first-position accuracy minus middle-position accuracy), recency bias intensity (last-position accuracy minus middle-position accuracy), and LiM intensity, the sum of the first two only when both are positive. The supporting object is a set of retrieval-reasoning minimal pairs: for each reasoning question a matched retrieval question targets exactly the single premise needed, so reasoning ac

Load-bearing premise

The causal reading of the retrieval-reasoning results rests on the assumption, stated only in the appendix, that every example in a dataset has the same logical structure and reasoning difficulty; if retrieval-easy instances are also reasoning-easy, the conditional-accuracy gap could be a composition effect rather than evidence that retrieval enables reasoning.

What would settle it

Run the same positional protocol on a model at $L_{\mathrm{rel}}$ values of 0.25, 0.5, and 0.9 while holding the absolute number of tokens between the relevant text and the end of the input constant: if accuracy tracks distance to the end, the distance-based bias is confirmed, and if it tracks $L_{\mathrm{rel}}$, the window-normalization claim is confirmed. For the retrieval-prerequisite claim, build difficulty-matched retrieval-reasoning pairs and test whether $P(RA=1|RT=0)$ stays measurably below $P(RA=1|RT=1)$ when reasoning complexity is held fixed.

Watch this falsifier. Get emailed when new claim-graph text bears on it.

If this is right

  • Positional-bias evaluations should report $L_{\mathrm{rel}}$ alongside token counts, because equal absolute lengths from different context windows are not comparable.
  • Prior conflicting results line up: studies below 50% of the window see LiM, while 100K-token studies mostly do not, because they are sampling different halves of the window.
  • Failures on long-context reasoning should be investigated as retrieval failures first; reasoning accuracy conditioned on successful retrieval is a cleaner measure of reasoning ability.
  • Long-context benchmarks need to include multiple relative lengths, since a single fixed length can conceal or fabricate a positional effect.
  • As inputs fill the window, distance from the end replaces first/middle/last as the organizing variable for accuracy.

Where Pith is reading between the lines

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

  • Beyond the paper: the distance-based-bias result suggests that model internals track absolute distance to the end rather than ordinal position; a natural follow-up is probing attention or hidden-state dynamics as a function of remaining tokens.
  • Beyond the paper: if retrieval is truly a prerequisite, retrieve-then-think prompting should improve reasoning more at high $L_{\mathrm{rel}}$ than at low $L_{\mathrm{rel}}$—a direct, testable prediction of the inheritance claim.
  • Beyond the paper: the causal reading could be checked by constructing difficulty-matched pairs where retrieval fails for positional reasons rather than complexity reasons; if the conditional gap persists, the prerequisite claim strengthens.
  • Beyond the paper: the window-normalization rule may transfer to multimodal or recurrent models whose effective horizons differ from nominal context windows, but the paper only tests transformer-based LLMs.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit.

Referee Report

3 major / 6 minor

Summary. The paper argues that positional biases in LLMs should be studied with input length normalized by each model's context window (Lrel), rather than with absolute token counts. Using four controlled synthetic datasets (MonoRel, PIR, RuleTaker, BoxTracker), six open-weight models with context windows from 8K to 128K, padding variants from prior work, and defined metrics PriMi, ReCi, and LiMi, the authors report that Lost in the Middle (LiM) appears consistently for Lrel ≤ 0.5 and fades beyond that point because primacy bias drops while recency bias persists, yielding a distance-based end preference. They additionally analyze retrieval and reasoning minimal pairs and claim that successful retrieval is a prerequisite for effective reasoning and that positional biases in reasoning are largely inherited from retrieval. The paper includes a real-world QA replication in Appendix A.2.1 and releases code.

Significance. If the relative-length finding holds, it would reconcile seemingly contradictory prior results (e.g., Liu et al. observing LiM at 6K tokens versus Zhang et al. not observing it at 100K tokens) and would provide a concrete methodological norm for future long-context evaluation. The study has notable strengths: controlled synthetic tasks that minimize reliance on in-weight knowledge, padding variants adopted from Levy et al. (2024), per-model and per-dataset results in appendices, a real-world QA check, and no fitted parameters in the core claims. The Lrel-based LiM pattern is credible and independently visible in the raw accuracy curves (Figures 3, 4, 13–16). However, the retrieval-reasoning 'prerequisite' claim is not established by the conditional-probability analysis; it requires either difficulty-matched instance controls or a substantial softening of the causal language. The paper is therefore promising but needs revision before the strongest claims can be accepted.

major comments (3)
  1. [Section 4.3, Table 4, Appendix A.4] The headline claim in Section 4.3 that 'successful retrieval is indeed a prerequisite for effective reasoning' is not supported by the conditional analysis. Tables 3–4 compare P(RA=1 | RT=1) with P(RA=1 | RT=0); this is observational conditioning, not an intervention, and the gap is equally consistent with retrieval-easy instances also being reasoning-easy instances. The defense offered in Appendix A.4 ('all examples within the same dataset follow the same logical structure and reasoning depth...') is asserted, not demonstrated, and the paper's own data strain it: Llama-3-70B on MonoRel has P(RA=1 | RT=0)=0.73, well above the chance floor of 0.5 that a strict prerequisite would imply, and the same quantity ranges from 0.15 to 0.73 across model-dataset cells. The paper should either match or stratify instance difficulty (e.g., by number of operations or by performance under retrieval-guar
  2. [Section 4.3, Figure 5] The claim that positional biases in reasoning are 'largely inherited from retrieval' is also not identified by conditioning on RT. Figure 5 plots P(RA=1 | RT=1) by position; if RT is a mediator of the position→RA path, conditioning on RT=1 blocks the very indirect effect the paper wants to measure, so flattened curves are expected even when reasoning itself has position effects. Conversely, if RT and RA share latent difficulty, conditioning on RT=1 selects a difficulty subset and can flatten any position curve. A mediation analysis, or a design where retrieval success is fixed by construction, is needed before inheritance can be concluded.
  3. [Eq. (2); Sections 4.1–4.2] LiMi is defined as a rectified sum that is set to 0 whenever either edge accuracy does not exceed the middle. Thus part of the message that 'the LiM effect disappears because primacy bias drops' is built into the metric: once PriMi goes non-positive, LiMi must be 0 even if recency is large. The raw curves in Figure 4 independently show the phenomenon, so the substantive claim is plausible, but the paper should separate the metric-defined quantity from the empirical accuracy pattern. Relatedly, the claim in Section 4.2 that primacy bias 'fades significantly' lacks significance tests; with 100 instances per dataset and six models, paired tests or confidence intervals across the seven Lrel levels would allow a quantitative check of the Lrel ≈ 0.5 crossover.
minor comments (6)
  1. [Introduction, Section 3] Typos: 'crutial' should be 'crucial' (fourth finding, Introduction); 'context widow' should be 'context window' (Introduction, third finding); 'adress' should be 'address' (start of Section 3).
  2. [Figure 5 caption] The caption says the right panel shows P(RA=1 | RT=1) but the surrounding text describes the right panel as P(RA=1 | RT=0). The right panel label is also inconsistent with the left-panel formula in the same sentence.
  3. [Table 4] The increase column is formatted inconsistently: most entries are percentages (e.g., 52, 42) but the Mistral-Small-24B MonoRel row reports 0.69 as if it were a proportion. The column header '∼Increase (%)' is also malformed.
  4. [Section 3.4] The sentence 'We use only the instruction-tuned versions of these models.' is repeated verbatim in the same paragraph.
  5. [Table 1 / Appendix A.1] In the BoxTracker false example, the name 'Hanna' appears in Table 1 but 'Hannah' appears in the full example in Appendix A.1; the name should be consistent.
  6. [Appendix A.2.1] Typo: 'ntheir' should be 'their'. Also, several reference titles contain spurious spaces (e.g., 'T ransactions', 'T ask'), likely from PDF extraction; these should be cleaned.

Circularity Check

2 steps flagged

Partial definitional coupling in the LiMi metric and the retrieval-reasoning minimal pairs, but the main Lrel finding is supported by raw accuracy curves.

specific steps
  1. self definitional [Section 3.3, Eq. (2); Section 4.1, paragraph 'Given LiMi’s definition']
    "LiMi = (accF − accM) + (accL − accM), if accF > accM and accL > accM, 0, otherwise. ... Since PriMi drops below zero, the fading LiM effect likely actually reflects a loss of primacy bias."

    By Eq. (2), LiMi is identically zero whenever accF ≤ accM (i.e., PriMi ≤ 0) or accL ≤ accM. Therefore the paper's explanation that LiM fades 'because primacy bias drops below zero' is a direct consequence of the metric's zeroing rule, not an independent empirical discovery: the moment PriMi becomes negative, the first condition in Eq. (2) fails and LiMi is set to 0 by construction. The raw accuracy curves in Fig. 4 do show first-position decline independently, so the effect is not fabricated; but the specific attribution that the LiM disappearance is 'driven by' the primacy drop is entailed by the definition of LiMi whenever the primacy term turns negative.

  2. self definitional [Section 3.1; Section 4.3, Evaluation paragraph]
    "Each retrieval question targets a specific piece of information required for the reasoning process. ... If successful retrieval is indeed a required precursor of successful reasoning, we expect a significant and constant performance increase of reasoning in case of sucessful retrieval, i.e. P(RA = 1 | RT = 1) > P(RA = 1 | RT = 0)."

    The retrieval question is, by construction, the atomic fact that the reasoning question requires ('targets the specific piece of information required'). Hence 'successful retrieval is a prerequisite for reasoning' is the dataset-design assumption restated as a conclusion: a reasoning answer that is faithful to the premises already entails having the targeted fact available. Because P(RA=1|RT=0) ranges up to 0.73 (Table 4), the entailment is empirically violated by guessing/other paths, so the conditional gap is not a mathematical identity; the causal reading additionally depends on the unverified equal-complexity assumption in Appendix A.4. This is definitional coupling plus confound, not a forced derivation.

full rationale

This is a measurement study with no fitted parameters, so the central quantitative claims are not predictions from a fitted model. The main Lrel finding (LiM strongest at Lrel ≤ 0.5, primacy fading beyond) is backed by raw position-specific accuracy curves (Figures 3–4) and by the real-world QA check (Appendix A.2.1), which are independent of the LiMi metric's zeroing rule. However, two components have definitional coupling. First, Eq. (2) hard-codes LiMi = 0 whenever either edge accuracy does not exceed the middle; the paper's statement that the LiM effect disappears 'because primacy bias drops below zero' is partly a consequence of this definition, as the paper itself acknowledges ('Given LiMi’s definition'). Second, the retrieval-reasoning minimal pairs define retrieval as the atomic sub-question of reasoning, so the 'prerequisite' conclusion is partly embedded in the construction; the causal interpretation additionally relies on the equal-complexity assumption stated in Appendix A.4 ('all examples within the same dataset follow the same logical structure and reasoning depth'), which is unverified and strained by Table 4's P(RA=1|RT=0)=0.73 for Llama-3-70B on MonoRel. This is a confound/limitation rather than a circular derivation. The only self-citation (Veseli et al. 2023, used for long-tail QA questions in Appendix A.2.1) is not load-bearing. No uniqueness theorems are imported from the authors. Overall, the central Lrel result has independent content, so the circularity score is moderate rather than high.

Axiom & Free-Parameter Ledger

4 free parameters · 5 axioms · 0 invented entities

The paper is an empirical measurement study: it introduces no fitted parameters in the derivation sense, but the central claims rest on hand-chosen design values (Lrel grid, middle placement, 100 instances per dataset), on vendor-declared context sizes as the normalization denominator, on position-neutral padding, and on an untested equal-complexity assumption that carries the causal 'prerequisite' claim. The bias metrics PriMi, ReCi, and LiMi are definitions, not new entities; no new forces, mechanisms, or conserved quantities are postulated.

free parameters (4)
  • Lrel grid = {0.06, 0.12, 0.25, 0.38, 0.5, 0.75, 1.0}
    Hand-chosen relative lengths aligned to Levy et al. (2024) absolute lengths (Section 3.2); grid resolution determines how precisely the 0.5 crossover can be located.
  • Observed crossover threshold = Lrel ≈ 0.5 (by inspection of averaged curves, Fig. 3)
    The central quantitative claim that LiM is strongest up to 50% of the context window is read off averaged curves without a fitted breakpoint model, a confidence interval, or a significance test.
  • Middle position placement = not precisely specified
    The exact location of the relevant text when labeled 'middle' is not defined in the main text; exact placement directly determines acc_M and hence all three bias metrics (Eqs. 2-4).
  • Instances per dataset = 100 per dataset
    Each dataset contains 100 instances (Section 3.1), which puts the standard error of an accuracy cell near five points and limits the resolution of per-model, per-position differences shown in Figure 4.
axioms (5)
  • domain assumption The advertised context window Lmax is the correct normalization denominator (Eq. 1)
    All results are stated in units of vendor-declared context sizes (8K, 32K, 128K). If a model's effective capacity is smaller than advertised, Lrel is miscalibrated and the observed 0.5 crossover shifts or vanishes.
  • domain assumption Distractor padding text is position-neutral and content-neutral with respect to the query
    Inputs are extended to target Lrel using Levy et al. (2024) style narrative padding (Section 3.2); any correlation between distractor content and position would be attributed to position.
  • domain assumption Within-dataset base instances have equal complexity, making the conditioning analysis causally interpretable
    Appendix A.4 states 'all examples within the same dataset follow the same logical structure and reasoning depth'; this is the basis for reading P(RA=1|RT=1) > P(RA=1|RT=0) as retrieval supporting reasoning rather than instance difficulty.
  • domain assumption Instruction-tuned checkpoints represent LLM positional behavior
    Section 3.4 restricts evaluation to instruction-tuned models and selects models with above-chance base accuracy; results may not transfer to base or continued-pretraining variants.
  • domain assumption The metric definitions in Eqs. 2-4 operationalize the phenomena (LiM requires both primacy and recency to beat the middle)
    LiMi is forced to zero unless acc_F > acc_M and acc_L > acc_M (Section 3.3), so conclusions about 'LiM disappearing' are partly inherited from this definitional choice.

reviewed 2026-08-05 · how reviews work

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Cite this review

Pith. "Pith review of Positional Biases Shift as Inputs Approach Context Window Limits." pith.science (2026). https://pith.science/paper/CGHB5RZ5

@misc{pith2026250807479,
  author       = {Pith},
  title        = {Pith review of: Positional Biases Shift as Inputs Approach Context Window Limits},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/CGHB5RZ5}},
  note         = {Machine review of arXiv:2508.07479}
}
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read the original abstract

Large Language Models (LLMs) often struggle to use information across long inputs effectively. Prior work has identified positional biases, such as the Lost in the Middle (LiM) effect, where models perform better when information appears at the beginning (primacy bias) or end (recency bias) of the input, rather than in the middle. However, long-context studies have not consistently replicated these effects, raising questions about their intensity and the conditions under which they manifest. To address this, we conducted a comprehensive analysis using relative rather than absolute input lengths, defined with respect to each model's context window. Our findings reveal that the LiM effect is strongest when inputs occupy up to 50% of a model's context window. Beyond that, the primacy bias weakens, while recency bias remains relatively stable. This effectively eliminates the LiM effect; instead, we observe a distance-based bias, where model performance is better when relevant information is closer to the end of the input. Furthermore, our results suggest that successful retrieval is a prerequisite for reasoning in LLMs, and that the observed positional biases in reasoning are largely inherited from retrieval. These insights have implications for long-context tasks, the design of future LLM benchmarks, and evaluation methodologies for LLMs handling extended inputs.

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This paper was first reviewed by deepseek-v4-flash on August 5, 2026.