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arxiv: 2605.10799 · v1 · submitted 2026-05-11 · 💻 cs.LG · cs.AI· cs.CL

Recognition: 1 theorem link

· Lean Theorem

The Last Word Often Wins: A Format Confound in Chain-of-Thought Corruption Studies

Gabriel Garcia
Authors on Pith no claims yet

Pith reviewed 2026-05-12 03:42 UTC · model grok-4.3

classification 💻 cs.LG cs.AIcs.CL
keywords chain-of-thoughtfaithfulness evaluationcorruption studiesformat confoundGSM8Kreasoning benchmarksmodel scale effects
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The pith

Corruption studies on chain-of-thought chains detect the location of explicit answer statements rather than actual reasoning steps.

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

Corruption studies test chain-of-thought faithfulness by swapping steps with errors and tracking accuracy loss. This paper demonstrates that when chains end with explicit terminal answers, the dominant format in benchmarks like GSM8K, the method mainly registers where the answer text sits instead of where computation happens. Removing only the answer statement while keeping all prior reasoning reduces suffix sensitivity by roughly 19 times at 3B scale. Experiments inserting conflicting answers show models follow the explicit text at high rates, with the effect appearing across model families and weakening only at 32B scale. The paper recommends a stricter three-step protocol to avoid the issue in future work.

Core claim

In chains with explicit terminal answer statements, corruption studies measure the placement of the answer text rather than the location of computational steps. Within-dataset ablations show that deleting only the answer statement collapses suffix sensitivity by a factor of 19 at 3B scale. Conflicting-answer insertions drive accuracy to near zero and produce followed-wrong rates from 0.63 to 1.00 at smaller scales, with the bias attenuating toward zero at 32B. The same protocol on suffix-free chains instead flags the prefix as load-bearing, and generation probes confirm the answer is not early-committed yet is followed at consumption time.

What carries the argument

The terminal answer statement in standard CoT formats, which serves as a positional cue that models follow at output time even when it contradicts prior reasoning.

If this is right

  • Standard GSM8K-style benchmarks systematically inflate apparent faithfulness for terminal positions.
  • Models follow explicit answer text at consumption time even when generation-time probes show no early commitment.
  • The bias appears across five architecture families at 7B scale and attenuates only beyond 14B.
  • Chains without answer suffixes shift load-bearing status to the prefix instead.
  • A minimum protocol of question-only control, format characterization, and full-position sweep is required for valid corruption studies.

Where Pith is reading between the lines

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

  • This format effect may explain why some faithfulness claims fail to generalize when answer phrasing changes across datasets.
  • Larger models may reduce reliance on the cue through scale alone, but the paper leaves open whether training interventions could eliminate it earlier.
  • The dissociation between generation and consumption suggests models treat the final statement as a separate retrieval cue rather than a recomputed result.

Load-bearing premise

The format ablations and conflicting-answer changes isolate the effect of the terminal statement without creating separate artifacts from chain editing or model training patterns.

What would settle it

Re-running the suffix corruption protocol on a new set of CoT chains that contain no explicit terminal answer statements and finding that suffix positions still show high sensitivity to corruption.

Figures

Figures reproduced from arXiv: 2605.10799 by Gabriel Garcia.

Figure 1
Figure 1. Figure 1: Within-dataset format ablation (Qwen 2.5-3B, N=300): same model, same exam￾ples, same reasoning, only the answer statement removed. Left: on standard GSM8K-v1 chains where the suffix reads “the answer is X”, suffix corruption collapses accuracy to 0.210 (∆=−0.760, p<10−6 ). Right: when only the explicit answer statement is removed from the same chains (GSM8K￾stripped-v1), suffix sensitivity collapses ≈19× … view at source ↗
Figure 2
Figure 2. Figure 2: Protocol schematic: five-condition experimental design. Each question is evaluated in [PITH_FULL_IMAGE:figures/full_fig_p007_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Format ablation: suffix sensitivity shrinks [PITH_FULL_IMAGE:figures/full_fig_p014_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Prefix-branch probe: answer accuracy when the model is stopped after each chain step. [PITH_FULL_IMAGE:figures/full_fig_p018_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Protocol-uniform conditioning summary on GSM8K-v1 conflicting-answer runs ( [PITH_FULL_IMAGE:figures/full_fig_p030_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Followed-wrong rate across model families and parameter scales. All conflicting-answer [PITH_FULL_IMAGE:figures/full_fig_p030_6.png] view at source ↗
read the original abstract

Corruption studies, the primary tool for evaluating chain-of-thought (CoT) faithfulness, identify which chain positions are "computationally important" by measuring accuracy when steps are replaced with errors. We identify a systematic confound: for chains with explicit terminal answer statements, the dominant format in standard benchmarks, corruption studies detect where the answer text appears, not where computation occurs. A within-dataset format ablation provides the key evidence: on standard GSM8K chains ending with "the answer is X," removing only the answer statement, preserving all reasoning, collapses suffix sensitivity ~19x at 3B (N=300, p=0.022). Conflicting-answer experiments quantify the causal mechanism: at 7B, CC accuracy drops to near-zero (<=0.02) across five architecture families; the followed-wrong rate spans 0.63-1.00 at 3B-7B and attenuates at larger scales (0.300 at Phi-4-14B, ~0.01 at 32B). A within-stable 7B replication (9.3x attenuation, N=76, p=7.8e-3; Qwen3-8B N=299, p=0.004) provides converging evidence, and the pattern replicates on MATH (DeepSeek-R1-7B: 10.9x suffix-survival recovery). On chains without answer suffixes the same protocol identifies the prefix as load-bearing (Delta=-0.77, p<10^-12). Generation-time probes confirm a dissociation: the answer is not early-determined during generation (early commitment <5%), yet at consumption time model outputs systematically follow the explicit answer text. The format-determination effect persists through 14B (8.5x ratio, p=0.001) and converges toward zero at 32B. We propose a three-prerequisite protocol (question-only control, format characterization, all-position sweep) as a minimum standard for corruption-based faithfulness studies.

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

0 major / 4 minor

Summary. The paper claims that corruption studies, the main method for probing chain-of-thought (CoT) faithfulness, are systematically confounded by chain format: when chains contain explicit terminal answer statements (the dominant format in benchmarks such as GSM8K), these studies measure sensitivity to the location of the answer text rather than the location of actual computation. Key evidence includes a within-dataset format ablation showing ~19x collapse in suffix sensitivity after answer-statement removal (3B scale, N=300), conflicting-answer manipulations yielding near-zero accuracy and high followed-wrong rates across five 7B-scale architectures (attenuating at 14B–32B), a within-stable replication, reversal to prefix sensitivity on no-suffix chains (Delta=-0.77), generation-time dissociation (<5% early commitment), and replication on MATH. The authors recommend a three-prerequisite protocol (question-only control, format characterization, all-position sweep) as a minimum standard.

Significance. If the central claim holds, the result is significant for LLM reasoning evaluation: it indicates that much existing work on CoT faithfulness may be capturing format sensitivity rather than computational importance, with direct implications for benchmark design and interpretation. Strengths include converging empirical lines (ablations, causal interventions, cross-scale and cross-dataset tests) with explicit statistical reporting (p-values, N sizes) and no free parameters or circular derivations. The proposed protocol offers a concrete, falsifiable improvement that could raise standards in future corruption studies.

minor comments (4)
  1. The format ablation reports a ~19x collapse at 3B; the exact token-level editing procedure (how the terminal statement is excised while 'preserving all reasoning') should be described in a dedicated methods subsection or appendix to allow exact replication.
  2. Generation-time probes state early commitment <5%; the precise operationalization of 'early commitment' (e.g., first-token probability threshold or prefix-matching criterion) should be stated explicitly.
  3. The five architecture families tested at 7B are referenced but not enumerated in the main text; listing them (with exact model names and parameter counts) would improve reproducibility.
  4. The no-suffix reversal result (Delta=-0.77, p<10^-12) is striking; a supplementary figure showing position-wise accuracy curves for both suffix and no-suffix conditions would aid visual comparison.

Simulated Author's Rebuttal

0 responses · 0 unresolved

We thank the referee for their positive and accurate summary of our work, as well as the recommendation for minor revision. The report correctly captures the central claim that corruption studies on CoT chains with explicit terminal answer statements primarily detect answer-text location rather than computational steps, supported by the format ablation, conflicting-answer interventions, scale effects, and the proposed protocol. We have no major points of disagreement and will incorporate minor revisions to enhance clarity, presentation, and any additional details on the protocol as needed.

Circularity Check

0 steps flagged

No significant circularity

full rationale

The paper's claims rest entirely on empirical measurements from controlled interventions (format ablations, conflicting-answer manipulations, within-dataset controls, cross-architecture replications, and generation-time probes). No derivations, equations, fitted parameters, or self-citations are invoked as load-bearing steps; the central finding that corruption studies track answer-text location rather than computation is directly evidenced by the reported deltas, p-values, and attenuation patterns across scales and datasets. This is self-contained experimental work with no reduction of results to inputs by construction.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The paper is empirical and relies on standard assumptions from machine learning evaluation and statistics rather than new free parameters, ad-hoc axioms, or invented entities.

axioms (1)
  • domain assumption Accuracy under corruption accurately reflects computational dependence when format confounds are absent
    Central to interpreting the ablation and conflicting-answer results as evidence of the confound.

pith-pipeline@v0.9.0 · 5673 in / 1309 out tokens · 66525 ms · 2026-05-12T03:42:22.383729+00:00 · methodology

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Reference graph

Works this paper leans on

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