REVIEW 5 major objections 6 minor 1 cited by
SEPS: A Separability Measure for Robust Unlearning in LLMs
T0 review · 5 major / 6 minor · reviewed 2026-08-07 · deepseek-v4-flash
Pith's one-line read Existing LLM unlearning methods fail when forget and retain queries are mixed in a single prompt, and this paper shows a new separability score plus mixed-prompt training closes much of that gap.
desk verdict A useful mixed-prompt evaluation for unlearning, with a solid baseline-failure analysis and a training fix whose generalization claims are undercut by train/test overlap. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The central object is the Separability Score, $\text{SEPS}=\max(\text{RIS}-\text{FIS},0)$, built from Retain and Forget Inclusion Scores. RIS and FIS average each query's score over the two orderings of a two-question prompt, so a model scores well only when it answers retain questions and refuses forget questions in the same prompt, rather than leaking everything or refusing everything. The companion machinery is Mixed Prompt training: MP-ME applies a token-level Kullback-Leibler loss toward a uniform distribution on forget content and toward the reference model on retain content, while MP-IDK applies cross-entropy to "I don't know" labels for forget answers and ground-truth answers for retain queries; both losses are symmetrized over retain-first and forget-first orders.
What would settle it
Have human annotators score the same mixed-prompt outputs that the judge scored, especially outputs that say "I don't know" or that answer only one of two numbered questions; if the judge's ratings diverge from human judgments of information overlap, the SEPS comparisons do not measure separability. Alternatively, rerun the eight-question stress test with forget and retain questions never seen during MP training; a sharp drop in SEPS would show that the reported robustness is memorization of training prompts.
Extended reading notes
Core claim
The paper claims that unlearning quality should be judged by separability, not only by single-query forget efficacy and model utility. It defines the Separability Score as $\max(\text{RIS}-\text{FIS},0)$, where Retain Inclusion Score (RIS) and Forget Inclusion Score (FIS) average the score of the retain and forget halves of prompts that appear in both orders, retain-then-forget and forget-then-retain. With this measure, the paper reports that ten existing methods score near zero on SEPS even when their single-query scores look acceptable, and identifies two failure patterns: untargeted methods let the retain score collapse to the forget baseline as soon as a forget question appears, and targeted methods answer a first forget question or refuse a second retain question. The proposed Mixed Prompt unlearning, MP-ME and MP-IDK, trains on prompts containing both kinds of questions with a symmetrized loss, and attains higher SEPS while remaining competitive on utility and forget efficacy, including on a stress test with up to eight mixed questions.
Load-bearing premise
The whole comparison stands or falls on the language-model judge scoring mixed-prompt outputs by real information overlap rather than by format or refusal style, since every RIS, FIS, and SEPS number is derived from those ratings.
Editorial extensions
If this is right
- SEPS should be reported alongside model utility and forget efficacy, because high scores on the standard single-query metrics can coexist with near-zero separability.
- Untargeted unlearning methods that maximize loss or entropy on forget data are unsafe for mixed prompts, since a single forget question makes them erase retain content too.
- Targeted "I don't know" training needs mixed-prompt objectives, because models trained only on single queries answer a first forget question or refuse a second retain question.
- Mixed Prompt training with MP-ME and MP-IDK keeps forget and retain answers separated in prompts containing up to eight mixed questions.
- Aggregating model utility, forget efficacy, and SEPS by harmonic mean gives a more realistic ranking of unlearning methods than utility and efficacy alone.
Reading between the lines
- If SEPS becomes standard, unlearning training should include both prompt orders and repeated same-type queries, because the paper shows separability failures are order- and position-dependent.
- The stress test is built from the same forget and retain samples used for MP training, so the paper does not establish that separability transfers to unseen content; a natural next test is a held-out mixed-prompt split.
- Pairing judge-based SEPS with per-answer exact-match or retrieval checks would protect the metric from LLM-judge format bias and make scores easier to audit.
- The failure asymmetry (forget-first hurts untargeted methods, forget-last hurts targeted methods) suggests that any deployment of unlearning should be tested with both orderings before release.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes SEPS, an evaluation metric for machine unlearning that measures whether a model answers retain queries while refusing forget queries when both appear in the same prompt. Using TOFU, MUSE, and WMDP, it reports that existing unlearning methods have near-zero SEPS and identifies two failure modes: untargeted methods collapse retain scores when a forget query is present, and targeted methods overfit single-query behavior. It then proposes MP-ME and MP-IDK, which train on interleaved forget/retain prompts, and reports large SEPS improvements and robustness up to eight mixed queries in a stress test.
Significance. If the results hold, SEPS is a useful evaluation primitive and the mixed-prompt training strategy is a simple, practical remedy for a real failure mode. The paper includes a broad comparison across ten baselines and three benchmarks, releases code, and provides qualitative examples showing the 'bleeding' and single-query overfitting effects. The main uncertainty is whether the stress-test results measure generalization: the stress-test prompts reuse the exact forget samples used to train MP methods, and epoch selection includes SEPS on the reported evaluation set. These issues are fixable with a held-out split and a validation-based selection protocol, so the central idea remains defensible.
major comments (5)
- [Section 6.4, Appendix C.8] The stress-test prompts are built from the same 40 forget01 forget samples and randomly selected retain samples used to train MP-ME and MP-IDK in Sections 6.1-6.2, and no train/test split is reported. The abstract and Section 6.4 claim robustness 'even in complex settings with up to eight mixed forget and retain queries,' but with this overlap the high stress-test scores may largely reflect memorization of the exact question-answer pairs whose separability is being measured. Please rerun the stress test on held-out content (e.g., a disjoint split of authors or QA pairs, or paraphrased queries), or explicitly separate and report results for query content never used in MP training, and adjust the generalization claim accordingly.
- [Appendix C.3] The checkpoint is selected by the harmonic mean of MU, FE, and SEPS computed on the same evaluation set that is reported in Tables 1 and 7. Because SEPS is part of the selection criterion, the headline numbers are produced by test-set selection, which inflates apparent performance and makes comparisons between methods difficult to interpret. Please select epochs on a validation split disjoint from the reported evaluation, or report results for both 5 and 10 epochs so readers can assess the sensitivity of every table to this choice.
- [Section 6.3, Figure 5, Figure 6] The paper states that MP-ME performs well in the RF ordering but struggles in the FR ordering, yet the stress-test summary in Figure 6 reports only average retain and forget scores collapsed over both orderings and over the 1/2/4 query counts. This averaging can hide the FR failure mode that the two-query analysis identifies, so the claim of robust performance 'up to eight' queries is not supported by the displayed aggregation. Please report stress-test results separately for retain-first and forget-first orderings, and preferably broken down by query count as well.
- [Appendix C.6, Table 2] The validity of the LLM-as-Judge scores for SEPS is supported only by correlation with ROUGE and cosine similarity, but all three metrics could be correlated while still sharing a systematic bias induced by the numbered mixed-prompt format or by the style of 'I don't know' refusals. Please provide a direct validation of the judge against known ground-truth inclusion levels, for example outputs that contain known fractions of forget and retain content and refusal-style outputs with no content, and report per-item agreement or calibration metrics in addition to correlation.
- [Tables 1, 7, 8, Figure 6] All quantitative results are point estimates without error bars, confidence intervals, or repeated-seed variation. Since the central claim is that MP methods 'significantly improve' over baselines, please report at least the standard error over evaluation samples or multiple training seeds for the main SEPS and H-Avg. comparisons; this is particularly important for the smaller differences, such as MP-ME versus NPO+GD in the forget01 setting.
minor comments (6)
- [Section 4] The formulas for FIS and RIS are printed identically as 'FR Score + RF Score / 2' and differ only through the underlining convention. Please use explicit notation, such as FIS = (F_R + F_RF)/2 and RIS = (R_RF + R_FR)/2, to avoid confusion.
- [Section 4] The phrase 'SEPS is formularized as' should be 'SEPS is formulated as'.
- [Appendix C.3] The sentence 'In our experiments, we mainly employ LLM-as-Judge scores to quantify the MU, FE, and SEPS of unlearned models' is repeated verbatim in the same paragraph; please delete the duplicate.
- [Figure 6] The caption should state explicitly how many prompts contribute to each bar and whether the averages are over prompts or over individual queries; currently the reader cannot tell whether the collapsed scores are dominated by the two-query or eight-query configurations.
- [Section 2] The phrase 'task arithematic' should be 'task arithmetic.'
- [Appendix C.8] Please report the number of sampled retain questions per line and the exact random seed or sampling procedure, since the stress test currently uses only 10 samples per configuration and reproducibility depends on this detail.
Circularity Check
MP-ME/MP-IDK are trained on the same RF/FR mixed prompts whose retain-forget gap defines SEPS, and the stress test reuses the same forget/retain QA samples with epoch selection on the reported metric, so the headline multi-query robustness is partly a fit rather than an out-of-sample prediction.
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fitted input called prediction
[Sec. 6.1-6.2 (MP training objectives) and Sec. 4 (SEPS definition)]
"Since either the retain or forget query may appear first, we symmetrize the objective by summing over both orderings: Ltotal = E[ LMP-ME(P[qr, qf , ar, af ]) + LMP-ME(P[qf , qr, af , ar]) ], where expectation is with respect to (qr, ar) ∼ Dr and (qf , af ) ∼ Df. ... SEPS = max(RIS − FIS, 0), where a score of 1 reflects perfect separation, meaning the model consistently responds correctly to retain queries while refusing forget queries when both types are present in a single prompt."
MP-ME and MP-IDK are optimized on exactly the two-order mixed prompts (RF and FR) whose retain/forget behavior defines RIS, FIS, and SEPS. In MP-ME, forget tokens are driven toward a uniform distribution and retain tokens toward the reference model; in MP-IDK, forget answers are labeled 'I don't know' and retain answers are ground truth. The high SEPS reported for the MP methods is therefore the direct evaluation of the training objective on its own training distribution, not an independent discovery about mixed-prompt separability. The baseline failures remain informative; the MP success on the two-query RF/FR setting is a fit to the measured quantity.
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fitted input called prediction
[Appendix C.8 (stress-test construction) compared with Sec. 6.1-6.2 (training distribution)]
"We employ the forget01 unlearning scenario from the TOFU benchmark, which consists of 40 forget samples and 3,960 retain samples. To construct our stress test, we partition the 40 forget samples into 10 lines, each containing 4 distinct forget samples (i.e., no overlap). Additionally, we randomly select 4 retain samples for each line."
MP-ME and MP-IDK are trained with expectations over (qr, ar) ∼ Dr and (qf , af ) ∼ Df. The stress test is assembled from the same 40 forget samples that constitute Df in the forget01 scenario and from randomly selected retain samples drawn from the same Dr pool, with no train/test split reported. The central claim of robustness 'with up to eight mixed forget and retain queries in a single prompt' is therefore measured on the exact question-answer content used for training, so high scores can reflect memorization rather than generalization to unseen mixed prompts.
1 more flagged steps
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fitted input called prediction
[Appendix C.3 (TOFU experimental details)]
"We fine-tune each unlearning method for 5 and 10 epochs, selecting the model with the higher harmonic mean of MU, FE, and SEPS."
The reported MU, FE, and SEPS values are the best of two epoch choices selected using those very metrics on the same evaluation data, with no validation split described. The headline numbers are thus maxima over a hyperparameter choice on the test set, making the reported robustness partly an artifact of selection. This applies to all methods equally, so comparative baseline failures are less affected, but the absolute MP robustness numbers are not unbiased out-of-sample estimates.
full rationale
SEPS itself is not circular: it is a new measurement combining retain and forget inclusion in mixed prompts, and the observation that existing single-query-trained methods fail under mixed prompts is an independent, informative comparison. The proposed MP variants, however, are trained on exactly the two-order mixed-prompt templates that define SEPS, so their high two-query SEPS is the fitted objective rather than an external validation. More importantly, the Sec. 6.4 stress-test claim of robustness 'up to eight mixed queries' is not out-of-sample: Appendix C.8 builds the 180 prompts from the same 40 forget samples and randomly selected retain samples used in MP training, and Appendix C.3 selects reported epochs by the harmonic mean of MU, FE, and SEPS on that same evaluation data. These factors make the central generalization claim partially circular, while the baseline failure findings and the metric remain non-circular. No load-bearing self-citation or imported uniqueness theorem is present.
Assumptions & free parameters
free parameters (4)
- loss weighting coefficients =
1.0 for forget and regularization losses
- learning rate =
1e-5
- beta for NPO/DPO/AP =
0.1
- epoch selection =
5 or 10 by best H-Avg of MU/FE/SEPS
assumptions (4)
- domain assumption GPT-4 judges output content correctly in mixed-prompt settings
- domain assumption Arithmetic and harmonic means of raw scores produce meaningful composite metrics
- domain assumption TOFU, MUSE, and WMDP with the mixed-prompt templates represent real-world multi-query interactions
- domain assumption Reusing the same QA pairs in training and stress-test prompts measures unlearning rather than memorization
Cite this review
Pith. "Pith review of SEPS: A Separability Measure for Robust Unlearning in LLMs." pith.science (2026). https://pith.science/paper/DOD2QYNS
@misc{pith2026250514832,
author = {Pith},
title = {Pith review of: SEPS: A Separability Measure for Robust Unlearning in LLMs},
year = {2026},
howpublished = {\url{https://pith.science/paper/DOD2QYNS}},
note = {Machine review of arXiv:2505.14832}
}
read the original abstract
Machine unlearning aims to selectively remove targeted knowledge from Large Language Models (LLMs), ensuring they forget specified content while retaining essential information. Existing unlearning metrics assess whether a model correctly answers retain queries and rejects forget queries, but they fail to capture real-world scenarios where forget queries rarely appear in isolation. In fact, forget and retain queries often coexist within the same prompt, making mixed-query evaluation crucial. We introduce SEPS, an evaluation framework that explicitly measures a model's ability to both forget and retain information within a single prompt. Through extensive experiments across three benchmarks, we identify two key failure modes in existing unlearning methods: (1) untargeted unlearning indiscriminately erases both forget and retain content once a forget query appears, and (2) targeted unlearning overfits to single-query scenarios, leading to catastrophic failures when handling multiple queries. To address these issues, we propose Mixed Prompt (MP) unlearning, a strategy that integrates both forget and retain queries into a unified training objective. Our approach significantly improves unlearning effectiveness, demonstrating robustness even in complex settings with up to eight mixed forget and retain queries in a single prompt.
Figures
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Forward citations
Cited by 1 Pith paper
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DUSK: Do Not Unlearn Shared Knowledge
DUSK benchmarks machine unlearning under overlapping forget and retain documents, showing existing methods remove surface text but fail to preserve shared knowledge while erasing unique content.
Reference graph
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Reviewed August 7, 2026 · model on record in the stance chip above.
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