REVIEW 3 major objections 3 minor 42 references
The paper proves that a single pivot sequence, Z_t = -log(1-U_{t,W_t}), turns Gumbel-max watermark detection into a one-pass, anytime-valid sequential test.
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 →
T0 review · deepseek-v4-flash
2026-08-01 06:13 UTC pith:HEF3U222
load-bearing objection A sound but incremental one-pass e-process for Gumbel-max watermark detection; the math checks out under a pivot assumption that needs a rigorous proof for keyed reconstruction. the 3 major comments →
Efficient Online LLM Watermark Detection via Rao-Blackwellized E-Processes
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The central claim is that watermark detection reduces to testing whether a single pivot sequence behaves like standard exponential noise. For the Gumbel-max watermark, at each step the detector reconstructs the key-derived uniform U_{t,W_t} associated with the emitted token and forms Z_t = -log(1-U_{t,W_t}). The paper argues that under the null of no watermarking, Z_t is conditionally Exp(1) given the past pivots, so each one-step e-value (1-λ)exp(λ Z_t) has conditional expectation 1. Their product M_t(λ) is therefore an e-process, giving an anytime-valid level-α test via the stopping rule M_t ≥ 1/α. Under the Gumbel-max alternative, the paper proves positive asymptotic log-growth for a suit
What carries the argument
The load-bearing object is the normalized exponential score E_t = (1-λ)e^{λ Z_t}, whose product forms the e-process M_t(λ) = (1-λ)^t e^{λ S_t}. The pivot Z_t = -log(1-U_{t,W_t}) is the mechanism that strips away the unknown next-token distribution: under the null it is Exp(1), and under watermarking larger values signal dependence. Rao-Blackwellization appears through the surrogate exponential-tilt family q_κ(z) = (1-κ)exp(-(1-κ)z), under which the cumulative sum S_t is a sufficient statistic, so inference depends only on the scalar S_t rather than the full token history.
Load-bearing premise
The entire false-positive guarantee hangs on the premise that, whenever text is not watermarked, the pivot Z_t = -log(1-U_{t,W_t}) has conditional Exp(1) distribution given the past; if tokens are selected with even a small dependence on the key-derived uniforms, the guarantee fails.
What would settle it
Generate unwatermarked text by a sampling rule that makes the emitted token depend on the key-derived uniforms (for example, always pick the token with maximum U, or use a biased RNG), feed the pivots through the proposed stopping rule at α=0.05, and repeat many times. If the conditional distribution of Z_t genuinely departs from Exp(1), the empirical false-positive rate under optional stopping will exceed 5%, demonstrating that the anytime-valid guarantee depends on the independence premise.
If this is right
- A detector can monitor a live token stream and stop the moment evidence crosses 1/α, with no correction for multiple looks.
- Only a single accumulator M_t and the cumulative sum S_t need to be kept, so the method runs in constant memory per token.
- The plug-in and mixture versions automatically adapt the tilt λ to the unknown signal strength while preserving validity.
- Fixed-horizon watermark tests that reuse the same data for repeated testing lose Type I error control; this construction gives an online alternative.
- Under watermarking, the test is consistent: it rejects almost surely as text length grows.
Where Pith is reading between the lines
- The same pivot reduction could likely be built for green-red list watermarks if a pivot with an explicit null distribution can be defined, opening a general recipe for online detection.
- A practical deployment could use the pivot's empirical distribution as a monitoring diagnostic: if the null assumption fails on human text, the detector would need recalibration before use.
- Anytime-valid detection enables use cases where a decision must be made mid-generation, such as halting a chat model's output or flagging content before the full response is delivered.
- A natural extension is to turn the e-process into a confidence sequence for watermark strength, giving an anytime estimate of how strong the watermark signal is, not just a binary rejection.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes an online, anytime-valid watermark detector for the Gumbel-max watermark, based on the reduced pivot Z_t = -log(1 - U_{t,W_t}). Under the null that Z_t | G_{t-1} ~ Exp(1), it constructs the e-process M_t(λ) = (1-λ)^t e^{λ S_t}, and adds plug-in and mixture versions of λ. The main theoretical claims are anytime-valid Type I error control via Theorem 3.1 and positive asymptotic log-growth under watermarking via Theorem 4.1. The method is O(1) memory and updates token by token (Algorithm 1). Experiments on synthetic spike-model data and on OPT-1.3B/GPT-Neo-1.3B text compare power, sequential Type I error, and runtime against online and offline baselines.
Significance. If the pivot null premise is valid, the construction is elegant and practically attractive: the one-step factors are genuine conditional e-values, the proof of Theorem 3.1 is a one-line MGF calculation, and the O(1) recursive update is a concrete improvement over history-dependent e-process baselines. The mixture and plug-in extensions are standard and valid under predictability. However, the paper's central reduction is asserted rather than proved for the actual keyed reconstruction, and the proof of the consistency theorem is deferred to an appendix that is absent from the manuscript. The significance for the sequential-testing/LLM-watermark community is therefore real but conditional on filling these gaps.
major comments (3)
- [§3.2 and Algorithm 1] The load-bearing null premise, Z_t | G_{t-1} ~ Exp(1), is asserted but not proved for the keyed reconstruction. The argument says W_t is independent of U_t under H0; however, G_{t-1} = σ(Z_1,...,Z_{t-1}) contains functions of past keyed pseudorandom vectors, so it may carry information about the key/PRF state and future U_t. Under the concrete key schedule used in Algorithm 1, the conditional law of U_{t,W_t} given G_{t-1} is not shown to be Unif(0,1), nor is Z_t shown to be Exp(1). Since Theorem 3.1 and the anytime-valid Type I error claim depend on this conditional law (not just marginal independence), the paper needs either a proof under an explicit key/PRF model or a clearly stated additional assumption defining the null under which the guarantee holds.
- [§4, Theorem 4.1] The proof of Theorem 4.1 is not present; the text defers to an Appendix that is not included. The theorem is central to the consistency claim (positive asymptotic log-growth and a.s. rejection). As written, it is an unverified assertion. Please provide the proof, or state the theorem as a conjecture and adjust the claims. Also clarify whether the λ whose existence is asserted is a universal constant or can depend on the unknown NTP sequence/P_t; if the latter, the practical relevance of the existence result is limited.
- [§3.4, Eq. (5)-(6)] The validity of the plug-in and mixture e-processes is stated to be proved in an absent Appendix. For the mixture this follows by convexity from Theorem 3.1, and for the plug-in it follows if λ_s is predictable and in (0,1); however, the clipping constants λ_min, λ_max are introduced without discussion of their effect on asymptotic log-growth, and Eq. (4)'s oracle λ* = 1 - μ^{-1} requires μ>1. Please include the deferred arguments and discuss the clipping/μ≤1 boundary case.
minor comments (3)
- [§3.3 / Proposition 3.2] The label 'Rao-Blackwellized' is not justified as stated: Proposition 3.2 only shows that S_t is sufficient under the surrogate family and that M_t(λ) is a function of S_t. Please explain how Eq. (2) is obtained by Rao-Blackwellizing a full-data e-process, or soften the terminology.
- [§3.4, Eq. (4)] The unclipped oracle λ* = 1 - μ^{-1} is not in (0,1) unless μ>1. Please add this condition or explicitly define the maximizer over (0,1) for μ≤1.
- [Throughout] The term 'unwatermarking language model' is used repeatedly; it should be 'unwatermarked'. Also, the references to an Appendix should be resolved: the manuscript contains no appendix, so all 'Appendix states...' passages are currently unverifiable.
Circularity Check
No significant circularity: the e-process is derived from an explicit pivot-null assumption, and no fitted parameter is renamed as a prediction.
full rationale
The derivation chain is self-contained apart from external pivot results. Theorem 3.1 explicitly assumes Z_t | G_{t-1} ~ Exp(1) under H0 as its premise; it does not claim to prove this from a self-citation. The construction M_t(λ) = (1-λ)^t e^{λ S_t} is the standard likelihood-ratio e-process for Exp(1) versus Exp(1−λ), normalized by the null MGF, and its anytime-validity follows from the stated conditional law. No parameter is fitted to data and then called a prediction: the plug-in rule estimates μ online, but validity holds for any predictable λ_s ∈ (0,1), so the e-process property does not depend on the fit. The oracle λ* = κ0 in Proposition 4.2 is derived in closed form from the surrogate family and is used only to motivate practical choices, not to force the consistency result. Citations to Li et al. (2025), Fernandez et al. (2023), and de Roos et al. (2025) are external and not self-citations by the present authors, so the load-bearing pivot null is not an imported uniqueness claim. The 'Rao-Blackwellized' framing is presentation rather than the argument for validity. The reviewer's concern that the Exp(1) pivot may fail under a concrete keyed reconstruction is a correctness or assumption gap, not a circularity: the paper's theorem states the pivot null as a supposition, and Algorithm 1's reconstruction details are left unspecified, but the claimed e-process property does not reduce to its own input.
Axiom & Free-Parameter Ledger
free parameters (3)
- λ (fixed sensitivity parameter) =
any value in (0,1) is valid; oracle under surrogate q_κ is λ*=κ0; in practice selected via plug-in μ̂ or mixture prior
- λmin, λmax (truncation constants) =
unspecified constants in (0,1)
- Mixture prior π (grid weights) =
discrete grid with weights π_k, unspecified
axioms (5)
- standard math Ville's inequality and e-process supermartingale properties
- domain assumption Under H0, W_t is sampled from Pt and independent of the key-generated U_t, so Y_t = U_{t,W_t} ~ Unif(0,1) and Z_t | G_{t-1} ~ Exp(1)
- domain assumption Assumption 4.1: max_w Pt(w) bounded away from 1
- domain assumption Assumption 4.2: (Pt) is m-dependent
- ad hoc to paper Surrogate reduced family q_κ(z) = (1-κ)e^{-(1-κ)z}
read the original abstract
As large language models (LLMs) are increasingly deployed, reliable and efficient mechanisms for distinguishing AI-generated text from human-written content have become essential. Statistical watermarking has emerged as a promising solution, yet most existing methods are typically fixed-horizon procedures, precluding valid early stopping in streaming generation. In this paper, we develop an efficient online watermark detection framework with anytime-valid inference based on Rao-Blackwellized e-processes, enabling recursive token-level evidence updates without storing the full history. In particular, we instantiate the framework for the Gumbel-max watermark and reduce the original token-level dependence testing problem to a pivot-induced sequential testing problem with an explicit null distribution. Theoretically, we prove anytime-valid Type I error control under arbitrary optional stopping and establish positive asymptotic log-growth under watermarking, implying consistency of the proposed stopping rules. Simulations and experiments on real LLM-generated text demonstrate efficient online detection with rigorous anytime-valid guarantees.
Figures
Reference graph
Works this paper leans on
-
[1]
(2023), ‘Watermarking of large language models’
Aaronson, S. (2023), ‘Watermarking of large language models’. https://simons. berkeley.edu/talks/scott-aaronson-ut-austin-openai-2023-08-17
2023
-
[2]
Altenschmidt, J., Altman, S., Anadkat, S. et al. (2023), ‘Gpt-4 technical report’, arXiv preprint arXiv:2303.08774
Pith/arXiv arXiv 2023
-
[3]
Bao, G., Zhao, Y., Teng, Z., Yang, L. and Zhang, Y. (2023), ‘Fast-detectgpt: Efficient zero-shot detection of machine-generated text via conditional probability curvature’, arXiv preprint arXiv:2310.05130
Pith/arXiv arXiv 2023
-
[4]
and Biderman, S
Black, S., Leo, G., Wang, P., Leahy, C. and Biderman, S. (2021), ‘Gpt-neo: Large scale autoregressive language modeling with mesh-tensorflow’, Zenodo
2021
-
[5]
Shyam, P., Sastry, G., Askell, A. et al. (2020), ‘Language models are few-shot learners’, Advances in Neural Information Processing Systems33, 1877–1901
2020
-
[6]
Lee, Y. T., Li, Y., Lundberg, S. et al. (2023), ‘Sparks of artificial general intelligence: Early experiments with gpt-4’, arXiv preprint arXiv:2303.1271210
Pith/arXiv arXiv 2023
-
[7]
Chen, C. and Wang, J.-K. (2024), ‘Online detection of llm-generated texts via sequential hypothesis testing by betting’, arXiv preprint arXiv:2410.22318
Pith/arXiv arXiv 2024
-
[8]
and Zamir, O
Christ, M., Gunn, S. and Zamir, O. (2024), Undetectable watermarks for language models, in ‘The Thirty Seventh Annual Conference on Learning Theory’, PMLR, pp. 1125–1139
2024
-
[9]
Kaskasoli, A., Stanforth, R., Matejovicova, T. et al. (2024), ‘Scalable watermarking for identifying large language model outputs’, Nature634(8035), 818–823. de Roos, D., Chugg, B., Gr¨ unwald, P. and Ramdas, A. (2025), ‘Rao-blackwellized e-variables’, arXiv preprint arXiv:2512.16759
arXiv 2024
-
[10]
and Furon, T
Fernandez, P., Chaffin, A., Tit, K., Chappelier, V. and Furon, T. (2023), Three bricks to consolidate watermarks for large language models, in ‘2023 IEEE International Workshop on Information Forensics and Security (WIFS)’, IEEE, pp. 1–6
2023
-
[11]
Mathur, A., Schelten, A., Vaughan, A. et al. (2024), ‘The llama 3 herd of models’, arXiv preprint arXiv:2407.21783 . Gr¨ unwald, P., de Heide, R. and Koolen, W. M. (2020), Safe testing,in ‘2020 Information Theory and Applications Workshop (ITA)’, IEEE, pp. 1–54
Pith/arXiv arXiv 2024
-
[12]
and Zhang, X
Guo, H., Cheng, S., Jin, X., Zhang, Z., Zhang, K., Tao, G., Shen, G. and Zhang, X. (2024), ‘Biscope: Ai-generated text detection by checking memorization of preceding tokens’, Advances in Neural Information Processing Systems37, 104065–104090
2024
-
[13]
R., Ramdas, A., McAuliffe, J
Howard, S. R., Ramdas, A., McAuliffe, J. and Sekhon, J. (2021), ‘Time-uniform, nonpara- metric, nonasymptotic confidence sequences’, The Annals of Statistics 49(2), 1055–1080
2021
-
[14]
Hu, Z., Chen, L., Wu, X., Wu, Y., Zhang, H. and Huang, H. (2023), ‘Unbiased watermark for large language models’, arXiv preprint arXiv:2310.10669
Pith/arXiv arXiv 2023
-
[15]
Huang, B., Xu, E., Ramchandran, K., Jiao, J. and Jordan, M. I. (2026), ‘Towards anytime- valid statistical watermarking’, arXiv preprint arXiv:2602.17608
arXiv 2026
-
[16]
and Laks Lakshmanan, V
Jawahar, G., Abdul-Mageed, M. and Laks Lakshmanan, V. (2020), Automatic detection of machine generated text: A critical survey, in ‘Proceedings of the 28th International Conference on Computational Linguistics’, pp. 2296–2309
2020
-
[17]
and Walsh, D
Johari, R., Koomen, P., Pekelis, L. and Walsh, D. (2022), ‘Always valid inference: Continuous monitoring of a/b tests’, Operations Research70(3), 1806–1821
2022
-
[18]
Kasneci, E., Seßler, K., K¨ uchemann, S., Bannert, M., Dementieva, D., Fischer, F., Gasser, U., Groh, G., G¨ unnemann, S., H¨ ullermeier, E. et al. (2023), ‘Chatgpt for good? on opportunities and challenges of large language models for education’, Learning and 22 Individual Differences103, 102274
2023
-
[19]
and Goldstein, T
Kirchenbauer, J., Geiping, J., Wen, Y., Katz, J., Miers, I. and Goldstein, T. (2023), A watermark for large language models, in ‘International Conference on Machine Learning’, PMLR, pp. 17061–17084
2023
-
[20]
and Su, W
Li, X., Ruan, F., Wang, H., Long, Q. and Su, W. J. (2025), ‘A statistical framework of watermarks for large language models: Pivot, detection efficiency and optimal rules’, The Annals of Statistics53(1), 322–351
2025
-
[21]
and Su, W
Li, X., Ruan, F., Wang, H., Long, Q. and Su, W. J. (2026), ‘Robust detection of watermarks for large language models under human edits’, Journal of the Royal Statistical Society Series B: Statistical Methodology88(2), 491–515
2026
-
[22]
Liu, A., Feng, B., Xue, B., Wang, B., Wu, B., Lu, C., Zhao, C., Deng, C., Zhang, C., Ruan, C. et al. (2024), ‘Deepseek-v3 technical report’, arXiv preprint arXiv:2412.19437
Pith/arXiv arXiv 2024
-
[23]
Mao, C., Vondrick, C., Wang, H. and Yang, J. (2024), ‘Raidar: generative ai detection via rewriting’, arXiv preprint arXiv:2401.12970
Pith/arXiv arXiv 2024
-
[24]
Milano, S., McGrane, J. A. and Leonelli, S. (2023), ‘Large language models challenge the future of higher education’, Nature Machine Intelligence5(4), 333–334
2023
-
[25]
Mitchell, E., Lee, Y., Khazatsky, A., Manning, C. D. and Finn, C. (2023), Detectgpt: Zero-shot machine-generated text detection using probability curvature, in ‘International Conference on Machine Learning’, PMLR, pp. 24950–24962
2023
-
[26]
Nakano, R., Hilton, J., Balaji, S., Wu, J., Ouyang, L., Kim, C., Hesse, C., Jain, S., Kosaraju, V., Saunders, W. et al. (2021), ‘Webgpt: Browser-assisted question-answering with human feedback’, arXiv preprint arXiv:2112.09332
Pith/arXiv arXiv 2021
-
[27]
and Wagner, D
Piet, J., Sitawarin, C., Fang, V., Mu, N. and Wagner, D. (2025), Markmywords: Analyzing and evaluating language model watermarks, in ‘2025 IEEE Conference on Secure and Trustworthy Machine Learning (SaTML)’, IEEE, pp. 68–91
2025
-
[28]
and Shafer, G
Ramdas, A., Gr¨ unwald, P., Vovk, V. and Shafer, G. (2023), ‘Game-theoretic statistics and safe anytime-valid inference’, Statistical Science38(4), 576–601
2023
-
[29]
and Koolen, W
Ramdas, A., Ruf, J., Larsson, M. and Koolen, W. M. (2022), ‘Testing exchangeability: Fork-convexity, supermartingales and e-processes’, International Journal of Approximate 23 Reasoning141, 83–109
2022
-
[30]
and Wang, R
Ramdas, A. and Wang, R. (2025), ‘Hypothesis testing with e-values’, Foundations and Trends® in Statistics1(1-2), 1–390
2025
-
[31]
(2022), ‘Ai bot chatgpt writes smart essays-should professors worry?’, Nature614(7947), 214–216
Stokel-Walker, C. (2022), ‘Ai bot chatgpt writes smart essays-should professors worry?’, Nature614(7947), 214–216
2022
-
[32]
and Nakov, P
Su, J., Zhuo, T., Wang, D. and Nakov, P. (2023), Detectllm: Leveraging log rank information for zero-shot detection of machine-generated text, in ‘Findings of the Association for Computational Linguistics: EMNLP 2023’, pp. 12395–12412
2023
-
[33]
Su, W., Wang, R. and Zhao, Z. (2026), ‘Online llm watermark detection via e-processes’, arXiv preprint arXiv:2602.14286
Pith/arXiv arXiv 2026
-
[34]
and Hu, X
Tang, R., Chuang, Y.-N. and Hu, X. (2024), ‘The science of detecting llm-generated text’, Communications of the ACM67(4), 50–59
2024
-
[35]
Touvron, H., Martin, L., Stone, K., Albert, P., Almahairi, A., Babaei, Y., Bashlykov, N., Batra, S., Bhargava, P., Bhosale, S. et al. (2023), ‘Llama 2: Open foundation and fine-tuned chat models’, arXiv preprint arXiv:2307.09288
Pith/arXiv arXiv 2023
-
[36]
and Wang, R
Vovk, V. and Wang, R. (2021), ‘E-values: Calibration, combination and applications’, The Annals of Statistics49(3), 1736–1754
2021
-
[37]
Xi, Z., Chen, W., Guo, X., He, W., Ding, Y., Hong, B., Zhang, M., Wang, J., Jin, S., Zhou, E. et al. (2025), ‘The rise and potential of large language model based agents: A survey’, Science China Information Sciences68(2), 121101
2025
-
[38]
Yao, S., Zhao, J., Yu, D., Du, N., Shafran, I., Narasimhan, K. and Cao, Y. (2022), ‘React: Synergizing reasoning and acting in language models’, arXiv preprint arXiv:2210.03629
Pith/arXiv arXiv 2022
-
[39]
Zhang, S., Roller, S., Goyal, N., Artetxe, M., Chen, M., Chen, S., Dewan, C., Diab, M., Li, X., Lin, X. V. et al. (2022), ‘Opt: Open pre-trained transformer language models’, arXiv preprint arXiv:2205.01068
Pith/arXiv arXiv 2022
-
[40]
and LeCun, Y
Zhang, X., Zhao, J. and LeCun, Y. (2015), ‘Character-level convolutional networks for text classification’, Advances in Neural Information Processing Systems28
2015
-
[41]
Zhou, H., Zhu, J., Su, P., Ye, K., Yang, Y., Gavioli-Akilagun, S. A. and Shi, C. (2025), ‘Adadetectgpt: Adaptive detection of llm-generated text with statistical guarantees’, 24 arXiv preprint arXiv:2510.01268
arXiv 2025
-
[42]
Zhou, H., Zhu, J., Yang, Y. and Shi, C. (2026), ‘Detecting llm-generated text with perfor- mance guarantees’, arXiv preprint arXiv:2601.06586 . 25
arXiv 2026
discussion (0)
Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.