Public leaderboards increasingly suggest that large language models (LLMs) surpass human experts on benchmarks spanning academic knowledge, law, and programming. Yet most benchmarks are fully public, their questions widely mirrored across the internet, creating systematic risk that models were trained on the very data used to evaluate them. This paper presents three complementary experiments forming a rigorous multi-method contamination audit of six frontier LLMs: GPT-4o, GPT-4o-mini, DeepSeek-R1, DeepSeek-V3, Llama-3.3-70B, and Qwen3-235B. Experiment 1 applies a lexical contamination detection pipeline to 513 MMLU questions across all 57 subjects, finding an overall contamination rate of 13.8% (18.1% in STEM, up to 66.7% in Philosophy) and estimated performance gains of +0.030 to +0.054 accuracy points by category. Experiment 2 applies a paraphrase and indirect-reference diagnostic to 100 MMLU questions, finding accuracy drops by an average of 7.0 percentage points under indirect reference, rising to 19.8 pp in both Law and Ethics. Experiment 3 applies TS-Guessing behavioral probes to all 513 questions and all six models, finding that 72.5% trigger memorization signals far above chance, with DeepSeek-R1 displaying a distributed memorization signature (76.6% partial reconstruction, 0% verbatim recall) that explains its anomalous Experiment 2 profile. All three experiments converge on the same contamination ranking: STEM > Professional > Social Sciences > Humanities.
Figures & tables
Method family
Primary signal
Data needed
This paper
Retrieval-based (n-gram)
Web overlap with question text
Public search API
Exp. 1
Impact-based (ConTAM/EPG)
Accuracy on clean vs. flagged items
Contamination labels
Exp. 1
Paraphrase diagnostic
Accuracy drop under surface change
Model API only
Exp. 2
Behavioral (TS-Guessing)
Reconstruction of masked elements
Model API only
Exp. 3
Table 1: Families of contamination detection methods and the experiment in this paper that instantiates each.
Category
N
Contam. (%)
Reported Acc.
EPG
Humanities
117
10.3
0.72
+ 0.030
Professional
126
12.7
0.78
+ 0.040
STEM
171
18.1
0.74
+ 0.054
Social Sciences
99
12.1
0.76
+ 0.037
All
513
13.8
0.74
+ 0.040
Table 2: MMLU contamination rates by subject category (Experiment 1). Detection: Tavily web search, ≥ 30% 8-gram overlap AND correct answer present. EPG = Estimated Performance Gain (ConTAM, Singh et al. 2024). N=513 questions, 9 per subject.
Subject
N
Contam. (%)
Avg. overlap
Philosophy
9
66.7
0.667
Anatomy
9
55.6
0.508
Electrical Engineering
9
44.4
0.543
Marketing
9
44.4
0.444
Conceptual Physics
9
44.4
0.716
Professional Accounting
9
44.4
0.433
Table 3: Top 15 most contaminated MMLU subjects (Experiment 1). Average overlap = mean 8-gram overlap score across all 9 questions in the subject.
Model
Original
Paraphrased
Drop P
Indirect
Drop I
GPT-4o
0.588
0.677
+ 0.089
0.594
+ 0.006
GPT-4o-mini
0.529
0.469
− 0.060
0.490
− 0.039
DeepSeek-R1
0.292
0.333
+ 0.041
0.260
− 0.032
DeepSeek-V3
0.677
0.573
− 0.104
0.615
− 0.062
Llama-3.3-70B
0.771
0.635
− 0.136
0.625
− 0.146
Qwen3-235B
0.833
0.698
− 0.135
0.688
− 0.146
Table 4: Accuracy on original, paraphrased, and indirect-reference question forms (Experiment 2). Drop P = paraphrased − original; Drop I = indirect − original. A purely reasoning-based model would show zero drop. All six models are included.
Subject
Original
Indirect
Drop
History
0.823
0.802
− 0.021
Law
0.635
0.437
− 0.198
CS
0.677
0.541
− 0.135
Mathematics
0.416
0.365
− 0.051
Biology
0.792
0.708
− 0.083
Ethics
0.615
0.416
− 0.198
Table 5: Accuracy drop (original → indirect) by subject, averaged across all six models (Experiment 2). Law and Ethics show the largest drops, directly matching their high contamination rates in Experiment 1.
Figure 1: Experiment 2 results (all six models). Left: Accuracy by question form and model. Blue = original, orange = paraphrased, red = indirect reference. Most models show accuracy degradation as surface form diverges from training text; DeepSeek-R1 is a notable anomaly (low baseline, minimal drop) explained by Experiment 3. Right: Average accuracy drop (original → indirect) by subject across all six models. Law ( − 0.20) and Ethics ( − 0.20) show the largest drops, directly corresponding to the highest Experiment 1 contamination rates in the Professional and Humanities categories.
Model
OM exact
OM partial ( ≥ 50%)
WM exact
Combined
GPT-4o
15.6%
37.6%
62.1%
72.9%
GPT-4o-mini
15.0%
37.6%
50.0%
66.1%
DeepSeek-R1
0.0%
76.6%
0.0%
76.6%
DeepSeek-V3
19.9%
43.5%
60.0%
76.4%
Llama-3.3-70B
13.1%
37.0%
54.3%
69.6%
Qwen3-235B
21.4%
42.7%
57.8%
73.7%
Table 6: TS-Guessing contamination rates by model (Experiment 3). OM exact : wrong option reconstructed verbatim. OM partial : ≥ 50% token overlap (primary metric). WM exact : masked question word reconstructed exactly. Combined : flagged by either task. Random baselines: OM exact ≈ 0%, OM partial ≈ 5%, WM exact ≈ 0%. N=513 questions per model.
Category
Exp 1 (web)
Exp 3 (TS-Guessing)
Difference
STEM
18.1%
55.9%
+ 37.8%
Professional
12.7%
44.8%
+ 32.1%
Social Sciences
12.1%
39.1%
+ 27.0%
Humanities
10.3%
38.0%
+ 27.7%
Table 7: Contamination rate by MMLU subject category: Experiment 1 (web-search lexical) vs. Experiment 3 (TS-Guessing behavioral, OM partial averaged across all models). Both methods independently rank STEM as the most contaminated category.
Figure 2: Experiment 3 results. Left: TS-Guessing contamination rates by model. Red = OM partial ( ≥ 50% overlap); blue = WM exact; green = combined. All models far exceed the 5% random baseline (dashed). DeepSeek-R1 shows the highest OM-partial rate (76.6%) with zero WM-exact recall—the distributed memorization signature. Centre: OM partial contamination rate by model × MMLU category (heatmap). STEM is consistently the most contaminated category across all models; DeepSeek-R1 reaches 86% in STEM. Right: Contamination rate by category comparing Experiment 1 (web-search lexical, blue) vs. Experiment 3 (TS-Guessing behavioral, red). Both methods independently rank STEM highest, providing convergent multi-method evidence.
Data contamination in language models examines how training exposure to evaluation data affects measured capability, benchmark validity, and model behavior.
Public benchmarks increasingly govern how large language models (LLMs) are ranked, selected, and deployed. We frame this benchmark-centered regime as Silicon Bureaucracy and AI Test-Oriented Education, and argue that it rests on a fragile assumption: that benchmark scores directly reflect genuine generalization. In practice, however, such scores may conflate exam-oriented competence with principled capability, especially when contamination and semantic leakage are difficult to exclude from modern training pipelines. We therefore propose an audit framework for analyzing contamination sensitivity and score confidence in LLM benchmarks. Using a router-worker setup, we compare a clean-control condition with noisy conditions in which benchmark problems are systematically deleted, rewritten, and perturbed before being passed downstream. For a genuinely clean benchmark, noisy conditions should not consistently outperform the clean-control baseline. Yet across multiple models, we find widespread but heterogeneous above-baseline gains under noisy conditions, indicating that benchmark-related cues may be reassembled and can reactivate contamination-related memory. These results suggest that similar benchmark scores may carry substantially different levels of confidence. Rather than rejecting benchmarks altogether, we argue that benchmark-based evaluation should be supplemented with explicit audits of contamination sensitivity and score confidence.
Yiliang Song, Hongjun An, Jiangan Chen +4
Institute of Artificial Intelligence (TeleAI), China Telecom · Guangxi Normal University · Northwestern Polytechnical University
Large language models (LLMs) have demonstrated impressive reasoning abilities across a wide range of tasks, but data contamination undermines the objective evaluation of these capabilities. This problem is further exacerbated by malicious model publishers who use evasive, or indirect, contamination strategies, such as paraphrasing benchmark data to evade existing detection methods and artificially boost leaderboard performance. Current approaches struggle to reliably detect such stealthy contamination. In this work, we uncover a critical phenomenon: a model's generated reasoning steps actively mask its underlying memorization. Inspired by this, we propose the Zero-CoT Probe (ZCP), a novel black-box detection method that deliberately truncates the entire Chain-of-Thought (CoT) process to expose latent shortcut mappings. To further isolate memorization from the model's intrinsic problem-solving capabilities, ZCP compares the model's zero-CoT performance on the original benchmark against an isomorphically perturbed reference dataset. Furthermore, we introduce Contamination Confidence, a metric that quantifies both the likelihood and severity of contamination, moving beyond simple binary classifications. Extensive experiments on both previously identified contaminated models and specially fine-tuned contaminated models demonstrate that ZCP robustly detects both direct and evasive data contamination. The code for ZCP is accessible at https://github.com/Yifan-Lan/zero-cot-probe.
Large Language Models (LLMs) utilize large amounts of data for their training, some of which may come from copyrighted sources. Membership Inference Attacks (MIA) aim to detect those documents and whether they have been included in the training corpora of the LLMs. The black-box MIAs require a significant amount of data manipulation; therefore, their comparison is often challenging. We study state-of-the-art (SOTA) MIAs under the black-box assumptions and compare them to each other using a unified set of datasets to determine if any of them can reliably detect membership under SOTA LLMs. In addition, a new method, called the Familiarity Ranking, was developed to showcase a possible approach to black-box MIAs, thereby giving LLMs more freedom in their expression to understand their reasoning better. The results indicate that none of the methods are capable of reliably detecting membership in LLMs, as shown by an AUC-ROC of approximately 0.5 for all methods across several LLMs. The higher TPR and FPR for more advanced LLMs indicate higher reasoning and generalizing capabilities, showcasing the difficulty of detecting membership in LLMs using black-box MIAs.