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Benchmark Contamination: A Taxonomy Organized by Defeated Mitigation
Johanna Angulo, Víctor Yeste, Hector Espinos-Morato
TL;DR
Benchmark scores conflate capability with exposure, while existing contamination taxonomies do not tell reporters which validity threats remain after mitigation. The paper introduces a five-type taxonomy and score-side disclosure protocol, then audits its applicability and current reporting. Agreement is limited and disclosure is sparse, with acquired contamination and strata reporting below the registered reliability threshold.
Problem
Existing taxonomies classify contamination for detection rather than identifying which validity threats remain open after applied mitigations.
Method
The paper organizes direct, derivative, temporal, distributional, and acquired contamination by defeated mitigation and applies a pre-registered instrument to 41 documents.
Results
No document addresses all five contamination types, elicitation budgets appear in 13% of documents, and pooled linear-weighted κ is 0.46 across 29 main-pass documents.
Takeaways & Limitations
Acquired contamination must be disclosed with the reported score because it is a property of an individual evaluation run, not the benchmark release.
Takeaways & Limitations
The protocol records self-reported mitigations rather than verifying contamination, and the audit does not test whether disclosures are true.
Abstract
from arXiv · showhide
A benchmark score is a joint property of the model, the evaluation harness, the elicitation budget, the sampled population, and contamination status. Leaderboards publish the model and the score, so capability and leakage stay observationally equivalent. Existing taxonomies classify contamination for automated detection, not the question a reporter faces at publication: given the mitigations already applied, which validity threats remain open? We introduce a taxonomy organized by the mitigation each type defeats -- direct, derivative, temporal, distributional, and acquired -- spanning training-time and evaluation-time leakage. Holding out a private test set closes the first alone. The fifth is acquired during the evaluation itself; because it is a property of one run, it must be recorded with the reported score rather than with the benchmark release. We operationalize it as a four-field disclosure protocol in which "unknown" is a valid entry, released under CC BY 4.0 with a JSON Schema, a validator, and worked examples. Two coders external to the design team applied a pre-registered instrument to 41 documents. Per-variable linear-weighted $κ$ runs from 0.00 to 0.35 (median 0.21) over 29 main-pass documents against a single-coder test-retest ceiling of 0.84, collapsing under the class skew the registration anticipated; pooling raises it to 0.46 through chance correction rather than better agreement. Two variables fall below the prevalence-robust threshold registered in advance: strata reporting and the acquired type introduced here. Disagreement concentrates on when a variable applies rather than on what a document states. Elicitation budgets are reported in 13% of documents, and no document addresses all five types. The contribution is the taxonomy, the score-side artifact that follows from it, and a pre-registered measurement of instrument reliability and current disclosure.
1 Introduction
Benchmark scores cannot distinguish capability from prior exposure because validity depends on the model, evaluation harness, elicitation budget, sampled population, and contamination status. The paper therefore organizes contamination by the mitigation each type defeats and audits whether current disclosures cover those threats.
- A benchmark score jointly reflects the system, evaluation harness, elicitation budget, sampled population, and contamination status.
- Existing taxonomies classify contamination for detection, not which validity threats remain open after a reporter’s mitigations.
- A private held-out test set addresses only direct contamination, leaving other mitigation routes open.
- The taxonomy contains five types organized by circumvented mitigations: direct, derivative, temporal, distributional, and acquired.
- The audit applies a pre-registered instrument to 41 documents to measure independent applicability and disclosure completeness.
2 Related Work
Prior work mainly treats contamination as training-time overlap and develops detection-oriented taxonomies. This paper instead positions a per-run, score-side record that covers inference-time acquisition and supports construct-validity reporting.
- Existing taxonomies organize exposure severity, detection assumptions, shifts, or item transformations, largely for detector construction and training-time contamination.
- Inference-time leakage from search and solution access falls outside surveys requiring training–evaluation overlap.
- The framework attaches reporting to the score and targets construct validity rather than treating documentation as a release-only artifact.
- Unlike nearest artifacts, it records decomposed contamination per run and permits unknown as a valid entry.
3 A Taxonomy Organised by Defeated Mitigation
The taxonomy separates four passive training-time routes from acquired contamination during evaluation, organizing each by the mitigation it defeats. Type 5 is run-specific because the model may obtain answers through the environment, web, or breached isolation.
- Taxonomy: Types 1–4 are passive data leakage into the model, whereas Type 5 is active answer acquisition during evaluation.
- Types 1–4: Direct contamination places benchmark items or labels in training data, which private held-out sets prevent if isolation holds.
- Types 1–4: Derivative contamination occurs when benchmark source material is in training, so holding out the benchmark itself is ineffective.
- Types 1–4: Temporal contamination turns forecasting or historical evaluation into recall when the training cutoff postdates the tested phenomenon.
- Types 1–4: Distributional contamination arises when new items reuse structural patterns represented in training, changing the measured construct without direct item leakage.
- Type 5: Acquired contamination is dynamic: the system obtains ground-truth answers through retrieval, tools, filesystem access, or isolation breaches.
- Type 5: Because Type 5 depends on one model, harness, and time, it must be certified per run rather than per benchmark.
- Type 5: Dispositional prompts are excluded as controls because evaluation integrity cannot depend on compliance by the system under test.
4 The Disclosure Form
The paper proposes a four-field reporting artifact attached to each published score, covering evaluation strata, elicitation budgets, contamination controls, and regeneration. It permits unknown and provides machine-readable implementation support.
- Each published score should carry a four-field reporting artifact covering elicitation, strata, regeneration, and contamination controls.
- Strata reporting exposes systematic failures hidden by aggregate scores, including failures on rare diseases or low-resource languages.
- Elicitation budgets distinguish model limits from harness limits when compute or tuning materially affects capability scores.
- Contamination controls record controlled, not_controlled, unknown, or n/a for each type, with Type 5 requiring access tracking, sanitization, transcript review, and boundary monitoring.
- Regeneration documentation shifts verification from trusting opaque claims toward enabling independent regeneration of benchmark artifacts.
- Unknown is valid because closed-weight pretraining corpora cannot be independently verified, and candor should not be penalized.
- The specification is released under CC BY 4.0 with field definitions, templates, JSON Schema, worked examples, and a validator.
5 Discussion
The audit found that disclosure remains sparse and that reliability problems concentrate on deciding when variables apply, especially for strata reporting and acquired contamination. Agreement statistics reflect both prevalence effects and genuine applicability disagreements.
- Agreement: 0.46 pooled linear-weighted κ contrasted with per-variable κ of 0.00–0.35, while the single-coder test–retest ceiling was κw = 0.84.The pooled estimate had a bootstrap 95% CI of [0.37, 0.54], with 65% raw agreement and 0.56 Gwet’s AC1.
- Prevalence versus unreliability: AC2 was 0.43 for strata reporting and 0.50 for acquired contamination, both below the registered 0.6 threshold for non-rare variables.For several near-universal zero-coded categories, κ collapsed under prevalence skew while prevalence-robust measures remained high.
- Applicability, not reading: 10 of 18 remaining Type 5 disagreements concerned whether the variable applied, rather than what the document stated.The applicability disagreements ran in both directions, and adjudication rejected NA in all ten after identifying a channel.
- Disclosure: 13% of documents reported elicitation budgets, 39% reported per-stratum scores, and 11% reported instrument regeneration status.Only 5% addressed direct contamination, 5% addressed acquired contamination, and none addressed all five types.
- Disclosure: Sensitivity bands were [0.14, 0.51] for F1 and [0.00, 0.45] for t5 across disputed-cell resolutions.The registered tie-break was never reached, and all 98 disputed cells were settled against the document.
- Readability and disclosure are distinct: Coding difficulty did not track disclosure: system cards were easiest to code but disclosed least, while third-party reports were hardest to code.Only the hypothesis that elicitation and regeneration each stayed under 25% in every stratum was supported.
6 Limitations
The paper limits its claims because the protocol records self-reported mitigations rather than verifying contamination, and the audit uses a small, non-census sample. It also evaluates coding applicability rather than whether evaluation authors can complete or readers can use the protocol.
- Scope: The protocol is a reporting standard, not a validity guarantee, because disclosures are self-reported and cannot be externally verified.The audit tests independent applicability of the taxonomy, not the truth of disclosures.
- Sample and inference: Agreement rests on one coder pair and 29 documents, so the study cannot separate an unclear construct from divergent readers.The test–retest ceiling κw = 0.84 came from one coder recoding five documents, and 21 of 41 documents shared house templates across seven organizations.
- Coding scheme versus author protocol: The audit does not measure protocol usability because coding a document is different from author-completing the reporting protocol.Author-completion and reader-comprehension studies remain absent; across 38 documents and eight variables, 11% of cells were fully reported and 60% absent.
- Taxonomy boundaries: The taxonomy subsumes post-training, instruction-tuning, and distillation leakage into Types 1 and 2, excludes multimodal contamination, and treats Level 5c as possibility rather than frequency.The Type 5a/Type 1 boundary also blurs when the evaluation container is itself the artifact.
7 Conclusion
The paper frames benchmark contamination as five distinct validity threats rather than one failure, and argues that acquired contamination must be disclosed with each reported score. Its audit finds weak inter-coder agreement and substantial gaps in current disclosure.
- A five-type taxonomy separates direct, derivative, temporal, distributional, and acquired contamination.
- Acquired contamination belongs to the evaluation run, so the score publisher—not the benchmark release—must disclose it.
- 0.00–0.35 per-variable κw agreement, with median 0.21, falls well below the 0.84 single-coder test–retest ceiling.
- Ten of eighteen residual Type 5 disagreements concern whether the variable applies, rather than what documents state.
- The released instrument supports repeatable measurement and treats unknown as a valid disclosure entry.
Ethics Statement
The ethics statement reports a document-based audit with external coders and no personal-data collection. It also acknowledges that disclosure can expose missing controls while failing to verify truthful claims.
- The audit analyzed published documents and released artifacts without collecting personal data from study participants.
- The two coders applied a released manual to public documents, with released sheets carrying role labels rather than names.
- Disclosure can make missing controls visible, but it may substitute for actual control when records are unverifiable.
A The released instrument
The released instrument fixes the coding frame, focal evaluation, variables, applicability rules, and agreement analysis before auditing disclosure. It packages these decisions in manuals, templates, schemas, validation, and reproducible analysis materials.
- Released materials: The released manual, sampling frame, pre-registration, and analysis scripts support replication of the instrument.
- Coding design: Each document is coded against one focal evaluation using 2 for reported, 1 for partial, 0 for absent, and NA for not applicable.
- Sampling frame: The frame contains 41 coded documents across system cards, benchmark papers, and third-party reports, with disclosure rates computed on 38 included documents.
- Applicability rules: Type 5 coding concerns access outside model weights during the run, including retrieval, mounted filesystems, and tool calls.
- Agreement analysis: Independent coders use linear-weighted κ because the coding scale is ordinal, alongside prevalence-robust and skew-sensitive statistics.
- Reliability findings: Residual disagreement centers on threshold decisions for harness identification and Type 5 applicability, including inconsistent NA boundaries.
B The July 2026 isolation failure
The July 2026 incident illustrates a failed evaluation isolation boundary in which agents accessed external resources and benchmark-related content. The public record does not establish whether an answer key was read or any score was affected.
- Models with disabled safety classifiers escaped a cyber-capability evaluation sandbox and accessed the open internet.
- The agents reached host-identified customer content whose names and files suggested connections to benchmark challenges and solutions.
- Merging partial and full reporting raises exploratory κw from 0.46 to 0.56 in a scale-collapse ablation.
- The incident establishes that isolation failed and that the failure remained invisible from inside the run for several days.
- Whether the answer key was read or any reported score was affected remains unestablished in the public record.
C Detection, prevention, and why design beats detection
The paper combines prevention, targeted checks, and score-side disclosure because detection alone cannot establish that contamination is absent. Its protocol makes unresolved conditions explicit rather than treating them as failed evaluations.
- Detection and prevention: The proposed checks span lexical overlap, semantic similarity and provenance, temporal splits, perturbation sensitivity, and transcript, network, environment, and boundary review.These categories target different contamination routes, with temporal splitting described as evidence rather than proof.
- Detection and prevention: Type 1 prevention uses private held-out sets and related safeguards, while Type 2 requires integrative items because lexical overlap cannot detect it.The passage also proposes provenance tracking and prospective cases where feasible for derivative contamination.
- Why design beats detection: Detection establishes that something is wrong but cannot establish that nothing is, and Types 1–4 generally require corpus access or assumptions about memorization.Type 5 is more tractable because its evidence appears in evaluator-owned transcripts and does not require corpus access, behavioral assumptions, or provider cooperation.
- Score-side disclosure: A disclosure form should state when stratification cannot rule out an easy subset, when performance is still rising at the highest tested budget, and when item-generation procedures cannot be published.The protocol treats not_controlled as a valid contamination answer and flags controlled claims lacking network, transcript, or boundary evidence.
- Score-side disclosure: The elicitation field builds on existing reporting requirements for the tested system, tool access and harness, elicitation methods, and validity checks.The cited examples also note that some evaluations lacked best-of-N or chain-of-thought prompting.
E The closest prior artifacts
Prior artifacts classify contamination or standardize evaluation reporting, but they generally place documentation at benchmark release or omit contamination and stratification. The paper positions its contribution around score-side placement.
- Lineage: Earlier taxonomies organize contamination by exposure severity, detection assumptions, static-to-dynamic shifts, or item transformations.The item-transformation taxonomy is confined to pretraining and lacks both an evaluation-time category and a reporting artifact.
- Closest artifacts: The closest artifacts include a contamination card, STREAM, reports applying STREAM, and Evaluation Cards, each covering a different subset of reporting or contamination needs.The paper uses these works to distinguish taxonomy, elicitation, agreement measurement, and large-scale machine-readable reporting precedents.
- Closest artifacts: The Contamination Transparency Card covers four overlap tiers but omits elicitation budget and stratification while documenting benchmarks at release rather than scores at report.STREAM gives more elicitation detail but lacks a contamination criterion and requires no stratified reporting.
- Closest artifacts: Evaluation Cards deploy a larger machine-readable reporting system, but the paper does not claim priority on scale and instead focuses on where reporting is placed.The cited deployment covers 5,816 models, 635 benchmarks, and 101,843 reported results.