Yevhen Kostiuk
Author directory2026
Belief Propagation in LLM World Models: Measuring Strategic Information Bias with Prediction Markets
Mykola Khandoga | Yevhen Kostiuk | Anton Polishko | Yurii Filipchuk | Kostiantyn Kozlov | Dmytro Zamriy | Artur Kiulian
Proceedings of the Fifth Ukrainian Natural Language Processing Conference (UNLP 2026)
Mykola Khandoga | Yevhen Kostiuk | Anton Polishko | Yurii Filipchuk | Kostiantyn Kozlov | Dmytro Zamriy | Artur Kiulian
Proceedings of the Fifth Ukrainian Natural Language Processing Conference (UNLP 2026)
Every information ecosystem produces beliefs that shape strategic decisions. Both human analysts and AI systems inherit the blind spots of their information sources. We show that LLMs, combined with prediction markets, function as a calibrated instrument for measuring how far ecosystem-induced beliefs fall from reality: LLMs extract the beliefs a text corpus implies, and prediction markets provide a ground truth proxy against which to quantify the error.We isolate the bias contribution of specific text through ablation: varying information context while holding the model fixed, with a contaminated model that knows actual outcomes as control. Applied to 111 Ukraine-related prediction markets (~93,000 predictions, four models), we find that English news context systematically biases territorial predictions, wrong 64–72% of the time (p 10{-6}). A contaminated model that knows actual outcomes shows the same error rate, indicating the bias originates primarily in the text. Supplementing with Ukrainian military-analytical sources partially corrects the distortion.We show that the distortion originates primarily in the sources, not the models. Consistent across four architectures, it will persist in any system that processes them and propagate into downstream decisions.
Naturalistic Measure of Social Norms Alignment
Yevhen Kostiuk | Kenneth Enevoldsen | Peter Bjerregaard Vahlstrup | Márton Kardos | Kristoffer Nielbo
The Proceedings of the Workshop on Open Reasoning Across Cultures Languages (ORACLE)
Yevhen Kostiuk | Kenneth Enevoldsen | Peter Bjerregaard Vahlstrup | Márton Kardos | Kristoffer Nielbo
The Proceedings of the Workshop on Open Reasoning Across Cultures Languages (ORACLE)
Social norms reflect shared expectations on acceptable behavior. Measuring social norms alignment remains challenging, with existing approaches typically relying on artificial closedform evaluations such as multiple-choice questionnaires or measuring agreement with predefined statements. In the context of this work, social norms alignment refers to measuring an agreement between solutions with respect to the social problem or dilemma. We propose a framework for measuring social norm alignment in naturalistic, free-form settings through solution matching. The framework enables us to measure alignment between any two dilemma responses e.g., LLMs to a human, LLMs to LLMs, or human to human. We introduce two metrics: stated and explicit agreement accuracy, and construct a dataset of 3k nontrivial social dilemmas in Danish. All dilemmas are assigned reference solutions derived from three panelists, who serve as culturally grounded judges. We evaluate the agreement of several LLMs and human responses in an interaction setup that resembles natural user–model conversations. Our results show that the proposed metrics produce consistent model rankings and reveal variation in agreement across different types of dilemmas, with higher agreement observed for topics such as neighbor conflicts and shared living situations. Overall, our work introduces a dataset and evaluation framework for studying culturally grounded social reasoning in naturalistic open-ended conversations.
One prompt is not enough: Instruction Sensitivity Undermines Embedding Model Evaluation
Yevhen Kostiuk | Kenneth Enevoldsen
Findings of the Association for Computational Linguistics: EMNLP 2026
Yevhen Kostiuk | Kenneth Enevoldsen
Findings of the Association for Computational Linguistics: EMNLP 2026
Instruction embedding models have become common among state-of-the-art models, however are evaluated using a single prompt per task. The single-point evaluation ignores a main problem of the instruction-based approach namely: sensitivity to the phrasing of the instruction. We present an empirical study of prompt sensitivity across 6 embedding models and 11 datasets. We show that reported scores misrepresent the distribution of scores over plausible prompts. The default prompt can both systematically understate or overstate performance. Furthermore, we show that the leaderboard ranking is not robust to prompt selection: a developer improve their rank by favorably selecting prompts, and under adversarial prompt selection any model can be promoted to first place. Our findings suggest that single-prompt evaluation is insufficient for instruction-tuned embedding models and that benchmarks should incorporate prompt robustness, either by evaluating over multiple prompts or by reporting sensitivity alongside point estimates.
2025
Framing the Language: Fine-Tuning Gemma 3 for Manipulation Detection
Mykola Khandoga | Yevhen Kostiuk | Anton Polishko | Kostiantyn Kozlov | Yurii Filipchuk | Artur Kiulian
Proceedings of the Fourth Ukrainian Natural Language Processing Workshop (UNLP 2025)
Mykola Khandoga | Yevhen Kostiuk | Anton Polishko | Kostiantyn Kozlov | Yurii Filipchuk | Artur Kiulian
Proceedings of the Fourth Ukrainian Natural Language Processing Workshop (UNLP 2025)
In this paper, we present our solutions for the two UNLP 2025 shared tasks: manipulation span detection and manipulation technique classification in Ukraine-related media content sourced from Telegram channels. We experimented with fine-tuning large language models (LLMs) with up to 12 billion parameters, including both encoder- and decoder-based architectures. Our experiments identified Gemma 3 12b with a custom classification head as the best-performing model for both tasks. To address the limited size of the original training dataset, we generated 50k synthetic samples and marked up an additional 400k media entries containing manipulative content.
From English-Centric to Effective Bilingual: LLMs with Custom Tokenizers for Underrepresented Languages
Artur Kiulian | Anton Polishko | Mykola Khandoga | Yevhen Kostiuk | Guillermo Gabrielli | Łukasz Gagała | Fadi Zaraket | Qusai Abu Obaida | Hrishikesh Garud | Wendy Wing Yee Mak | Dmytro Chaplynskyi | Selma Amor | Grigol Peradze
Proceedings of the Fourth Ukrainian Natural Language Processing Workshop (UNLP 2025)
Artur Kiulian | Anton Polishko | Mykola Khandoga | Yevhen Kostiuk | Guillermo Gabrielli | Łukasz Gagała | Fadi Zaraket | Qusai Abu Obaida | Hrishikesh Garud | Wendy Wing Yee Mak | Dmytro Chaplynskyi | Selma Amor | Grigol Peradze
Proceedings of the Fourth Ukrainian Natural Language Processing Workshop (UNLP 2025)
In this paper, we propose a model-agnostic cost-effective approach to developing bilingual base large language models (LLMs) to support English and any target language. The method includes vocabulary expansion, initialization of new embeddings, model training and evaluation. We performed our experiments with three languages, each using a non-Latin script—Ukrainian, Arabic, and Georgian.Our approach demonstrates improved language performance while reducing computational costs. It mitigates the disproportionate penalization of underrepresented languages, promoting fairness and minimizing adverse phenomena such as code-switching and broken grammar. Additionally, we introduce new metrics to evaluate language quality, revealing that vocabulary size significantly impacts the quality of generated text.
Evaluating LLM Judgment on Latvian and Lithuanian Short Answer Matching
Yevhen Kostiuk | Oxana Vitman | Łukasz Gągała | Artur Kiulian
Proceedings of the 1st Workshop on Nordic-Baltic Responsible Evaluation and Alignment of Language Models (NB-REAL 2025)
Yevhen Kostiuk | Oxana Vitman | Łukasz Gągała | Artur Kiulian
Proceedings of the 1st Workshop on Nordic-Baltic Responsible Evaluation and Alignment of Language Models (NB-REAL 2025)
Towards Multilingual LLM Evaluation for Baltic and Nordic languages: A study on Lithuanian History
Yevhen Kostiuk | Oxana Vitman | Łukasz Gągała | Artur Kiulian
Proceedings of the 1st Workshop on Nordic-Baltic Responsible Evaluation and Alignment of Language Models (NB-REAL 2025)
Yevhen Kostiuk | Oxana Vitman | Łukasz Gągała | Artur Kiulian
Proceedings of the 1st Workshop on Nordic-Baltic Responsible Evaluation and Alignment of Language Models (NB-REAL 2025)
Automating Alternative Generation in Decision-Making
Yevhen Kostiuk | Clara Seyfried | Chris Reed
Findings of the Association for Computational Linguistics: EMNLP 2025
Yevhen Kostiuk | Clara Seyfried | Chris Reed
Findings of the Association for Computational Linguistics: EMNLP 2025
In decision making, generating alternative solutions is crucial for solving a problem. However, cognitive biases can impede this process by constraining individual decision makers’ creativity. To address this issue, we introduce a new task for automatically generating alternatives, inspired by the process of human “brainstorming”. We define alternative options based on atomic action components and present a dataset of 106 annotated Reddit r/Advice posts containing unique alternative options extracted from users’ replies. We also introduce new metrics to assess the quality of generated components, including distinctiveness, creativity, upvote-weighted, crowd intersection, and final commit intersection scores. As a baseline, we evaluated the large language models (LLMs) LLaMa3:8b, LLaMa3.1:8b, and Gemma 2:9b on the alternative component generation task. On the one hand, models demonstrated high creativity (ability to generate options beyond what Reddit users suggested) and performed well at proposing distinct alternatives. A subset of generated components was manually evaluated and found overall useful. This indicates that LLMs might be used to extend lists of alternative options, helping decision makers consider a problem from different perspectives. On the other hand, LLMs’ outputs often failed to align with human suggestions, implying that they still tend to miss important components.