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The paper introduces NOLLI, a benchmark specifically designed to diagnose the performance gaps between English and Korean in puzzle-solving tasks, comprising 15 types of puzzles and 7,500 items. By behaviorally calibrating difficulty rather than relying solely on task size, the authors reveal that while overall accuracy between matched English and Korean models is statistically equivalent, significant performance disparities arise in writing-system-intensive tasks. Notably, the Korean Cipher task exhibits a stark accuracy drop compared to English, highlighting the nuanced challenges faced by models in processing Korean sub-syllabic structures.
Korean models struggle significantly with writing-system-intensive tasks, revealing a 68.7 pp accuracy gap in the Korean Cipher compared to English.
We introduce NOLLI, a procedurally generated English-Korean puzzle benchmark designed to diagnose where Korean performance gaps arise. It comprises 15 puzzle types (25 tasks; 7,500 items), with every instance seed-regenerable, verified to have a unique solution, and scored deterministically. Rather than equating harder with bigger, we calibrate difficulty behaviorally, tuning each generator until a fixed reference model lands in target accuracy bands. Its three-level design combines matched direct translations, script adaptations over Hangul jamo (sub-syllabic letters), and Korean-only tasks grounded in Korean culture or orthography. We evaluate 15 frontier, open-weight, and Korean-developed models; among the 12 above a 3% overall-accuracy floor, matched English-Korean accuracy is statistically equivalent within a +/- 10 pp margin (TOST), suggesting little cost from presentation language alone. Writing-system-intensive tasks show sharper gaps: Korean Cipher falls behind English by up to 68.7 pp, whereas Cryptarithmetic over the same jamo shows no systematic penalty, and Jamo Composition accuracy predicts Korean Cipher accuracy. These contrasts are diagnostic rather than causal, consistent with difficulty in multi-step sub-syllabic execution. Korean-only tasks separate rule-application deficits, which vary in sign, from a Kinship deficit positive in all 12. Finally, a salient size measure fails to grow from Easy to Hard in 7 of 15 types, making structural size an unreliable proxy for empirical difficulty.
