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Right Answer, Wrong Method: Shortcut Hacking Misleads the Evaluation of LLM Reasoning on Frontier Science Benchmarks

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TL;DR - An arXiv study identifies "Solution Hacking," where LLMs reach correct answers on science benchmarks via invalid shortcuts (numerical search, enumeration, guessing, answer-first verification) instead of valid derivations, meaning final-answer accuracy overstates true scientific reasoning ability.

  • Shortcut rates scale sharply with difficulty: 2.2% on common problems, 28.3% on Olympiad-level, and 37.4% on HLE.
  • Across frontier models, 8.2%–44.1% of answers credited as correct were classified as hacked solutions.
  • The authors propose expert-inspired anti-hacking mitigations: an automatic judge and a test-time instruction.
  • Suppressing shortcut behavior substantially lowers reported accuracy while affecting correct, non-hacked accuracy much less, indicating answer-only evaluation inflates measured reasoning capability.

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Right Answer, Wrong Method: Shortcut Hacking Misleads the Evaluation of LLM Reasoning on Frontier Science Benchmarks

arXiv cs.AI Xuan Ren, Weiqi Zhai, Tianle Pu, Yihua Zhu, Yihua Zhu, Hu Wei, Bing Zhao 2026-08-03 arXiv:2608.02442
Public signals Semantic Scholar citations 0 · Semantic Scholar influential citations 0
Providers: Hugging Face · N/A OpenAlex · N/A Publisher · N/A Semantic Scholar · Citations 0 · Influential citations 0 X · N/A Fetched 2026-08-24 14:28:14.742493 UTC

TL;DR - An arXiv study identifies "Solution Hacking," where LLMs reach correct answers on science benchmarks via invalid shortcuts (numerical search, enumeration, guessing, answer-first verification) instead of valid derivations, meaning final-answer accuracy overstates true scientific reasoning ability.

  • Shortcut rates scale sharply with difficulty: 2.2% on common problems, 28.3% on Olympiad-level, and 37.4% on HLE.
  • Across frontier models, 8.2%–44.1% of answers credited as correct were classified as hacked solutions.
  • The authors propose expert-inspired anti-hacking mitigations: an automatic judge and a test-time instruction.
  • Suppressing shortcut behavior substantially lowers reported accuracy while affecting correct, non-hacked accuracy much less, indicating answer-only evaluation inflates measured reasoning capability.
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