Prompt-Robust Language Models: Which Training Strategies Work?

Frederic Sadrieh, Michal Štefánik


Abstract
Despite their strong performance, large language models remain highly sensitive to prompt formulation. Prior work has addressed this issue through refined data construction or more complex robustness-oriented training methods. In this work, we reproduce and compare these strategies under controlled conditions, evaluating their effects on worst-case prompt sensitivity and average performance. We find that conventional instruction fine-tuning offers limited reductions in prompt sensitivity: across methods, the average gap between the best- and worst-performing prompts remains 40–57% of performance. Moreover, recent robustness-enhancing methods often fail to outperform simpler data construction strategies; in worst-case settings, at least one simple method performs better in 5 of 8 evaluations. Overall, our findings show that prompt sensitivity remains an open problem, and motivate practitioners to prioritize simpler data construction techniques before adopting more sophisticated robustness methods.
Anthology ID:
2026.findings-emnlp.1900
Volume:
Findings of the Association for Computational Linguistics: EMNLP 2026
Month:
October
Year:
2026
Address:
Budapest, Hungary
Editors:
Hung-Yi Lee, Sunipa Dev, Jessy Li, Desmond Elliott
Venue:
Findings
SIG:
SIGDAT
Publisher:
Association for Computational Linguistics
Note:
Pages:
40490–40507
Language:
URL:
https://aclanthology.org/2026.findings-emnlp.1900/
DOI:
10.18653/2026.findings-emnlp.1900
Bibkey:
Cite (ACL):
Frederic Sadrieh and Michal Štefánik. 2026. Prompt-Robust Language Models: Which Training Strategies Work?. In Findings of the Association for Computational Linguistics: EMNLP 2026, pages 40490–40507, Budapest, Hungary. Association for Computational Linguistics.
Cite (Informal):
Prompt-Robust Language Models: Which Training Strategies Work? (Sadrieh & Štefánik, Findings 2026)
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PDF:
https://aclanthology.org/2026.findings-emnlp.1900.pdf