One Token Away from Collapse: The Fragility of Instruction-Tuned Helpfulness
Researchers demonstrate that instruction-tuned large language models suffer severe performance degradation when subject to simple lexical constraints like banning a single punctuation mark or common word, losing 14-48% of response quality. This fragility stems from a planning failure where models couple task competence to narrow surface-form templates, affecting both open-weight and commercially deployed closed-weight models like GPT-4o-mini.