AbsenceBench: Language Models Can't Tell What's Missing

TL;DR

AbsenceBench evaluates large language models' ability to detect deliberately omitted information across various domains, revealing significant limitations in their capacity to identify missing content despite their strengths in recalling known information.

Contribution

This paper introduces AbsenceBench, a new benchmark specifically designed to assess LLMs' ability to detect missing information, highlighting a fundamental limitation of transformer attention mechanisms.

Findings

State-of-the-art models achieve only 69.6% F1-score on AbsenceBench.

Models struggle to attend to gaps in documents due to attention mechanism limitations.

Performance drops are consistent across different domains and context lengths.

Abstract

Large language models (LLMs) are increasingly capable of processing long inputs and locating specific information within them, as evidenced by their performance on the Needle in a Haystack (NIAH) test. However, while models excel at recalling surprising information, they still struggle to identify clearly omitted information. We introduce AbsenceBench to assesses LLMs' capacity to detect missing information across three domains: numerical sequences, poetry, and GitHub pull requests. AbsenceBench asks models to identify which pieces of a document were deliberately removed, given access to both the original and edited contexts. Despite the apparent straightforwardness of these tasks, our experiments reveal that even state-of-the-art models like Claude-3.7-Sonnet achieve only 69.6% F1-score with a modest average context length of 5K tokens. Our analysis suggests this poor performance stems from a fundamental limitation: Transformer attention mechanisms cannot easily attend to "gaps" in documents since these absences don't correspond to any specific keys that can be attended to. Overall, our results and analysis provide a case study of the close proximity of tasks where models are already superhuman (NIAH) and tasks where models breakdown unexpectedly (AbsenceBench).