UCSD paper: context rot is attention competition, not length — rival statements slash accuracy to 11-21%

dair_ai · x · 2026-11-04

A UC San Diego paper shows long-context retrieval failures aren't about length. Adding 32K neutral filler tokens left Qwen2.5-1.5B at 97-100% accuracy, but a few same-format statements with rival values dropped accuracy to 0.11-0.21, multiplying loss 9-110x. Across 126,473 trials, attention mass left on the evidence explained up to 58% of loss variance vs at most 8% for length. A masked-filler protocol confirms causality: masking fillers hurt MMLU but in retrieval masking cost 46 points while visible filler cost none. Context rot is attention competition. Code and data released.

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