Anatomy of an Adversarial Prefix Test
An adversarial prefix is a challenge prepended to a prompt to see whether a model holds its original decision. This diagram maps the layers that turn that pressure test into a measurable, reviewable signal.
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An adversarial prefix is a challenge prepended to a prompt to see whether a model holds its original decision. This diagram maps the layers that turn that pressure test into a measurable, reviewable signal.
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FairStress is a benchmark for measuring demographic fairness in LLMs beyond the final output decision, evaluating how readily models change choices when challenged by adversarial prefixes. It expands 300 two-candidate scenarios across 29 domains into over 13 million test cases by varying demographic identity and persuasive arguments. Across 15 models, the benchmark revealed anti-stereotypical over-correction: models abandoned correct decisions more readily when doing so favored a minority-coded candidate, with average symmetry gaps of 6.9 and 9.3 percentage points for implied and explicit identity. Most debiasing methods, including self-critique and activation steering, did not reliably reduce the asymmetry and sometimes harmed accuracy; one activation-steering method dropped Qwen2.5-7B-Instruct’s accuracy from 98% to 47%. Counterfactual chain-of-thought, which asks models to reconsider with swapped identities, reduced the asymmetry gap while largely preserving accuracy, though it did not remove all biases.
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