Same Question, Different Lies:\texorpdfstring{\\}{ } Cross-Context Consistency (C\textsuperscript{3}) for Black-Box Sandbagging Detection

As language models grow more capable, accurate capability evaluation becomes essential for safety decisions. If models can deliberately underperform on dangerous capability evaluations---a behavior known as \emph{sandbagging}---they may evade safety measures designed for their true capability level. We introduce Cross-Context Consistency (C³), a general framework for unsupervised black-box sandbagging detection that exploits a fundamental asymmetry: when a model truly lacks capability, its confusion manifests consistently across paraphrased questions, but when a capable model feigns incompetence, its strategic choices about \emph{how} to appear weak create detectable inconsistencies. Our framework is agnostic to the specific consistency metric and aggregation method; we present a simple instantiation using embedding cosine distance and mean aggregation across samples that requires no training data or model-specific adaptation. We evaluate C³ across prompted and fine-tuned sandbagging scenarios, across instructed and more naturalistic setups, maintaining a classification signal where other black-box methods fail. Our findings show the limitations of existing sandbagging detection methods, and reveal the efficacy of consistency-checking as a detection mechanism for dangerous capabilities. Crucially, C³ and trusted monitoring detect complementary sandbagging strategies---no single method dominates---motivating defense-in-depth deployment where layered detectors cover each other's blind spots.