Viet Nguyen, Herman Bergström, Stephan Rabanser, Rahul Krishnan
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Tabular prior-fitted networks (PFNs) work well across tabular classification problems but can fail silently at deployment when distributions shift. We propose **CALD** (Calibrated Anchor-Loss Disagreement), a lightweight unsupervised monitor that triggers an alarm to warn when a PFN might fail. By finetuning the decoder to maximize disagreement with the test-set prediction while maintaining accuracy on an ID anchor set, **CALD** identifies harmful OOD batches through the resulting anchor loss, requiring no labels on test batches. We prove that the post-finetune anchor loss has a strictly lower expectation under harmful shift than under ID eval, yielding a one-sided test with a formal power guarantee. Across 14 datasets covering tabular and structured distribution shift, on both TabPFN and TabICL, **CALD** matches or outperforms baselines on harmful shifts and is markedly more resilient to benign-shift false alarms.