MMLU-Pro Under Admissible Interface Perturbations: A Three-Family Stress Test with Prediction-Space Canonicalization

This technical note presents a controlled three-family stress test on MMLU-Pro under admissible answer-interface perturbations (baseline, choiceₛhuffle, labelᵣemap). Three model families are evaluated on a locked subset of 140 items spanning 14 categories. A methodological caveat affecting prediction comparability is explicitly identified and corrected through full prediction-space canonicalization with exact decoder recovery. The family-level perturbation signatures remain unchanged after canonicalization, while part of the raw prediction-level instability is reduced but not eliminated. The result is diagnostic and local in scope: under the tested setup, MMLU-Pro remains locally usable but exhibits interface-sensitive evaluative closure and limited global neutrality under the tested perturbations.