arXiv:2505.21857v2 Announce Type: replace-cross Abstract: Foundation model ensembles can improve accuracy and robustness, but full fine-tuning and parameter-space merging are often expensive or impossible when candidate models have different architectures. We propose Optimizable Model Averaging (OMA), a black-box approach that learns nonnegative model-combination weights, one scalar coefficient per model, to average the candidate models' predictive probabilities, leaving all model parameters frozen, by minimizing regularized predictive entropy. A central challenge is that minimizing predictive entropy can favour confidently incorrect models: increased confidence need not imply improved accuracy or calibration. We therefore examine regularization as a mechanism for controlling adaptation and empirically evaluate its effect on calibration, observing lower expected calibration error in selected configurations, alongside better performance-calibration trade-offs than model output averaging. As foundation models continue to advance, these approaches will enable the incorporation of future, potentially much better models to improve performance on challenging text and image classification tasks.