StepFun's P²-MOPD beats multi-teacher distillation with half the training steps

teortaxesTex · x · 2026-11-12

StepFun's new paper P²-MOPD improves multi-teacher on-policy distillation (MOPD).

Key idea: unlike task arithmetic, which stays within the span of frozen student/expert parameters, P²-MOPD treats the linear combination of student and expert parameters as a single differentiable, trainable model, jointly optimizing mixing coefficients and student weights — expert logits supervise behavior while expert weights act as differentiable parameter priors.

Results: across four domains at 1.7B and 4B scales, it reaches more balanced recovery faster than MOPD from the same initialization, surpassing MOPD's final average recovery in half the training steps. At end of training the 1.7B model hits 94.7% average recovery vs 80.9% for MOPD. Code is open-sourced on GitHub.

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