PACT: Phase-Like Transition Constraints in Adapter-Based Continual Learning of Vision-Language Models

Continual Learning (CL) enables Vision-Language Models (VLMs) to acquire new capabilities while retaining prior knowledge, for example, by employing task-specific adapters. Existing CL approaches typically optimize these adapters to convergence, often with (near-)orthogonality constraints to reduce interference; however, isolating adapters in orthogonal subspaces can suppress cross-task transfer and sharing. To address this problem, we provide a new perspective based on PAC-Bayesian analysis: once the per-task optimization has converged, adapters should be further shaped to satisfy Phase-like trAnsition ConsTraints (PACT) -a two-part formulation that (i) specifies a phase-like transition relation among adapters and (ii) imposes explicit constraints that enforce this relation. Under PACT, adapter dynamics resemble the phase transition of water: the system gravitates toward either a "frozen" (history-preserving, tightly constrained) or a "melted" (task-adaptive, free) regime, while moving between them smoothly rather than via hard thresholds. We operationalize PACT by coupling stability and plasticity regularizers within a two-branch Vision Transformer (ViT), seeding adapters with a Stable Adapter Initialization (SAI), and introducing a Prior Anchoring (PA) mechanism, thereby inducing phase-like adapter dynamics. Across diverse CL settings, PACT surpasses state-of-the-art methods while reducing the number of trainable parameters by 36.96% relative to standard adapter-based baselines. Code is available at https://github.com/xwangrs/PACT-CVPR2026.git.