ZeroSiam: An Efficient Siamese for Test-Time Entropy Optimization without Collapse

Test-time entropy minimization helps adapt a model to novel environments and incentivize its reasoning capability, unleashing the model's potential during inference by allowing it to evolve and improve in real-time using its own predictions. However, pure test-time entropy minimization can favor non-generalizable shortcuts, such as inflating the logit norm and driving all predictions to a dominant class to reduce entropy, risking collapsed solutions (e.g., constant one-hot outputs) that trivially minimize the objective without meaningful learning. In this paper, we introduce ZeroSiam, an efficient asymmetric Siamese architecture tailored for test-time entropy minimization. ZeroSiam prevents collapse through asymmetry learning, which is efficiently achieved by a learnable predictor and a stop-gradient operator before the classifier. Beyond the collapse prevention effect, we provide empirical and theoretical evidence that ZeroSiam also absorbs and regularizes biases at testing, enhancing TTA effectiveness even when no collapse occurs. Despite its simplicity, extensive results show that ZeroSiam can avoid collapse and perform more stably over prior methods using negligible overhead, demonstrating efficacy on both vision and language tasks across challenging test scenarios and diverse models, e.g., tiny based models that are particularly collapse-prone.