Evaluating the behavioral frontier of deep learning (DL) systems is crucial for understanding their generalizability and robustness. However, boundary testing is challenging due to their high-dimensional input space. Generative artificial intelligence offers a promising solution by modeling data distribution within compact latent space representations, thereby facilitating finer-grained explorations. In this work, we introduce MIMICRY, a novel black-box system-agnostic test generator that leverages these latent representations to generate frontier inputs for the DL systems under test. Specifically, MIMICRY uses style-based generative adversarial networks trained to learn the representation of inputs with disentangled features. This representation enables embedding style-mixing operations between a source and a target input, combining their features to explore the boundary between them. We evaluated the effectiveness of different MIMICRY configurations in generating boundary inputs for four popular DL image classification systems. Our results show that manipulating the latent space allows for effective and efficient exploration of behavioral frontiers. As opposed to a model-based baseline, MIMICRY generates a higher quality frontier of behaviors which includes more and closer inputs. Additionally, we assessed the validity of these inputs, revealing a high validity rate according to human assessors.
