VLMs Trace Without Tracking: Diagnosing Failures in Visual Path Following

Abstract

Vision-language models (VLMs) perform strongly on multimodal benchmarks, yet their ability to reliably follow continuous visual structure remains unclear. We focus on path following, a basic operation that requires following a selected path through successive visual continuations. Before considering complex factors such as crossings or overlaps, we first test whether current VLMs can robustly follow a simple continuous line under controlled settings that remove such confounds. Across frontier models, we find that tracing is already fragile, with this vulnerability manifesting as unstable path preservation in the presence of other nearby paths. Analysis of internal representations shows that information needed to determine the correct path continuation becomes weaker, with this degradation already emerging in the visual encoding stage. Common remedies do not resolve this weakness: model scaling provides limited improvement, additional reasoning provides uneven gains at substantial cost, and explicit tracing instructions fail to stabilize performance. The same path-preservation failures and sensitivity to nearby paths persist in visually richer and more realistic settings, suggesting that current VLMs lack a robust mechanism for following a selected path.