A Unified Uncertainty Representation for Graph Neural Networks via Doubly-Spectral Stochastic Expansion

Reliable deployment of graph neural networks requires calibration, out-of-distribution (OOD) detection, and robustness to distribution shift, yet existing methods typically address these requirements with separate models and objectives. A key obstacle is the lack of a single learned representation whose structure can serve all three tasks through different readouts. Inspired by spectral approaches in graph signal processing and polynomial chaos theory, we model uncertainty-bearing node embeddings as random graph signals: graph Fourier filters capture structural variation, while a scalar orthogonal-polynomial chaos coordinate captures latent stochastic variation. This yields a \emph{doubly-spectral stochastic} (DSS) expansion in which the mean coefficient carries evidence for classification and energy-based OOD scoring, higher-order coefficients encode structured logit variation, and quadrature averaging turns that variation into a calibration-sensitive predictive distribution. Calibration thus uses the quadrature-averaged predictive distribution, OOD detection uses the mean-logit energy score, and distribution-shift robustness uses the corresponding DSS branch as a regularized spectral residual when paired with a deterministic encoder. Under mild conditions this representation universally approximates Gaussian-latent random graph signals with exponentially decaying truncation error. The resulting model, \emph{DSS-GNN} (Doubly-Spectral Stochastic GNN), can be used standalone or as a residual branch alongside a deterministic encoder (\emph{DSS-Hybrid}). In standalone form, DSS-GNN achieves the lowest Brier score among the compared uncertainty-aware baselines on all 14 node classification benchmarks without post-hoc correction; in hybrid form, DSS-Hybrid achieves the best AUROC on most node-OOD settings, competitive cross-graph OOD performance, and the strongest shifted accuracy among compared baselines on all 7 GOOD concept-shift benchmarks under standard ERM training.