Reducing Trace-Complexity Bias in Microservice Anomaly Scoring via Weighted Residual Service Trace Vectors

Distributed trace anomaly scoring must distinguish unusual service-path latency behavior from structural differences among traces. However, raw Service Trace Vector (STV) scores may increase with trace size and feature occupancy, potentially prioritizing structurally larger traces even when no anomaly has been injected. We evaluate this structural score dependence on held-out, non-injected traces from three TrainTicket configurations using five train–test splits per configuration. Raw STV + Isolation Forest scores show strong mean Pearson correlations with service count (r=0.868), span count (r=0.820), and the number of observed STV features (r=0.960). Local Outlier Factor closely reproduces this dependence, whereas One-Class SVM reduces its Pearson magnitude, indicating a representation–scorer interaction rather than an effect specific to Isolation Forest. We propose WR-STV, which applies clipped feature-wise standardized residuals and log-frequency reliability weighting. Validation selects k=1; therefore, the final configuration scores each trace using its maximum reliability-weighted residual. WR-STV reduces the mean Pearson correlations with service and span count to 0.296 and 0.238, respectively. Under the original controlled 30×, three-span perturbation setting, WR-STV improves within-dataset ROC-AUC from 0.605 to 0.814. An expanded 1.10×–30× study shows that subtle perturbations remain difficult and that ranking performance generally increases with severity. WR-STV requires approximately 0.022 ms of mean batched scoring time per trace, approximately 16× lower than the evaluated Isolation Forest baselines. These results support WR-STV for reducing structural score dependence in controlled STV-visible latency ranking; they do not establish performance for naturally occurring incidents or non-latency anomaly types.