Half-period harmonics can be identified at the cost of completeness. Top left: reliability, defined here as the fraction of retained measurements corresponding to the true rotation period rather than a harmonic, remains uniformly high across the period range probed by TESS. Our ground-truth periods are taken from A. McQuillan et al. (2014). Top right: this gain in reliability comes at the expense of completeness, which decreases steadily for more stringent cuts. Bottom left: the random forest classifier likewise identifies half-period harmonics robustly across period space, with high reliability indicating that most measurements flagged as harmonics are indeed harmonics. Bottom right: as for the true-period selection, imposing a high-probability threshold for harmonic identification reduces completeness, removing a large fraction of potential harmonic detections.