Spectral variability of PKS 2155−304. Left: photon index vs. photon flux, showing a positive correlation (ρ ≈ 0.3), weak but robust, indicative of an atypical softer-when-brighter trend. Right: phase-resolved spectral evolution folded at the 1.7 yr period, with the 17.4-year baseline variability (green dots) consistent with an achromatic trend given the lack of significant correlation between photon index and phase (ρ = 0.036, p = 0.60), and the two consecutive 30-day bins of the extreme spectral hardening event reported by Dinesh et al. (2025, red dots).