The validity test of WKB evaluated along LOSs for PSR J1856−3754 (blue solid) and PSR J0720−3125 (red dashed). The left four panels ((a)–(d)) correspond to 1 MHz, the right four ((e)–(h)) to 1 GHz. For each frequency, the top panels show the effective transverse magnetic field ∣B_eff∣ and the effective photon-mass term ﹩| {{\rm{\Delta }}}_{\omega }^{2}| ﹩ along the LOS, while the bottom panels show the associated WKB diagnostics Q_B ≡ ∣∂_sB_eff∣/(k_em∣B_eff∣) and ﹩{Q}_{{\rm{\Delta }}}\equiv | {\partial }_{s}{{\rm{\Delta }}}_{\omega }^{2}| /{k}_{{\rm{em}}}^{2}﹩. Even at the lower end of the radio band at 1 MHz, both Q_B and Q_Δ remain below 1, and they drop by several orders of magnitude at 1 GHz.