Comparison of the observed and modelled bolometric light curve and photospheric velocity of SN 2021tsz. Top: Observed bolometric light curve alongside the best-fit SNEC model (solid line), obtained by simultaneously fitting the light curve and photospheric velocity up to 70 days. The dashed line shows the model without CSM. The shaded region indicates the fitting interval. Bottom: Model photospheric velocities plotted against observed values, derived from the minima of the Fe IIλ5169 absorption line. An explosion epoch offset of −0.9 days is applied.