Comparison of the true and derived stellar masses with (left; intS_Z_true) and without (right; intS_Z) fixing the stellar metallicity. Different SFH models are shown in different colours, as indicated in the legend. The right panel also displays the fitted Z_★ values for each SFH model, while the lower panels show the differences relative to the true quantities. When metallicity is treated as a free parameter, M_★ is overestimated by 0.2 dex for both parametric and non-parametric SFH models. This bias reflects the age–metallicity degeneracy, which tends to yield posteriors with underestimated metallicity and overly high fractions of old stars relative to the true values.