The Cepheid distance scale relies on a period–luminosity (P–L) relation that is known to depend on metallicity, but both the size and even the sign of this dependence have been debated, with different studies finding metal-rich Cepheids either brighter or fainter at fixed period (Sasselov et al. 1997; Storm et al. 2004). Within ΛCDM this metallicity sensitivity propagates directly into the calibration of Type Ia supernovae and thus into local H0, so the fact that plausible choices for the metallicity correction can shift H0 by several percent means that resolving the Hubble tension requires exceptionally tight control of stellar-physics systematics that current models and data have not yet definitively achieved (Efstathiou 2020; Ripepi et al. 2022).