Semileptonic $\tau$ decays: powerful probes of non-standard charged current weak interactions

When looking for heavy ($\mathcal{O}$(few TeV)) New Physics, the most efficient way to benefit from both high and low-energy measurements simultaneously is the use of the Standard Model Effective Field Theory (SMEFT). In this talk I highlight the importance of semileptonic $\tau$ decays in complementing, in this respect, the traditional low-energy precision observables and high-energy measurements. This is yet another reason for considering hadronic tau decays as golden channels at Belle-II beyond the unquestionable interest of the CP violation anomaly in $\tau\to K_S\pi\nu_\tau$ decays, that I also discuss within the effective theory. A couple of new results for $\tau^-\to K^-\nu_\tau$ decays are also included.