  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.
