We calculate the dephasing rate due to magnetic impurities in a weakly
disordered metal as measured in a weak localization experiment. If the
density $n_S$ of magnetic impurities is sufficiently low and the
(non-magnetic) disorder sufficiently small, the dephasing rate
$1/\tp$ is a universal function, $1/\tau_\varphi =
( n_S/\nu) f\!\left(T/T_\text{K}\right)$, where $T_\text{K}$ is the Kondo
temperature and $\nu$ the density of states. We show that inelastic vertex corrections
with a typical energy transfer $\Delta E$ are suppressed by powers of
$1/(\tp \Delta E) \propto n_S$. Therefore the dephasing rate can be calculated from the
{\em inelastic cross section} which is evaluated numerically exactly using the
numerically renormalization group.