  Proto-neutron stars forming a few seconds after core-collapse supernovae are
hot and dense environments where hyperons can be efficiently produced by weak
processes. By making use of various state-of-the-art supernova simulations
combined with the proper extensions of the equations of state including
$\Lambda$ hyperons, we calculate the cooling of the star induced by the
emission of dark bosons $X^0$ through the decay $\Lambda\to n X^0$. Comparing
this novel energy-loss process to the neutrino cooling of SN 1987A allows us to
set stringent constraints on massless dark photons and axions with
flavor-violating couplings to quarks. We find that this new supernova bound can
be orders of magnitude stronger than other limits in dark-sector models.
