  Understanding the fracture toughness of glasses is of prime importance for
science and technology. We study it here using extensive atomistic simulations
in which the interaction potential, glass transition cooling rate and loading
geometry are systematically varied, mimicking a broad range of experimentally
accessible properties. Glasses' nonequilibrium mechanical disorder is
quantified through $A_{\rm g}$, the dimensionless prefactor of the universal
spectrum of nonphononic excitations, which measures the abundance of soft
glassy defects that affect plastic deformability. We show that while a
brittle-to-ductile transition might be induced by reducing the cooling rate,
leading to a reduction in $A_{\rm g}$, iso-$\!A_{\rm g}$ glasses are either
brittle or ductile depending on the degree of Poisson contraction under
unconstrained uniaxial tension. Eliminating Poisson contraction using
constrained tension reveals that iso-$\!A_{\rm g}$ glasses feature similar
toughness, and that varying $A_{\rm g}$ under these conditions results in
significant toughness variation. Our results highlight the roles played by both
soft defects and loading geometry (which affects the activation of defects) in
the toughness of glasses.
