  We discuss the construction of the chiral Lagrangian for a light spin-1
boson, here denoted as $X$, featuring both vector and axial-vector couplings to
light $u,d,s$ quarks. Focusing on $\Delta S = 1$ transitions, we show that
there are model-independent tree-level contributions to $K^\pm \to \pi^\pm X$,
sourced by Standard Model charged currents, which receive an $m^2_K / m_X^2$
enhancement from the emission of a longitudinally polarized $X$. This flavour
observable sets the strongest to date model-independent bound on the diagonal
axial-vector couplings of $X$ to $u,d,s$ quarks for $m_X < m_K - m_\pi$,
superseding the bounds arising from beam-dump and collider searches.
