The phonon-assisted emission of GaAs _{1 x}Bi _x quantum wells with Bi concentrations up to x=0.055 is investigated by continuous-wave photoluminescence as a function of lattice temperature and excitation density. Strong carrier-phonon coupling is observed manifesting itself in a Huang-Rhys factor of up to 0.3 depending on the lattice temperature and Bi content. Carriers bound to Bi-clusters are identified to be the origin of the efficient carrier-phonon scattering.