We use a superconducting microresonator as a cavity to sense absorption of microwaves by a
superconducting quantum point contact defined by surface gates over a proximitized two-dimensional
electron gas. Renormalization of the cavity frequency with phase difference across the point contact is
consistent with coupling to Andreev bound states. Near π phase difference, we observe random
fluctuations in absorption with gate voltage, related to quantum interference-induced modulations in the
electron transmission. Close to pinch-off, we identify features consistent with the presence of a single
Andreev bound state and describe the Andreev-cavity interaction using a dispersive Jaynes-Cummings
model. By fitting the weak Andreev-cavity coupling, we extract ~GHz decoherence consistent with charge
noise and the transmission dispersion associated with a localized state.