We investigate a coherently driven exciton--photon microcavity with Kerr nonlinearity in the ultrastrong-coupling regime. When the lower and upper polariton branches are well separated in energy, the full Hopfield--Rabi--Kerr model reduces to an effective single-mode description of the lower polariton. We analyze the stability of the lower-polariton steady states. We show that the resulting bistability is qualitatively similar to that in the strong-coupling regime. However, in the ultrastrong-coupling regime counter-rotating processes and the diamagnetic
A2 term renormalize the polariton spectrum and composition, changing the effective detuning
Δ~1 and nonlinearity
ULP g-dependency beyond the strong-coupling (RWA) picture. As a result, although the semiclassical bistability criterion keeps its standard Kerr--oscillator form, the turning points and hysteresis window are shifted relative to the strong-coupling prediction.