<p>Compact photonic systems that offer high frequency stability and low noise are of increasing importance to applications in precision metrology, quantum computing, communication and advanced sensing technologies. However, on-chip resonators comprising dielectrics cannot match the frequency stability and noise characteristics of Fabry–Pérot cavities, whose electromagnetic modes live almost entirely in vacuum. Here we present a novel strategy to interface microfabricated Fabry–Pérot cavities with photonic integrated circuits to realize compact, high-performance integrated systems. Using this new integration approach, we demonstrate the self-injection locking of an on-chip laser to a millimetre-scale vacuum-gap Fabry–Pérot cavity using a circuit interface that transforms the reflected cavity response to enable efficient feedback to the laser. This system achieves a phase noise of –97 dBc Hz^–1 at 10-kHz offset frequency, a fractional frequency stability of 5 × 10⁻¹³ at 10 ms, a 150-Hz 1/π integral linewidth and a 35-mHz fundamental linewidth. We also present a complementary integration strategy that utilizes a vertical-emission grating coupler and a back-reflection cancellation circuit to realize a fully co-integrated module that effectively redirects the reflected signals and isolates back-reflections with a 10-dB suppression ratio, serving as a key for on-chip Pound–Drever–Hall locking. Together, these results highlight how vacuum-gap Fabry–Pérot reference cavities can be harnessed for ultrastable, low-noise photonic systems.</p>