<p>Optical microcavities are widely used as transducers for gas sensing, yet their performance is constrained by a natural trade-off between sensitivity and dynamic range. Here we introduce a laser-tagging optofluidic microcavity that overcomes this limitation, enabling hydrogen detection across a concentration range spanning from the single-molecule level to 1.53 × 10⁵ ppm. The architecture features a hollow whispering-gallery-mode microcavity, functionally coated on its interior surface with a Pt/WO₃ nanofilm. Gas detection is mediated via thermal phonon transfer, which allows efficient gas–material interaction without perturbing the optical field, preserving an ultrahigh intrinsic <i>Q</i> factor of 1.89 × 10⁹ during sensing. Through so-called laser tagging, a probe laser is dynamically locked to the microcavity’s optimal operating point, enabling real-time resonance tracking. This scheme not only suppresses phase noise by more than three orders of magnitude but also facilitates wide-bandwidth optoelectronic heterodyne demodulation. We achieve hertz-level frequency-shift resolution and a measurable resonance shift of up to 1 GHz, allowing the sensor to detect hydrogen concentrations from 3 × 10⁻⁵ ppm to 1.53 × 10⁵ ppm. With lock-in amplification, even individual molecular dynamics can be resolved. The device’s integrated, centimetre-scale footprint ensures robust operation outside the laboratory, offering a universal strategy to advance optical microcavities towards ultraprecise metrology applications.</p>