The cosmic microwave background (CMB) anisotropies remain the cleanest, most powerful probe of fundamental physics in the cosmos. Measurements of the small-scale CMB temperature and polarization fields have recently undergone transformative improvements with Data Release 6 (DR6) of the Atacama Cosmology Telescope (ACT) and will soon improve further with the Simons Observatory, which will open new windows into physics beyond the standard models (BSM) of particle physics and cosmology. I will first discuss our recent cosmological parameter constraints from the ACT DR6 CMB power spectra, with a particular emphasis on constraining BSM physics operating just prior to recombination, including new relativistic particles and new pseudo-scalar fields. I will then turn to novel searches for BSM physics in CMB secondary anisotropies, as could be imprinted by the screening of CMB photons by massive dark photons (DPs) or axion-like particles. I will show the first results of searches for these signals in CMB data, enabled by our state-of-the-art needlet internal linear combination code, yielding leading bounds on kinetically mixed DPs and axion-photon couplings covering two decades in DP or axion particle mass. I will conclude with a look ahead to the prospects for BSM physics from the Simons Observatory.