Cosmic birefringence—the rotation of the polarization plane of light as it traverses the Universe—offers a direct observational window into parity-violating physics beyond the Standard Model. In this work, we revisit the anisotropic component of cosmic birefringence, which leads to the generation of $B$-mode polarization in the cosmic microwave background (CMB). Using an exact theoretical treatment beyond the thin last-scattering surface approximation, we constrain the amplitude of anisotropic birefringence with combined polarization data from SPTpol, ACT, POLARBEAR, and BICEP. The joint analysis yields a best-fit amplitude of $A_{\mathrm{CB}}=0.42_{-0.34}^{+0.40}\times {10}^{-4}$, consistent with zero within $2\sigma$, and we place a 95% confidence-level upper bound of $A_{\mathrm{CB}}<1\times {10}^{-4}$. The constraint is not dominated by any single experiment and remains robust under the inclusion of a possible isotropic rotation angle. These results provide leading constraints on anisotropic cosmic birefringence from CMB $B$-mode polarization and illustrate the potential of upcoming experiments to improve sensitivity to parity-violating effects in the early Universe.

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• Cosmic microwave background