  The Cabibbo Angle Anomaly, an apparent deficit in first-row CKM unitarity,
can be addressed by leptophilic Standard Model extensions that generate new
contributions to the Fermi constant and affect the determination of the CKM
element $V_{ud}$. We focus on simplified models with this property, including
the Standard Model extended by vectorlike leptons, by the singly charged scalar
singlet, or by a leptophilic $Z'$ boson.
