|Δ c |=|Δ u |=1 processes are unique probes of flavor physics in the up-sector within and beyond the Standard Model (SM). SM tests with rare semileptonic charm meson decays are based on an approximate CP-symmetry, a superior GIM-mechanism, angular distributions, lepton-universality and lepton flavor conservation. We analyze the complete set of null test observables in D →π ℓ ℓ(') and Ds→K ℓ ℓ(') decays, ℓ(')=e ,μ , and find large room for new physics safely above the SM contribution. We identify signatures of supersymmetry, leptoquarks and anomaly-free U (1) ' -models with generation-dependent charges, for which we provide explicit examples. Z'-effects in c →u ℓ ℓ(') transitions can be sizable if both left-handed and right-handed couplings to quarks are present.