Solute segregation to planar defects, also called Suzuki effect, affects the stacking fault energy and alters mechanical properties. Although several studies have used theoretical or experimental work to quantify the effect for certain alloy composition, there has been no comprehensive study providing an overview of the Suzuki effect in a alloy across the composition and temperature spectrum. In this study, we present a predictive model of the Suzuki effect in the Co-Ni binary system and its extension to ternary systems. This prediction is based on long-range diffusion simulations implemented in the MatCalc software. For the present investigation, the authors revised the thermodynamic modeling of the Co-Ni system basing it on current DFT calculations of the enthalpy of mixing of the FCC and HCP phases as well as experimental work from the literature. The thermodynamic parameters obtained are then used to run long-range diffusion simulations and to investigate the effect of solute atoms on planar defects. The effect of Co in the Co-Ni FCC matrix system was evaluated for different compositions and temperatures. Increasing the temperature, result in a stronger segregation tendency of Co at the defect. According to the present study, the amount of segregation is primarily driven by the system thermodynamics, with diffusion having a limited impact. The present contribution demonstrates how solute atoms influence planar defects, and it establishes trends across chemical composition and temperature. These insights are essential for understanding the effect of such defects on plastic deformation and, consequently, the mechanical properties of alloys. This analysis is the first step toward developing a physical model to predict stacking fault energies (SFE) in metals and to assess the interfacial energies of planar defects in metallic alloys.