Well spacing and hydraulic fracture design optimization are among the most important challenges confronting companies operating in unconventional reservoirs. Field trials are time-consuming and expensive. Reservoir simulation and/or rate transient analysis can help guide development decisions, but these calculations can be affected by non-uniqueness. For example, it is not possible to resolve permeability and fracture geometry using only production and pressure data in rate transient analysis. This work demonstrates that tracers can be used to reduce non-uniqueness. We quantitatively apply tracer measurements as part of the calibration and history matching of a fully coupled 3D hydraulic fracturing, geomechanics, and reservoir simulator. With the use of calibrated models, forward modeling and sensitivity analysis can be used more accurately to guide better decisions about well spacing and hydraulic fracture design. Tracers are complementary to data sources such as microseismic and distributed acoustic sensing, which focus on hydraulic fracture creation but provide less constraint on the producing behavior of wells, which ultimately drives asset financial performance.