Diagnostic fracture-injection tests (DFIT) are small-volume fracturing tests used to estimate the minimum principal stress and other formation properties. Conventional DFIT interpretation techniques assume that injection forms a single, planar fracture perpendicular to the minimum principal stress. However, abundant evidence exists that hydraulic fracturing causes the generation of complex networks, especially in shale. A computational simulation of a DFIT test in a “complex” fracture network that includes both new and preexisting fractures was performed with a discrete fracture-network (DFN) algorithm that fully couples fluid flow with the stresses induced by fracture deformation and propagation. Results show that with fracture network complexity, standard DFIT interpretation methods might give erroneous results. More broadly, results suggest that hydraulic-fracture models and interpretation techniques need to be updated to include the effects of fracture-network complexity. The results are preliminary because of several simplifying assumptions made by the simulator. In future work, those assumptions will be relaxed, the simulator will be used to develop applied interpretation techniques, and the techniques will be applied to field data.