A common requirement to solve EM-based optimization problems consists of implementing numerical drivers to perform multiple calls to highly accurate commercial CAD tools, such as HFSS. Usually, this multiple calls to the tools cause a high consumption of computational resources and dedicated usage of licenses to manage the software. In environments such as academy and industry, sometimes it is not possible to have unlimited resources of these kind, being important to optimize the usage of them. This article proposes a technique which allows the engineer to avoid a continuous usage of a GUI environment while designing a high-frequency structure by means of an optimization algorithm. Our method consists of on-line editing the structure under study and collecting the EMresponses without ever opening or using the HFSS’s GUI. HFSS is especially suitable to this approach since it handles two separate licenses for its GUI and solving engine. The proposed method yields considerable savings in computational resources and processing time. Our technique is illustrated by designing a microstrip low-pass filter structure using a space mapping technique with HFSS and APLAC, as fine and coarse models, respectively.ITESO, A.C.