The study aims to investigate prospective middle school mathematics teachers' mental actions related to debugging, which is one of the computational thinking skills in the modeling process. The study was conducted with a single-case embedded model. The collaborative working group consisted of three prospective mathematics teachers selected by criterion sampling. The data were collected from the video analysis, screen excerpts, and GeoGebra files explaining the solution process of three prospective mathematics teachers for the designed two mathematical modeling problems (experimental and theoretical). According to the results obtained from the data through content analysis based on the theoretical framework, it was identified that the prospective teachers conducted sub-activities such as recognizing/detecting the error, extracting the error, and correcting the error, which is one of the dimensions of computational thinking in technology-supported mathematical modeling. These skills as the basic steps of interpretation, verification, and revision were developed in the process of technology-supported mathematical modeling. GeoGebra was involved as an important mental trigger in the debugging process. In further studies, computational thinking studies describing all the components in the process of technology-supported mathematical modeling can be conducted, and computational thinking skills can be revealed in the process of mathematical modeling in non-computerized environments.