This is a a newly developed ray tracing code called mmpxrt, dedicated to study and design x-ray crystal optics, with a special focus on mosaic crystal spectrometers. Its main advantage over other currently available ray tracing codes is that it includes detailed and benchmarked algorithm to treat mosaic crystals, especially HOPG and HAPG (Highly Oriented / Annealed Pyrolitic Graphite). The code is dedicated primarily to study crystal spectrometers, therefore their implementation is very straightforward, and the code has mostly automatic evaluation of their performance. It can, however, be used universally to study other crystal instruments, like monochromators, mirrors, and analyzers. The code is publicly available in HZDR's Gitlab, written in Python3 and is distributed as a Python library with test cases included.