We present 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 a detailed and benchmarked algorithm to treat mosaic crystals, especially HOPG and HAPG (Highly Oriented/Annealed Pyrolitic Graphite). The code is primarily designed to study crystal spectrometers, therefore their implementation is very straightforward and includes the automated evaluation of their performance. It can, however, be used universally to study other Bragg crystal based instruments, such as monochromators, mirrors, and analyzers. The code is publicly available, written in Python3 and is distributed as a Python library with test cases and user manual included. Program summary: Program title: mmpxrt CPC Library link to program files: https://dx.doi.org/10.17632/dkpbzvtz3b.1 Developer's repository link: https://gitlab.hzdr.de/smid55/mmpxrt Licensing provisions: MIT Programming language: Python 3 Nature of problem: Mosaic crystals are commonly used for X-ray spectroscopy and similar applications. However, the complicated structure of such crystals makes their function non-trivial and often counter-intuitive, therefore a proper simulation tool is needed to design and understand such instruments. Solution method: We have developed a Monte-Carlo X-ray ray tracing code which simulates the setup of given spectrometer, analyzes the results and provides the performance of the spectrometer.