  We describe the special relativistic extension of the CRONOS code, which has
been used for studies of gamma-ray binaries in recent years. The code was
designed to be easily adaptable, allowing the user to easily change existing
functionalities or introduce new modules tailored to the problem at hand.
Numerically, the equations are treated using a finite-volume Godunov scheme on
rectangular grids, which currently support Cartesian, spherical, and
cylindrical coordinates. The employed reconstruction technique, the approximate
Riemann solver and the equation of state can be chosen dynamically by the user.
Further, the code was designed with stability and robustness in mind, detecting
and mitigating possible failures early on. We demonstrate the code's
capabilities on an extensive set of validation problems.
