CalculiX [1] is a mature open-source finite element software that has been developed and maintained for over 25 years. It supports a wide range of classical structural and thermo mechanical analyses and is central within MTU Aero Engines’ simulation workflows. It also enjoys strong adoption among private users and open-source communities, often integrated into CAD/CAE workflows via FreeCAD’s FEM workbench, used through GUIs like Mecway and PrePoMax and integrated via a plugin into Coreform Cubit. In academia, CalculiX serves as a versatile tool for teaching and research in multiphysics applications - for example, in fluid-structure interaction studies via coupling to preCICE [2] and in multiscale composite modeling through NASA’s MAC/GMC framework [3]. This contribution addresses the dual challenge of maintaining and modernizing a production code while preserving its long term stability and backward compatibility - requirements that are central in industrial settings. It touches on the typical challenges involved in transitioning from a single author codebase to team based development. Furthermore, technical advancements are discussed: replacing POSIX threads with OpenMP, integrating modern sparse and eigen solvers, interfacing with linear algebra libraries and strengthening testing to detect numerical and scalability regressions early. A governance model is outlined to foster collaboration via community contributions while ensuring code quality and long-term maintainability. These efforts aim to grow CalculiX as a reliable, industry hardened yet welcoming platform within the open source mechanics ecosystem. [1] CalculiX: A Three-Dimensional Structural Finite Elemente Program. https://www.calculix.de/. [2] Chourdakis G, Davis K, Rodenberg B et al. preCICE v2: A sustainable and user-friendly coupling library. Open Res Europe 2022, 2:51 (https://doi.org/10.12688/openreseurope.14445.2) [3] Gustafson, Peter & Yapor Genao, Francisco & Bednarcyk, Brett & Pineda, Evan. (2019). Integration of MAC/GMC into CalculiX, an open source finite element code. 10.2514/6.2019-0775.