Reproducibility in HPC remains difficult under the constraints of production supercomputers: no root access, limited internet, and software stacks that increasingly span C/C++, Fortran, Python, MPI, and GPU runtimes. Traditional approaches based on environment modules and Conda require manual intervention to locate dependencies, leak system libraries into builds, and fail to compose across projects. Containers help with deployment but do not by themselves guarantee reproducibility. We report on our experience building a hybrid HPC/AI software stack with Nix, covering local development on a workstation without root and remote deployment as an Apptainer image on a production cluster. Nix's consistent package layout, full environment isolation, and flake-based composition resolve the dependency discovery, leakage, and composition problems we encountered, while unifying C/C++ and Python management under a single declarative specification that also generates the deployment container. We discuss trade-offs against Spack and Guix, the development-versus-production split addressed via CMake presets, and current gaps in ML package coverage in Nixpkgs.

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