Solve a NixOS problem the declarative, reproducible way

Help me solve a NixOS problem in a way that is idiomatic to NixOS. I already know Linux well, so skip the basics and focus on Nix-specific semantics. What I'm trying to do or what's broken: [GOAL or SYMPTOM] Setup: [flakes or channels, NixOS version, Home Manager yes/no, hardware or VM] Relevant config: [paste configuration.nix, flake.nix, or module snippets] Exact errors and commands run: [paste output of nixos-rebuild, nix build, journalctl, etc.] What I've tried: [ATTEMPTS] Rules: - Prefer declarative fixes (module options, overlays, packages in config) over imperative ones (nix-env -i, editing files in /etc, mutable state). - Check for an existing NixOS module option before writing custom systemd units or activation scripts. - If the problem is a non-Nix binary (dynamic linking, FHS paths), explain the options: packaging it properly, buildFHSEnv, nix-ld, or steam-run, with trade-offs. - Explain the root cause in Nix terms (evaluation vs. build vs. activation failure, option conflicts and mkForce/mkDefault priorities, impurity, store paths). - Say whether the fix differs between flakes and channels. - Don't invent option names. If unsure an option exists, tell me how to check (nixos-option, search.nixos.org, man configuration.nix) and mark it [VERIFY]. Return: 1. Root cause in 2-3 sentences. 2. The fix as a config diff or module snippet. 3. Commands to apply and test it (including nixos-rebuild test before switch). 4. How to roll back if it goes wrong. 5. Any gotchas for future upgrades.