programs.gnupg.agent: boolean flags in settings render as invalid gpg-agent.conf

Nixpkgs version

Unstable (26.05)

Describe the bug

programs.gnupg.agent.settings accepts boolean values but renders them incorrectly for gpg-agent.conf. Flag-style options like disable-scdaemon must appear as bare keywords with no argument, but the module renders disable-scdaemon = true; as disable-scdaemon true, which gpg-agent rejects on startup.

The root cause is in the settings format definition in nixos/modules/programs/gnupg.nix:

agentSettingsFormat = pkgs.formats.keyValue {
  mkKeyValue = lib.generators.mkKeyValueDefault { } " ";
};

mkKeyValueDefault stringifies booleans, producing disable-scdaemon true instead of bare disable-scdaemon.

Steps to reproduce

1. Add to NixOS configuration:

programs.gnupg.agent = {
  enable = true;
  settings = {
    disable-scdaemon = true;
  };
};

2. Rebuild (nixos-rebuild switch)
3. Observe /etc/gnupg/gpg-agent.conf contains disable-scdaemon true
4. gpg-agent crashes on startup

Expected behaviour

disable-scdaemon = true; should render as bare disable-scdaemon in gpg-agent.conf, or the type system should reject boolean values if they can't be rendered correctly.

Relevant log output

gpg-agent[48772]: /etc/gnupg/gpg-agent.conf:2: argument not expected
systemd[2147]: gpg-agent.service: Main process exited, code=exited, status=2/INVALIDARGUMENT
systemd[2147]: gpg-agent.service: Failed with result 'exit-code'.

Additional context

Workaround: disable-scdaemon = ""; renders as bare disable-scdaemon and works.

Related: #271436 (filed as a docs issue for the same underlying problem)

System metadata

• system: "x86_64-linux"
• host os: Linux 6.12.68, NixOS, 26.05 (Yarara), 26.05.20260204.00c21e4
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.31.3
• channels(root): "nixos-24.05"
• nixpkgs: /nix/store/hiyg9ihp2i3ncldhcjm2vv81zn8gnb2i-nixpkgs/nixpkgs

Notify maintainers

---

Note for maintainers: Please tag this issue in your pull request description. (i.e. Resolves #ISSUE.)

Start in nixos/modules/programs/gnupg.nix at the agentSettingsFormat definition and reproduce the boolean rendering described in the issue. Determine whether boolean flags should render as bare keywords or be rejected by the settings type, then verify that the generated gpg-agent.conf is accepted by gpg-agent and add regression coverage wherever the module's existing tests belong.