Mesa unstable on nixos-stable in 2026?

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Hi everybody,

The nixos-25.05 release notes mention “The global Mesa version can now be managed without a mass rebuild by setting hardware.graphics.package.”

Reading this I thought this could solve the usecase of using mesa from unstable on a nixos stable system. So I’ve gone ahead and created a simple module as follows:

# pre-steps:
# sudo nix-channel --add https://nixos.org/channels/nixos-unstable nixos-unstable
# sudo nix-channel --update

let
  unstable = import <nixos-unstable> { };
in
{
  hardware.graphics = {
    enable = true;
    package = unstable.mesa;
    package32 = unstable.pkgsi686Linux.mesa;
  };
}

While the update worked and mesa (from unstable) seemed multiple applications started acting weird.
Firefox reports the “new” mesa unstable version string but reports no hardware acceleration available.
Steam reports the “old” mesa stable version string.
Gnome DE constantly keeps the fans spinning.

Am I missing something obvious in the module above or is this just not possible without breakage?