Fix `Did you mean devShell?` by exporting `devShells.default`

March 06, 2026 nix

CI failed with:

`{3}txt error: flake '...' does not provide attribute 'devShells.x86_64-linux.default' or 'packages.x86_64-linux.default' Did you mean devShell?


`devShells.<system>.default` is expected here. `flake.nix` was exporting `devShell`, which [is a legacy flake output key since Nix 2.7](https://nix.dev/manual/nix/2.24/release-notes/rl-2.7).

So change:

```nix
devShell = pkgs.mkShell {

Into:

devShells.default = pkgs.mkShell {

Inside a per-system output block, devShells.default means the default development shell for that system (which becomes devShells.<system>.default at the top-level flake output).

Why this started failing now (likely): in my CI (early 2026), Determinate Nix behavior exposed the legacy key usage, so applying the Nix 2.7 rename fixed it.