treefmt

Set up

Add treefmt-nix to your inputs:

$ devenv inputs add treefmt-nix github:numtide/treefmt-nix

Check the available integrations in the list of all available integrations.

Add your desired integration to your devenv.nix file. For example, the following code would enable treefmt with the nixpkgs-fmt and rustfmt integrations:

{ inputs, ... }:


{
  treefmt = {
    enable = true;
    config.programs = {
      nixfmt.enable = true;
      rustfmt.enable = true;
    };
  };
}

In Action:

$ devenv shell
Building shell ...
Entering shell ...


treefmt # This would run treefmt on all files.

Additional Devenv Integrations

Git Hooks

If you would like to enable treefmt in your git-hooks hooks, simply add:

{ inputs, ... }:


{
  git-hooks.hooks = {
    treefmt.enable = true;
  };
}

This will enable treefmt hooks and automatically change the default package to the one you have defined in your devenv.

Using a Custom Formatter

It is also possible to use custom formatters with treefmt-nix. For example, the following custom formatter formats JSON files using yq-go:

{ pkgs, lib, ... }


{
  treefmt.config.settings.formatter = {
    "yq-json" = {
      command = "${lib.getExe pkgs.bash}";
      options = [
        "-euc"
        ''
          for file in "$@"; do
            ${lib.getExe yq-go} -i --output-format=json $file
          done
        ''
        "--" # bash swallows the second argument when using -c
      ];
      includes = [ "*.json" ];
      excludes = [ ".git/*" ".devenv/*" ]; # don't mess with git and devenv files
    };
  };
}

Last updated Oct 11, 2026