Formatter does not work in a git worktree

[kuruczgy]

Describe the bug

Running nix fmt (set up as recommended in the README) fails with

Error: failed to load config: failed to determine tree root: failed to find tree-root based on tree-root-file: could not find [.git/config] in /path/to/your/worktree

To Reproduce

Steps to reproduce the behavior:

1. Create a git worktree using git worktree add
2. Try to run nix fmt in the worktree.

Expected behavior

Formats as normal.

System information

treefmt-nix version: a05be41
nixpkgs version: 56b033fe4f9da755b1872466f24b32df7cfc229e

nix-shell -p nix-info --run "nix-info -m":

 - system: `"aarch64-linux"`
 - host os: `Linux 6.16.0-rc3, NixOS, 25.11 (Xantusia), 25.11.20250615.cbfb263`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Lix, like Nix) 2.91.1
System type: aarch64-linux
Additional system types: 
Features: gc, signed-caches
System configuration file: /etc/nix/nix.conf
User configuration files: /home/user/.config/nix/nix.conf:/etc/xdg/nix/nix.conf:/home/user/.nix-profile/etc/xdg/nix/nix.conf:/nix/profile/etc/xdg/nix/nix.conf:/home/user/.local/state/nix/profile/etc/xdg/nix/nix.conf:/etc/profiles/per-user/user/etc/xdg/nix/nix.conf:/nix/var/nix/profiles/default/etc/xdg/nix/nix.conf:/run/current-system/sw/etc/xdg/nix/nix.conf
Store directory: /nix/store
State directory: /nix/var/nix
Data directory: /nix/store/g9clkwljsncp6fadzc105ay78b7l6mg2-lix-2.91.1/share`
 - nixpkgs: `/nix/store/nki040zkkasz5mnd7c6jv0gn3sipmc30-source`

[kuruczgy]

I suspect the issue is that we unconditionally set --tree-root-file to .git/config by default here:

treefmt-nix/module-options.nix

Line 205 in a05be41

--tree-root-file=${config.projectRootFile} \

I think if we just omitted this option by default treefmt would be smart enough to figure out the root on its own.

[jfly]

I suspect the issue is that we unconditionally set --tree-root-file to .git/config by default here

@kuruczgy that used to be the case, but we recently changed this to flake.nix for flakes. See 6a846d3. However, you say you're using a05be41, which does include that commit.

Are you able to share a repro?

[kuruczgy]

@jfly I am using default.nix as an entry point, so that explains why the commit you linked does not affect my project.

Here is my project, specifically my treefmt-nix instantiation: https://github.com/kuruczgy/x1e-nixos-config/blob/c37010a5569bbf51e07631d7094ae3d036dd50a6/flake.nix#L106

[jfly]

Got it! Then I'd suggest that you set projectRootFile. Something like this:

diff --git a/flake.nix b/flake.nix
index d6c9d1c..9b82024 100644
--- a/flake.nix
+++ b/flake.nix
@@ -113,6 +113,8 @@
             );
           in
           (treefmt-nix.evalModule pkgs {
+            projectRootFile = "flake.nix";
+
             programs.nixfmt.enable = true;
             settings.on-unmatched = "info";
             programs.mdformat = {

[kuruczgy]

Thanks, that seems like a pragmatic workaround for my project.

Do you believe this issue should stay open? This workaround will not work for projects without a flake.nix.

[jfly]

Good question. My (not very strong) opinion is that we provide a good experience for folks using flakes, and for folks not using flakes, they may be choosing a bit more of an adventure.

If you're correct that core treefmt would handle this correctly, I suppose there's an argument for getting rid of this logic and just letting treefmt figure it out.