overriding a non-flake input with a subdirectory of a git repository does not work

[marienz]

Describe the bug

Pointing --override-input at a subdirectory of a git repository results in a git+file: url with a dir attribute pointing at the subdirectory. For a non-flake input, this subdirectory seems to be ignored.

Steps To Reproduce

Create a flake like this:

.
├── data
│   └── file
├── flake.lock
└── flake.nix

with flake.nix containing:

{
  inputs = {
    data = {
      url = ./data;
      flake = false;
    };
  };

  outputs = {data, ...}: {
    packages.x86_64-linux = {
      default = builtins.toFile "demo" (builtins.readFile "${data}/file");
    };
  };
}

The contents of file do not matter. (And what we do with data does not really matter.)

Running nix build . copies the contents of file, as expected.

If the flake is not in a git repository, running nix build . --override-input data ./data does the same thing, as expected, and we can see the input being overridden to a path: url:

$ nix build . --override-input data ./data
warning: not writing modified lock file of flake 'path:/tmp/flake-demo':
• Updated input 'data':
    'path:data'
  → 'path:/tmp/flake-demo/data?lastModified=1788008901&narHash=sha256-sz6PxoNpM5SL6UiBvOFFvSFSEf2UsjfWX1w8r2bPenc%3D' (2026-08-29)

But if the flake is in a git repository, we get:

$ nix build . --override-input data ./data
warning: Git tree '/home/marienz/tmp/flake-demo' is dirty
warning: not writing modified lock file of flake 'git+file:///home/marienz/tmp/flake-demo':
• Updated input 'data':
    'path:data'
  → 'git+file:///home/marienz/tmp/flake-demo?dir=data' (1970-01-01)
error:
       … while calling the 'toFile' builtin
         at /home/marienz/tmp/flake-demo/flake.nix:11:17:
           10|     packages.x86_64-linux = {
           11|       default = builtins.toFile "demo" (builtins.readFile "${data}/file");
             |                 ^
           12|     };

       … while evaluating the second argument passed to builtins.toFile

       … while calling the 'readFile' builtin
         at /home/marienz/tmp/flake-demo/flake.nix:11:41:
           10|     packages.x86_64-linux = {
           11|       default = builtins.toFile "demo" (builtins.readFile "${data}/file");
             |                                         ^
           12|     };

       error: Path 'file' does not exist in Git repository "/home/marienz/tmp/flake-demo".

Similarly, if we print "${data}" or use it in a derivation, we get a store path that contains the entire git repository, not the subdir we wanted.

Expected behavior

The behavior when not in a git repository is the behavior I expect: when overriding a non-flake input with a local path, the local path should be used.

I'm not sure if it makes more sense for --override-input on a non-flake input to use a path: instead of git+file: url, or if the dir attribute should work.

Metadata

nix-env (Nix) 2.34.8

Additional context

Explicitly passing a path: url to --override-input works around the problem:

nix build . --override-input data path:./data
warning: Git tree '/home/marienz/tmp/flake-demo' is dirty
warning: not writing modified lock file of flake 'git+file:///home/marienz/tmp/flake-demo':
• Updated input 'data':
    'path:data'
  → 'path:/home/marienz/tmp/flake-demo/data?lastModified=1788007640&narHash=sha256-sz6PxoNpM5SL6UiBvOFFvSFSEf2UsjfWX1w8r2bPenc%3D' (2026-08-29)

But this is not very intuitive.

Checklist

• checked latest Nix manual (source)
• checked open bug issues and pull requests for possible duplicates

#5401 looks related (but I don't think it's a duplicate.)

• reviewed the contributing guide

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