`nix flake prefetch --json` reports inconsistent information after Nix 2.27

[getchoo]

Describe the bug

The "locked" attribute of the output will only sometimes contain the narHash of the Flake

Steps To Reproduce

1. Run nix flake prefetch --json on a local path: type Flake (e.g., the nixpkgs registry entry in NixOS by default)
2. Run nix flake prefetch --json on a remote Flake (e.g., github;)

Expected behavior

locked.narHash is consistently reported

Metadata

nix-env (Nix) 2.28.1

Additional context

With the following `nix flake metadata` output for different Flakes

$ nix flake metadata nixpkgs
Resolved URL:  path:/nix/store/5ds20jm3x2s4z7wn3581r6lc9ybmh45b-source?lastModified=1744463964&narHash=sha256-LWqduOgLHCFxiTNYi3Uj5Lgz0SR%2BXhw3kr/3Xd0GPTM%3D&rev=2631b0b7abcea6e640ce31cd78ea58910d31e650
Locked URL:    path:/nix/store/5ds20jm3x2s4z7wn3581r6lc9ybmh45b-source?lastModified=1744463964&narHash=sha256-LWqduOgLHCFxiTNYi3Uj5Lgz0SR%2BXhw3kr/3Xd0GPTM%3D&rev=2631b0b7abcea6e640ce31cd78ea58910d31e650
Description:   A collection of packages for the Nix package manager
Path:          /nix/store/5ds20jm3x2s4z7wn3581r6lc9ybmh45b-source
Revision:      2631b0b7abcea6e640ce31cd78ea58910d31e650
Last modified: 2025-04-12 09:19:24
Fingerprint:   69bcf68d2f795758132d280b52882973cf81960aaa93af45417a6758921728ea
$ nix flake metadata github:NixOS/nix
Resolved URL:  github:NixOS/nix
Locked URL:    github:NixOS/nix/f683a555bf3e258ca7e3669d51993587f6a641d0?narHash=sha256-eErpyq4Fdvz%2Bcla8NmTiwJ0WdrYKDeJ0nlvion5FhmQ%3D
Description:   The purely functional package manager
Path:          /nix/store/mhdcdd15m3zmyw8xkzbw39h0alzpsp7y-source
Revision:      f683a555bf3e258ca7e3669d51993587f6a641d0
Last modified: 2025-04-18 01:27:04
Fingerprint:   d7e7a6c5fe739e2fa0689a2ca8ea4e8d277a7f2271725325564167f2722a4849
Inputs:
├───flake-compat: github:edolstra/flake-compat/ff81ac966bb2cae68946d5ed5fc4994f96d0ffec?narHash=sha256-NeCCThCEP3eCl2l/%2B27kNNK7QrwZB1IJCrXfrbv5oqU%3D (2024-12-04 16:08:25)
├───flake-parts: github:hercules-ci/flake-parts/205b12d8b7cd4802fbcb8e8ef6a0f1408781a4f9?narHash=sha256-4pDvzqnegAfRkPwO3wmwBhVi/Sye1mzps0zHWYnP88c%3D (2024-12-04 11:43:21)
│   └───nixpkgs-lib follows input 'nixpkgs'
├───git-hooks-nix: github:cachix/git-hooks.nix/aa9f40c906904ebd83da78e7f328cd8aeaeae785?narHash=sha256-NdaCraHPp8iYMWzdXAt5Nv6sA3MUzlCiGiR586TCwo0%3D (2024-12-15 16:26:21)
│   ├───flake-compat follows input ''
│   ├───gitignore follows input ''
│   ├───nixpkgs follows input 'nixpkgs'
│   └───nixpkgs-stable follows input 'nixpkgs'
├───nixpkgs: github:NixOS/nixpkgs/52faf482a3889b7619003c0daec593a1912fddc1?narHash=sha256-6hl6L/tRnwubHcA4pfUUtk542wn2Om%2BD4UnDhlDW9BE%3D (2025-03-30 06:12:12)
├───nixpkgs-23-11: github:NixOS/nixpkgs/a62e6edd6d5e1fa0329b8653c801147986f8d446?narHash=sha256-oamiKNfr2MS6yH64rUn99mIZjc45nGJlj9eGth/3Xuw%3D (2024-05-31 12:45:33)
└───nixpkgs-regression: github:NixOS/nixpkgs/215d4d0fd80ca5163643b03a33fde804a29cc1e2?narHash=sha256-uGJ0VXIhWKGXxkeNnq4TvV3CIOkUJ3PAoLZ3HMzNVMw%3D (2022-01-24 19:20:45)

You will get the following outputs (Note that both contain a `hash`, but not `locked.narHash`)

$ nix flake prefetch nixpkgs --json | jq '.'
{
  "hash": "sha256-LWqduOgLHCFxiTNYi3Uj5Lgz0SR+Xhw3kr/3Xd0GPTM=",
  "locked": {
    "__final": true,
    "lastModified": 1744463964,
    "narHash": "sha256-LWqduOgLHCFxiTNYi3Uj5Lgz0SR+Xhw3kr/3Xd0GPTM=",
    "path": "/nix/store/5ds20jm3x2s4z7wn3581r6lc9ybmh45b-source",
    "rev": "2631b0b7abcea6e640ce31cd78ea58910d31e650",
    "type": "path"
  },
  "original": {
    "lastModified": 1744463964,
    "narHash": "sha256-LWqduOgLHCFxiTNYi3Uj5Lgz0SR+Xhw3kr/3Xd0GPTM=",
    "path": "/nix/store/5ds20jm3x2s4z7wn3581r6lc9ybmh45b-source",
    "rev": "2631b0b7abcea6e640ce31cd78ea58910d31e650",
    "type": "path"
  },
  "storePath": "/nix/store/5ds20jm3x2s4z7wn3581r6lc9ybmh45b-source"
}
$ nix flake prefetch github:NixOS/nix --json | jq '.'
{
  "hash": "sha256-eErpyq4Fdvz+cla8NmTiwJ0WdrYKDeJ0nlvion5FhmQ=",
  "locked": {
    "__final": true,
    "lastModified": 1744954024,
    "owner": "NixOS",
    "repo": "nix",
    "rev": "f683a555bf3e258ca7e3669d51993587f6a641d0",
    "type": "github"
  },
  "original": {
    "owner": "NixOS",
    "repo": "nix",
    "type": "github"
  },
  "storePath": "/nix/store/mhdcdd15m3zmyw8xkzbw39h0alzpsp7y-source"
}

This doesn't occur in Nix < 2.27

After a bit of bisecting, it seems this was introduced in #12421 via c74ad6b.

Thanks to @isabelroses for noticing this in catppuccin/nix#542. The bump to Nix 2.28 in Nixpkgs revealed this by breaking our current parsing of this output

Checklist

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

---

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[roberth]

Discussed in Nix team meeting:

Should we even expose the narHash by default? We can only return a narHash if the whole input is fetched, potentially including submodules and Git LFS files, which is not desirable. (It may be in the lock, but doing this during locking is still undesirable for large repos, so that would only shift the problem)

Opened #13150

Did we change the output by accident? narHash -> hash
No, we're getting rid of the narHash at the fetchTree input attributes level, because the input is already locked by the commit hash rev.

{
  // prefetch output
  "hash": "sha256-LWqduOgLHCFxiTNYi3Uj5Lgz0SR+Xhw3kr/3Xd0GPTM=",
  "locked": {
    // fetchTree input attributes
    "__final": true,
    "lastModified": 1744463964,
    "narHash": "sha256-LWqduOgLHCFxiTNYi3Uj5Lgz0SR+Xhw3kr/3Xd0GPTM=",
    "path": "/nix/store/5ds20jm3x2s4z7wn3581r6lc9ybmh45b-source",
    "rev": "2631b0b7abcea6e640ce31cd78ea58910d31e650",
    "type": "path"
  },
  "original": {
    // fetchTree input attributes before locking
    "lastModified": 1744463964,
    "narHash": "sha256-LWqduOgLHCFxiTNYi3Uj5Lgz0SR+Xhw3kr/3Xd0GPTM=",
    "path": "/nix/store/5ds20jm3x2s4z7wn3581r6lc9ybmh45b-source",
    "rev": "2631b0b7abcea6e640ce31cd78ea58910d31e650",
    "type": "path"
  },
  "storePath": "/nix/store/5ds20jm3x2s4z7wn3581r6lc9ybmh45b-source"
}

So this behavior is as intended. Please use hash instead of narHash here.

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2025-05-07-nix-team-meeting-minutes-224/63973/1