- `builtins.fetchTree` is now its own experimental feature, [`fetch-tree`](@docroot@/development/experimental-features.md#xp-fetch-tree).
  This allows stabilising it independently of the rest of what is encompassed by [`flakes`](@docroot@/development/experimental-features.md#xp-fetch-tree).

- The interface for creating and updating lock files has been overhauled:

  - [`nix flake lock`](@docroot@/command-ref/new-cli/nix3-flake-lock.md) only creates lock files and adds missing inputs now.
    It will *never* update existing inputs.

  - [`nix flake update`](@docroot@/command-ref/new-cli/nix3-flake-update.md) does the same, but *will* update inputs.
    - Passing no arguments will update all inputs of the current flake, just like it already did.
    - Passing input names as arguments will ensure only those are updated. This replaces the functionality of `nix flake lock --update-input`
    - To operate on a flake outside the current directory, you must now pass `--flake path/to/flake`.

  - The flake-specific flags `--recreate-lock-file` and `--update-input` have been removed from all commands operating on installables.
    They are superseded by `nix flake update`.

- Commit signature verification for the [`builtins.fetchGit`](@docroot@/language/builtins.md#builtins-fetchGit) is added as the new [`verified-fetches` experimental feature](@docroot@/development/experimental-features.md#xp-feature-verified-fetches).

- [`nix path-info --json`](@docroot@/command-ref/new-cli/nix3-path-info.md)
  (experimental) now returns a JSON map rather than JSON list.
  The `path` field of each object has instead become the key in the outer map, since it is unique.
  The `valid` field also goes away because we just use `null` instead.

  - Old way:

    ```json5
    [
      {
        "path": "/nix/store/8fv91097mbh5049i9rglc73dx6kjg3qk-bash-5.2-p15",
        "valid": true,
        // ...
      },
      {
        "path": "/nix/store/wffw7l0alvs3iw94cbgi1gmmbmw99sqb-home-manager-path",
        "valid": false
      }
    ]
    ```

  - New way

    ```json5
    {
      "/nix/store/8fv91097mbh5049i9rglc73dx6kjg3qk-bash-5.2-p15": {
        // ...
      },
      "/nix/store/wffw7l0alvs3iw94cbgi1gmmbmw99sqb-home-manager-path": null,
    }
    ```

  This makes it match `nix derivation show`, which also maps store paths to information.

- When Nix is installed using the [binary installer](@docroot@/installation/installing-binary.md), in supported shells (Bash, Zsh, Fish)
  [`XDG_DATA_DIRS`](https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html#variables) is now populated with the path to the `/share` subdirectory of the current profile.
  This means that command completion scripts, `.desktop` files, and similar artifacts installed via [`nix-env`](@docroot@/command-ref/nix-env.md) or [`nix profile`](@docroot@/command-ref/new-cli/nix3-profile.md)
  (experimental) can be found by any program that follows the [XDG Base Directory Specification](https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html).

- A new command `nix store add` has been added. It replaces `nix store add-file` and `nix store add-path` which are now deprecated.

- A new option [`always-allow-substitutes`](@docroot@/command-ref/conf-file.md#conf-always-allow-substitutes) has been added.

  When set to `true`, Nix will always try to substitute a derivation, even if it has the [`allowSubstitutes`]{#adv-attr-allowSubstitutes} attribute set to `false`.