diff --git a/flake.lock b/flake.lock
index 832b3fe..ee8027c 100644
--- a/flake.lock
+++ b/flake.lock
@@ -17,11 +17,11 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1777954456,
-        "narHash": "sha256-hGdgeU2Nk87RAuZyYjyDjFL6LK7dAZN5RE9+hrDTkDU=",
+        "lastModified": 1784356753,
+        "narHash": "sha256-12KrbMiWLcf8m7pCvAtZh1ZrgF85ZXDXvfR/fWTKy84=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "549bd84d6279f9852cae6225e372cc67fb91a4c1",
+        "rev": "61b7c44c4073f0b827768aff0049561b5110ea5a",
         "type": "github"
       },
       "original": {
diff --git a/flake.nix b/flake.nix
index aa83a63..e592cbb 100644
--- a/flake.nix
+++ b/flake.nix
@@ -7,7 +7,11 @@
   };
 
   outputs =
-    { self, nixpkgs, crane }:
+    {
+      self,
+      nixpkgs,
+      crane,
+    }:
     let
       supportedSystems = [
         "x86_64-linux"
@@ -35,8 +39,8 @@
 
             package = lib.mkOption {
               type = lib.types.package;
-              default = self.packages.${pkgs.stdenv.hostPlatform.system}.ratty;
-              defaultText = lib.literalExpression "self.packages.\${pkgs.stdenv.hostPlatform.system}.ratty";
+              default = pkgs.ratty;
+              defaultText = lib.literalExpression "pkgs.ratty";
               description = "The ratty package to install.";
             };
 
@@ -57,7 +61,22 @@
             };
 
             gpuBackend = lib.mkOption {
-              type = lib.types.nullOr (lib.types.enum [ "vulkan" "gl" "gles" ]);
+              type = lib.types.nullOr (
+                lib.types.enum (
+                  if pkgs.stdenv.isDarwin then
+                    [
+                      "metal"
+                      "gl"
+                      "gles"
+                    ]
+                  else
+                    [
+                      "vulkan"
+                      "gl"
+                      "gles"
+                    ]
+                )
+              );
               default = null;
               description = ''
                 Force the wgpu backend.
@@ -79,6 +98,15 @@
               '';
               example = "RTX 3060";
             };
+            defaultShell = lib.mkOption {
+              type = lib.types.nullOr lib.types.package;
+              default = null;
+              description = ''
+                Default shell to use when no -e/--command flag is passed.
+                Set to null to use the package's built-in default (bash).
+              '';
+              example = lib.literalExpression "pkgs.zsh";
+            };
           };
         };
     in
@@ -130,6 +158,11 @@
 
       formatter = forEachSystem (system: nixpkgs.legacyPackages.${system}.nixfmt-rfc-style);
 
+      # Overlay — adds pkgs.ratty for use with the NixOS/HM modules.
+      overlays.default = final: prev: {
+        ratty = self.packages.${final.stdenv.hostPlatform.system}.ratty;
+      };
+
       checks = forEachSystem (
         system:
         let
@@ -173,6 +206,12 @@
         {
           inherit (opts) options;
           config = lib.mkIf cfg.enable {
+            assertions = [
+              {
+                assertion = cfg.package != null;
+                message = "programs.ratty.package must not be null when programs.ratty.enable is true";
+              }
+            ];
             home.packages = [ cfg.package ];
             xdg.configFile."ratty/ratty.toml" = lib.mkIf (cfg.settings != { }) {
               source = tomlFormat.generate "ratty.toml" cfg.settings;
@@ -180,6 +219,7 @@
             home.sessionVariables = lib.mkMerge [
               (lib.mkIf (cfg.gpuBackend != null) { WGPU_BACKEND = cfg.gpuBackend; })
               (lib.mkIf (cfg.gpuAdapter != null) { WGPU_ADAPTER_NAME = cfg.gpuAdapter; })
+              (lib.mkIf (cfg.defaultShell != null) { SHELL = lib.getExe cfg.defaultShell; })
             ];
           };
         };
@@ -215,26 +255,37 @@
         {
           inherit (opts) options;
           config = lib.mkIf cfg.enable {
+            assertions = [
+              {
+                assertion = cfg.package != null;
+                message = "programs.ratty.package must not be null when programs.ratty.enable is true";
+              }
+            ];
             environment.systemPackages = [
-              (let
-                hasSettings = cfg.settings != { };
-                hasGpuOpts = cfg.gpuBackend != null || cfg.gpuAdapter != null;
-              in
-              if !(hasSettings || hasGpuOpts) then
-                cfg.package
-              else
-                pkgs.symlinkJoin {
-                  name = "ratty-system-wrapped";
-                  paths = [ cfg.package ];
-                  nativeBuildInputs = [ pkgs.makeWrapper ];
-                  postBuild = ''
-                    rm -f $out/bin/ratty
-                    makeWrapper ${lib.getExe cfg.package} $out/bin/ratty \
-                      ${lib.optionalString hasSettings "--add-flags \"--config-file /etc/ratty/ratty.toml\""} \
-                      ${lib.optionalString (cfg.gpuBackend != null) "--set WGPU_BACKEND '${cfg.gpuBackend}'"} \
-                      ${lib.optionalString (cfg.gpuAdapter != null) "--set WGPU_ADAPTER_NAME '${cfg.gpuAdapter}'"}
-                  '';
-                })
+              (
+                let
+                  hasSettings = cfg.settings != { };
+                  hasGpuOpts = cfg.gpuBackend != null || cfg.gpuAdapter != null;
+                in
+                if !(hasSettings || hasGpuOpts) then
+                  cfg.package
+                else
+                  pkgs.symlinkJoin {
+                    name = "ratty-system-wrapped";
+                    paths = [ cfg.package ];
+                    nativeBuildInputs = [ pkgs.makeWrapper ];
+                    postBuild = ''
+                      rm -f $out/bin/ratty
+                      makeWrapper ${lib.getExe cfg.package} $out/bin/ratty \
+                        ${lib.optionalString hasSettings "--add-flags \"--config-file /etc/ratty/ratty.toml\""} \
+                        ${lib.optionalString (cfg.gpuBackend != null) "--set WGPU_BACKEND '${cfg.gpuBackend}'"} \
+                        ${lib.optionalString (cfg.gpuAdapter != null) "--set WGPU_ADAPTER_NAME '${cfg.gpuAdapter}'"} \
+                        ${lib.optionalString (
+                          cfg.defaultShell != null
+                        ) "--set-default SHELL '${lib.getExe cfg.defaultShell}'"}
+                    '';
+                  }
+              )
             ];
 
             environment.etc."ratty/ratty.toml" = lib.mkIf (cfg.settings != { }) {
diff --git a/nix/default.nix b/nix/default.nix
index 2c63e34..1ca8e7d 100644
--- a/nix/default.nix
+++ b/nix/default.nix
@@ -58,7 +58,7 @@ let
   # cache hit rate — dependency hashes don't change when assets/docs change.
   commonArgs = {
     pname = "ratty";
-    version = "0.4.1";
+    version = "0.4.2";
 
     src = craneLib.cleanCargoSource ../.;
 
@@ -95,62 +95,65 @@ let
 
 in
 # Build the full package, reusing cached dependency artifacts.
-craneLib.buildPackage (commonArgs // {
-  inherit cargoArtifacts;
+craneLib.buildPackage (
+  commonArgs
+  // {
+    inherit cargoArtifacts;
 
-  # The full source (including assets, config, website) is needed for
-  # postInstall below.  buildDepsOnly already handled the filtered source.
-  src = ../.;
+    # The full source (including assets, config, website) is needed for
+    # postInstall below.  buildDepsOnly already handled the filtered source.
+    src = ../.;
 
-  desktopItems = [
-    (makeDesktopItem {
-      name = "ratty";
-      desktopName = "Ratty";
-      comment = "A GPU-rendered terminal emulator with inline 3D graphics";
-      exec = "ratty";
-      terminal = false;
-      categories = [
-        "System"
-        "TerminalEmulator"
-        "Utility"
-      ];
-      icon = "ratty";
-    })
-  ];
+    desktopItems = [
+      (makeDesktopItem {
+        name = "ratty";
+        desktopName = "Ratty";
+        comment = "A GPU-rendered terminal emulator with inline 3D graphics";
+        exec = "ratty";
+        terminal = false;
+        categories = [
+          "System"
+          "TerminalEmulator"
+          "Utility"
+        ];
+        icon = "ratty";
+      })
+    ];
 
-  # Assets are embedded at compile time via rust-embed.
-  # Copy them to $out/share for reference and custom model discovery fallback.
-  postInstall = ''
-    # Step 1: Copy assets
-    mkdir -p $out/share/ratty
-    cp -r assets/objects $out/share/ratty/
-    install -Dm644 config/ratty.toml $out/share/ratty/ratty.toml
-    install -Dm644 website/assets/images/ratty-logo.png \
-      $out/share/icons/hicolor/512x512/apps/ratty.png
+    # Assets are embedded at compile time via rust-embed.
+    # Copy them to $out/share for reference and custom model discovery fallback.
+    postInstall = ''
+      # Step 1: Copy assets
+      mkdir -p $out/share/ratty
+      cp -r assets/objects $out/share/ratty/
+      install -Dm644 config/ratty.toml $out/share/ratty/ratty.toml
+      install -Dm644 website/assets/images/ratty-logo.png \
+        $out/share/icons/hicolor/512x512/apps/ratty.png
 
-    # Step 2: wrapProgram for env var management
-    wrapProgram $out/bin/ratty \
-      --set SHELL '${bash}/bin/bash' \
-      --prefix LD_LIBRARY_PATH : '${runtimeLibraryPath}' \
-      ${lib.optionalString stdenv.isDarwin ''
-        --prefix DYLD_LIBRARY_PATH : '${runtimeLibraryPath}' \
-        --prefix DYLD_FALLBACK_LIBRARY_PATH : '${runtimeLibraryPath}' \
-      ''}
+      # Step 2: wrapProgram for env var management
+      wrapProgram $out/bin/ratty \
+        --set-default SHELL '${bash}/bin/bash' \
+        --prefix LD_LIBRARY_PATH : '${runtimeLibraryPath}' \
+        ${lib.optionalString stdenv.isDarwin ''
+          --prefix DYLD_LIBRARY_PATH : '${runtimeLibraryPath}' \
+          --prefix DYLD_FALLBACK_LIBRARY_PATH : '${runtimeLibraryPath}' \
+        ''}
 
-    # Step 3: Thin wrapper for conditional -e "$SHELL" injection.
-    # The --run script is defined as a separate Nix string to avoid
-    # the nested double-tick problem (Nix does not support nested double-tick strings).
-    mv $out/bin/ratty $out/bin/.ratty-env-wrapped
-    makeWrapper $out/bin/.ratty-env-wrapped $out/bin/ratty \
-      --run '${runScript}'
-  '';
+      # Step 3: Thin wrapper for conditional -e "$SHELL" injection.
+      # The --run script is defined as a separate Nix string to avoid
+      # the nested double-tick problem (Nix does not support nested double-tick strings).
+      mv $out/bin/ratty $out/bin/.ratty-env-wrapped
+      makeWrapper $out/bin/.ratty-env-wrapped $out/bin/ratty \
+        --run '${runScript}'
+    '';
 
-  meta = {
-    description = "GPU-rendered terminal emulator with inline 3D graphics";
-    homepage = "https://github.com/orhun/ratty";
-    license = lib.licenses.mit;
-    maintainers = [ "daniejbolt" ];
-    mainProgram = "ratty";
-    platforms = lib.platforms.unix;
-  };
-})
+    meta = {
+      description = "GPU-rendered terminal emulator with inline 3D graphics";
+      homepage = "https://github.com/orhun/ratty";
+      license = lib.licenses.mit;
+      maintainers = [ "daniejbolt" ];
+      mainProgram = "ratty";
+      platforms = lib.platforms.unix;
+    };
+  }
+)