diff --git a/.gitignore b/.gitignore
index 07c6d80..b261cfc 100644
--- a/.gitignore
+++ b/.gitignore
@@ -24,4 +24,8 @@
 
 !/hosts/
 
+!/libs/
+
 !/modules/
+
+!/tests/
diff --git a/flake.lock b/flake.lock
index faca085..200067f 100644
--- a/flake.lock
+++ b/flake.lock
@@ -570,6 +570,27 @@
         "type": "github"
       }
     },
+    "nix-darwin": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1783744694,
+        "narHash": "sha256-2cp6N3rrwnGYLTx9l6N+NI+kwrCWxvJUbj5WJhvB29A=",
+        "owner": "nix-darwin",
+        "repo": "nix-darwin",
+        "rev": "c3e90c89649b07d1a96e4b9dd6cd0d6e44b91a74",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-darwin",
+        "ref": "nix-darwin-26.05",
+        "repo": "nix-darwin",
+        "type": "github"
+      }
+    },
     "nix-index-database": {
       "inputs": {
         "nixpkgs": [
@@ -962,6 +983,7 @@
         "lanzaboote": "lanzaboote",
         "llm-agents": "llm-agents",
         "niri-flake": "niri-flake",
+        "nix-darwin": "nix-darwin",
         "nix-index-database": "nix-index-database",
         "nixos-hardware": "nixos-hardware",
         "nixos-wsl": "nixos-wsl",
diff --git a/flake.nix b/flake.nix
index e2a3541..5ca0dd0 100644
--- a/flake.nix
+++ b/flake.nix
@@ -11,6 +11,11 @@
       inputs.nixpkgs.follows = "nixpkgs";
     };
 
+    nix-darwin = {
+      url = "github:nix-darwin/nix-darwin/nix-darwin-26.05";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+
     # Hardware / Platform
     nixos-hardware.url = "github:NixOS/nixos-hardware/master";
     nixos-wsl.url = "github:nix-community/NixOS-WSL";
diff --git a/flake/default.nix b/flake/default.nix
index ade85a1..d14de03 100644
--- a/flake/default.nix
+++ b/flake/default.nix
@@ -2,5 +2,6 @@
   imports = [
     ./formatter.nix
     ./git-hooks.nix
+    ./registry.nix
   ];
 }
diff --git a/flake/registry.nix b/flake/registry.nix
new file mode 100644
index 0000000..064836a
--- /dev/null
+++ b/flake/registry.nix
@@ -0,0 +1,42 @@
+{
+  inputs,
+  lib,
+  ...
+}:
+let
+  dotfilesLib = import ../libs {
+    inherit inputs lib;
+    root = ../.;
+  };
+  hostSpecsPath = ../hosts/default.nix;
+  hostSpecs =
+    if builtins.pathExists hostSpecsPath then
+      let
+        value = import hostSpecsPath;
+      in
+      if builtins.isFunction value then
+        value (
+          builtins.intersectAttrs (builtins.functionArgs value) {
+            inherit inputs lib;
+          }
+        )
+      else
+        value
+    else
+      { };
+  configurations = dotfilesLib.hosts.mkConfigurations hostSpecs;
+in
+{
+  flake = {
+    inherit (configurations) darwinConfigurations nixosConfigurations;
+    lib = dotfilesLib;
+  };
+
+  perSystem =
+    { pkgs, ... }:
+    {
+      checks.registry = import ../tests/registry.nix {
+        inherit inputs lib pkgs;
+      };
+    };
+}
diff --git a/libs/default.nix b/libs/default.nix
new file mode 100644
index 0000000..2528af7
--- /dev/null
+++ b/libs/default.nix
@@ -0,0 +1,17 @@
+{
+  inputs,
+  lib ? inputs.nixpkgs.lib,
+  root,
+}:
+let
+  registry = import ./registry.nix {
+    inherit inputs lib;
+    modulesRoot = root + "/modules";
+  };
+  hosts = import ./hosts.nix {
+    inherit inputs lib registry;
+  };
+in
+{
+  inherit hosts registry;
+}
diff --git a/libs/hosts.nix b/libs/hosts.nix
new file mode 100644
index 0000000..d69bfc1
--- /dev/null
+++ b/libs/hosts.nix
@@ -0,0 +1,168 @@
+{
+  inputs,
+  lib,
+  registry,
+}:
+let
+  ensure =
+    condition: message: value:
+    if condition then value else throw "host registry: ${message}";
+
+  isLinux = system: lib.hasSuffix "-linux" system;
+  isDarwin = system: lib.hasSuffix "-darwin" system;
+
+  hostFile =
+    spec: name:
+    let
+      path = spec.path + "/${name}";
+    in
+    if builtins.pathExists path then path else null;
+
+  selectedUnits =
+    spec:
+    [ "users.${spec.user}" ]
+    ++ map (name: "profiles.${name}") (spec.profiles or [ ])
+    ++ map (name: "applications.${name}") (spec.applications or [ ])
+    ++ (spec.units or [ ]);
+
+  validateSpec =
+    name: spec:
+    ensure (builtins.isAttrs spec) "${name}: host specification must be an attribute set" (
+      ensure (spec ? system && builtins.isString spec.system) "${name}: system is required" (
+        ensure (isLinux spec.system || isDarwin spec.system)
+          "${name}: unsupported system '${spec.system}'; expected a Linux NixOS or Darwin system"
+          (
+            ensure (spec ? user && builtins.isString spec.user && spec.user != "") "${name}: user is required" (
+              ensure (spec ? path && builtins.pathExists spec.path)
+                "${name}: path must name an existing host directory"
+                (
+                  ensure
+                    (lib.all
+                      (field: builtins.isList (spec.${field} or [ ]) && lib.all builtins.isString (spec.${field} or [ ]))
+                      [
+                        "profiles"
+                        "applications"
+                        "units"
+                      ]
+                    )
+                    "${name}: profiles, applications, and units must be lists of strings"
+                    (ensure (builtins.isBool (spec.homeManager or true)) "${name}: homeManager must be a boolean" spec)
+                )
+            )
+          )
+      )
+    );
+
+  mkSpecialArgs = name: spec: {
+    inherit inputs registry;
+    inherit (spec) system;
+    hostName = name;
+    primaryUser = spec.user;
+  };
+
+  mkHomeManagerModule =
+    name: spec: selected:
+    let
+      homePath = hostFile spec "home.nix";
+      homeModules = [
+        (registry.mkModule { class = "home"; })
+        (registry.mkSelectionModule selected)
+      ]
+      ++ lib.optional (homePath != null) homePath;
+    in
+    {
+      imports = [ inputs.home-manager.nixosModules.home-manager ];
+
+      home-manager = {
+        useGlobalPkgs = true;
+        useUserPackages = true;
+        extraSpecialArgs = mkSpecialArgs name spec;
+        users.${spec.user}.imports = homeModules;
+      };
+    };
+
+  mkDarwinHomeManagerModule =
+    name: spec: selected:
+    let
+      homePath = hostFile spec "home.nix";
+      homeModules = [
+        (registry.mkModule { class = "home"; })
+        (registry.mkSelectionModule selected)
+      ]
+      ++ lib.optional (homePath != null) homePath;
+    in
+    {
+      imports = [ inputs.home-manager.darwinModules.home-manager ];
+
+      home-manager = {
+        useGlobalPkgs = true;
+        useUserPackages = true;
+        extraSpecialArgs = mkSpecialArgs name spec;
+        users.${spec.user}.imports = homeModules;
+      };
+    };
+
+  mkNixos =
+    name: rawSpec:
+    let
+      spec = validateSpec name rawSpec;
+      selected = registry.validateUnitIds (selectedUnits spec);
+      nixosPath = hostFile spec "nixos.nix";
+      modules = [
+        (registry.mkModule { class = "nixos"; })
+        (registry.mkSelectionModule selected)
+        { networking.hostName = lib.mkDefault name; }
+      ]
+      ++ lib.optional (spec.homeManager or true) (mkHomeManagerModule name spec selected)
+      ++ lib.optional (nixosPath != null) nixosPath;
+    in
+    inputs.nixpkgs.lib.nixosSystem {
+      inherit (spec) system;
+      specialArgs = mkSpecialArgs name spec;
+      inherit modules;
+    };
+
+  mkDarwin =
+    name: rawSpec:
+    let
+      spec = validateSpec name rawSpec;
+      selected = registry.validateUnitIds (selectedUnits spec);
+      darwinPath = hostFile spec "darwin.nix";
+      modules = [
+        (registry.mkModule { class = "darwin"; })
+        (registry.mkSelectionModule selected)
+        { networking.hostName = lib.mkDefault name; }
+      ]
+      ++ lib.optional (spec.homeManager or true) (mkDarwinHomeManagerModule name spec selected)
+      ++ lib.optional (darwinPath != null) darwinPath;
+    in
+    ensure (inputs ? nix-darwin) "${name}: the nix-darwin input is required" (
+      inputs.nix-darwin.lib.darwinSystem {
+        inherit (spec) system;
+        specialArgs = mkSpecialArgs name spec;
+        inherit modules;
+      }
+    );
+
+  mkConfigurations =
+    hostSpecs:
+    let
+      validated = lib.mapAttrs validateSpec hostSpecs;
+    in
+    {
+      nixosConfigurations = lib.mapAttrs mkNixos (
+        lib.filterAttrs (_: spec: isLinux spec.system) validated
+      );
+      darwinConfigurations = lib.mapAttrs mkDarwin (
+        lib.filterAttrs (_: spec: isDarwin spec.system) validated
+      );
+    };
+in
+{
+  inherit
+    mkConfigurations
+    mkDarwin
+    mkNixos
+    selectedUnits
+    ;
+}
diff --git a/libs/registry.nix b/libs/registry.nix
new file mode 100644
index 0000000..d573ec1
--- /dev/null
+++ b/libs/registry.nix
@@ -0,0 +1,324 @@
+{
+  inputs,
+  lib,
+  modulesRoot,
+}:
+let
+  reservedFiles = {
+    common = "common.nix";
+    nixos = "nixos.nix";
+    darwin = "darwin.nix";
+    home = "home.nix";
+    meta = "meta.nix";
+  };
+
+  isFile = kind: kind == "regular" || kind == "symlink";
+
+  ensure =
+    condition: message: value:
+    if condition then value else throw "unit registry: ${message}";
+
+  callWithAvailableArgs =
+    value: availableArgs:
+    if builtins.isFunction value then
+      value (builtins.intersectAttrs (builtins.functionArgs value) availableArgs)
+    else
+      value;
+
+  pathFor =
+    relativePath:
+    if relativePath == [ ] then
+      modulesRoot
+    else
+      modulesRoot + "/${lib.concatStringsSep "/" relativePath}";
+
+  entryIsFile = entries: name: builtins.hasAttr name entries && isFile entries.${name};
+
+  normalizeMeta =
+    unit:
+    let
+      metaPath = unit.fragments.meta;
+      importedValue =
+        if metaPath == null then
+          { }
+        else
+          callWithAvailableArgs (import metaPath) {
+            inherit inputs lib unit;
+          };
+      imported =
+        ensure (builtins.isAttrs importedValue) "${unit.id}: meta.nix must return an attribute set"
+          importedValue;
+      allowedKeys = [
+        "description"
+        "includes"
+        "imports"
+      ];
+      unknownKeys = lib.filter (name: !(builtins.elem name allowedKeys)) (builtins.attrNames imported);
+      description = imported.description or null;
+      includes = imported.includes or [ ];
+      imports = imported.imports or { };
+      allowedImportKeys = [
+        "nixos"
+        "darwin"
+        "home"
+      ];
+      unknownImportKeys =
+        if builtins.isAttrs imports then
+          lib.filter (name: !(builtins.elem name allowedImportKeys)) (builtins.attrNames imports)
+        else
+          [ ];
+      normalized = {
+        inherit description includes;
+        imports = {
+          nixos = imports.nixos or [ ];
+          darwin = imports.darwin or [ ];
+          home = imports.home or [ ];
+        };
+      };
+    in
+    ensure (unknownKeys == [ ])
+      "${unit.id}: meta.nix has unsupported keys: ${lib.concatStringsSep ", " unknownKeys}"
+      (
+        ensure (description == null || builtins.isString description)
+          "${unit.id}: meta.description must be a string"
+          (
+            ensure (builtins.isList includes && lib.all builtins.isString includes)
+              "${unit.id}: meta.includes must be a list of fully qualified unit IDs"
+              (
+                ensure (lib.unique includes == includes) "${unit.id}: meta.includes contains duplicate unit IDs" (
+                  ensure (builtins.isAttrs imports) "${unit.id}: meta.imports must be an attribute set" (
+                    ensure (unknownImportKeys == [ ])
+                      "${unit.id}: meta.imports has unsupported classes: ${lib.concatStringsSep ", " unknownImportKeys}"
+                      (
+                        ensure (lib.all builtins.isList [
+                          normalized.imports.nixos
+                          normalized.imports.darwin
+                          normalized.imports.home
+                        ]) "${unit.id}: every meta.imports.<class> value must be a list" normalized
+                      )
+                  )
+                )
+              )
+          )
+      );
+
+  makeUnit =
+    relativePath: entries:
+    let
+      directory = pathFor relativePath;
+      id = lib.concatStringsSep "." relativePath;
+      fragments = lib.mapAttrs (
+        _class: fileName: if entryIsFile entries fileName then directory + "/${fileName}" else null
+      ) reservedFiles;
+      baseUnit = {
+        inherit
+          id
+          directory
+          fragments
+          relativePath
+          ;
+        optionPath = [ "my" ] ++ relativePath ++ [ "enable" ];
+        kind = builtins.head relativePath;
+        name = lib.last relativePath;
+      }
+      //
+        lib.optionalAttrs (builtins.length relativePath > 2 && builtins.head relativePath == "profiles")
+          {
+            group = builtins.elemAt relativePath 1;
+          };
+    in
+    ensure (relativePath != [ ]) "the modules root cannot itself be a unit" (
+      ensure (lib.all (component: component != "" && !(lib.hasInfix "." component)) relativePath)
+        "${id}: path components must be non-empty and must not contain dots"
+        (baseUnit // { meta = normalizeMeta baseUnit; })
+    );
+
+  walk =
+    relativePath:
+    let
+      directory = pathFor relativePath;
+      entries = builtins.readDir directory;
+      hasReservedFile = lib.any (fileName: entryIsFile entries fileName) (
+        builtins.attrValues reservedFiles
+      );
+      childDirectories = lib.filter (name: entries.${name} == "directory") (builtins.attrNames entries);
+      current = lib.optional hasReservedFile (makeUnit relativePath entries);
+      children = lib.concatMap (name: walk (relativePath ++ [ name ])) childDirectories;
+    in
+    current ++ children;
+
+  discoveredUnits =
+    ensure (builtins.pathExists modulesRoot) "modules root does not exist: ${toString modulesRoot}"
+      (walk [ ]);
+  unitsById = builtins.listToAttrs (map (unit: lib.nameValuePair unit.id unit) discoveredUnits);
+
+  dependencyValidation = lib.foldl' (
+    valid: unit:
+    lib.foldl' (
+      inner: includedId:
+      if builtins.hasAttr includedId unitsById then
+        inner
+      else
+        throw "unit registry: ${unit.id} includes missing unit '${includedId}'"
+    ) valid unit.meta.includes
+  ) true discoveredUnits;
+
+  units = builtins.seq dependencyValidation unitsById;
+  unitIds = builtins.attrNames units;
+
+  getUnit =
+    id: if builtins.hasAttr id units then units.${id} else throw "unit registry: unknown unit '${id}'";
+
+  validateUnitIds =
+    ids:
+    ensure (
+      builtins.isList ids && lib.all builtins.isString ids
+    ) "selected units must be a list of strings" (map (id: builtins.seq (getUnit id) id) ids);
+
+  optionDefinitions = lib.foldl' lib.recursiveUpdate { } (
+    map (
+      unit:
+      lib.setAttrByPath unit.optionPath (
+        lib.mkOption {
+          type = lib.types.bool;
+          default = false;
+          description =
+            if unit.meta.description == null then
+              "Whether to enable the ${unit.id} unit."
+            else
+              "Whether to enable ${unit.meta.description}.";
+        }
+      )
+    ) discoveredUnits
+  );
+
+  enabled = config: unit: lib.getAttrFromPath unit.optionPath config;
+
+  enableUnit = id: lib.setAttrByPath (getUnit id).optionPath true;
+
+  includeConfig =
+    config: unit: lib.mkIf (enabled config unit) (lib.mkMerge (map enableUnit unit.meta.includes));
+
+  fragmentClasses = {
+    nixos = [
+      "common"
+      "nixos"
+    ];
+    darwin = [
+      "common"
+      "darwin"
+    ];
+    home = [ "home" ];
+  };
+
+  applyFragment =
+    {
+      config,
+      fragmentPath,
+      options,
+      specialArgs,
+      unit,
+    }:
+    let
+      fragment = import fragmentPath;
+      directArgs = specialArgs // {
+        inherit
+          config
+          lib
+          options
+          specialArgs
+          unit
+          ;
+      };
+      fragmentArgSpec = builtins.functionArgs fragment;
+      fragmentArgs = builtins.listToAttrs (
+        lib.concatMap (
+          name:
+          if builtins.hasAttr name directArgs then
+            [ (lib.nameValuePair name directArgs.${name}) ]
+          else if fragmentArgSpec.${name} then
+            [ ]
+          else
+            [ (lib.nameValuePair name config._module.args.${name}) ]
+        ) (builtins.attrNames fragmentArgSpec)
+      );
+      resultValue = if builtins.isFunction fragment then fragment fragmentArgs else fragment;
+      result =
+        ensure (builtins.isAttrs resultValue)
+          "${unit.id}: ${builtins.baseNameOf fragmentPath} must return an attribute set"
+          resultValue;
+      forbiddenKeys = lib.filter (name: builtins.hasAttr name result) [
+        "imports"
+        "options"
+        "config"
+      ];
+    in
+    ensure (forbiddenKeys == [ ])
+      "${unit.id}: ${builtins.baseNameOf fragmentPath} is a configuration fragment and cannot define top-level ${lib.concatStringsSep ", " forbiddenKeys}"
+      result;
+
+  externalImports = class: lib.concatMap (unit: unit.meta.imports.${class}) discoveredUnits;
+
+  mkModule =
+    { class }:
+    ensure (builtins.hasAttr class fragmentClasses) "unsupported module class '${class}'" (
+      builtins.seq dependencyValidation (
+        {
+          config,
+          lib,
+          options,
+          specialArgs,
+          ...
+        }:
+        let
+          fragmentConfigs = lib.concatMap (
+            unit:
+            lib.filter (value: value != null) (
+              map (
+                fragmentClass:
+                let
+                  fragmentPath = unit.fragments.${fragmentClass};
+                in
+                if fragmentPath == null then
+                  null
+                else
+                  lib.mkIf (enabled config unit) (applyFragment {
+                    inherit
+                      config
+                      fragmentPath
+                      options
+                      specialArgs
+                      unit
+                      ;
+                  })
+              ) fragmentClasses.${class}
+            )
+          ) discoveredUnits;
+        in
+        {
+          imports = externalImports class;
+          options = optionDefinitions;
+          config = lib.mkMerge ((map (includeConfig config) discoveredUnits) ++ fragmentConfigs);
+        }
+      )
+    );
+
+  mkSelectionModule =
+    selectedIds:
+    let
+      checkedIds = validateUnitIds (lib.unique selectedIds);
+    in
+    {
+      config = lib.mkMerge (map enableUnit checkedIds);
+    };
+in
+{
+  inherit
+    getUnit
+    mkModule
+    mkSelectionModule
+    unitIds
+    units
+    validateUnitIds
+    ;
+}
diff --git a/tests/fixtures/registry-invalid-dependency/modules/applications/broken/meta.nix b/tests/fixtures/registry-invalid-dependency/modules/applications/broken/meta.nix
new file mode 100644
index 0000000..6349450
--- /dev/null
+++ b/tests/fixtures/registry-invalid-dependency/modules/applications/broken/meta.nix
@@ -0,0 +1,3 @@
+{
+  includes = [ "applications.missing" ];
+}
diff --git a/tests/fixtures/registry-invalid-fragment/modules/applications/broken/home.nix b/tests/fixtures/registry-invalid-fragment/modules/applications/broken/home.nix
new file mode 100644
index 0000000..e140d9d
--- /dev/null
+++ b/tests/fixtures/registry-invalid-fragment/modules/applications/broken/home.nix
@@ -0,0 +1,3 @@
+{
+  imports = [ ];
+}
diff --git a/tests/fixtures/registry/hosts/darwin.nix b/tests/fixtures/registry/hosts/darwin.nix
new file mode 100644
index 0000000..d7493a0
--- /dev/null
+++ b/tests/fixtures/registry/hosts/darwin.nix
@@ -0,0 +1,3 @@
+{
+  system.stateVersion = 6;
+}
diff --git a/tests/fixtures/registry/hosts/nixos.nix b/tests/fixtures/registry/hosts/nixos.nix
new file mode 100644
index 0000000..3df8577
--- /dev/null
+++ b/tests/fixtures/registry/hosts/nixos.nix
@@ -0,0 +1,9 @@
+{
+  fileSystems."/" = {
+    device = "/dev/null";
+    fsType = "ext4";
+  };
+
+  boot.loader.grub.enable = false;
+  system.stateVersion = "26.05";
+}
diff --git a/tests/fixtures/registry/modules/applications/alpha/common.nix b/tests/fixtures/registry/modules/applications/alpha/common.nix
new file mode 100644
index 0000000..998e720
--- /dev/null
+++ b/tests/fixtures/registry/modules/applications/alpha/common.nix
@@ -0,0 +1,7 @@
+{
+  marker ? "host",
+  ...
+}:
+{
+  test.systemValues = [ "common:${marker}" ];
+}
diff --git a/tests/fixtures/registry/modules/applications/alpha/darwin.nix b/tests/fixtures/registry/modules/applications/alpha/darwin.nix
new file mode 100644
index 0000000..c7fe75c
--- /dev/null
+++ b/tests/fixtures/registry/modules/applications/alpha/darwin.nix
@@ -0,0 +1,3 @@
+{
+  test.systemValues = [ "darwin" ];
+}
diff --git a/tests/fixtures/registry/modules/applications/alpha/home.nix b/tests/fixtures/registry/modules/applications/alpha/home.nix
new file mode 100644
index 0000000..f65f95a
--- /dev/null
+++ b/tests/fixtures/registry/modules/applications/alpha/home.nix
@@ -0,0 +1,3 @@
+{
+  test.homeValues = [ "home" ];
+}
diff --git a/tests/fixtures/registry/modules/applications/alpha/meta.nix b/tests/fixtures/registry/modules/applications/alpha/meta.nix
new file mode 100644
index 0000000..f5c2f6c
--- /dev/null
+++ b/tests/fixtures/registry/modules/applications/alpha/meta.nix
@@ -0,0 +1,39 @@
+{ lib, ... }:
+{
+  description = "alpha test application";
+
+  includes = [ "services.nested" ];
+
+  imports.nixos = [
+    {
+      options.test = {
+        external = lib.mkOption {
+          type = lib.types.str;
+          default = "external-default";
+        };
+        systemValues = lib.mkOption {
+          type = lib.types.listOf lib.types.str;
+          default = [ ];
+        };
+      };
+    }
+  ];
+
+  imports.darwin = [
+    {
+      options.test.systemValues = lib.mkOption {
+        type = lib.types.listOf lib.types.str;
+        default = [ ];
+      };
+    }
+  ];
+
+  imports.home = [
+    {
+      options.test.homeValues = lib.mkOption {
+        type = lib.types.listOf lib.types.str;
+        default = [ ];
+      };
+    }
+  ];
+}
diff --git a/tests/fixtures/registry/modules/applications/alpha/nixos.nix b/tests/fixtures/registry/modules/applications/alpha/nixos.nix
new file mode 100644
index 0000000..395eecc
--- /dev/null
+++ b/tests/fixtures/registry/modules/applications/alpha/nixos.nix
@@ -0,0 +1,4 @@
+{
+  test.systemValues = [ "nixos" ];
+  test.external = "set-by-nixos-fragment";
+}
diff --git a/tests/fixtures/registry/modules/applications/beta/home.nix b/tests/fixtures/registry/modules/applications/beta/home.nix
new file mode 100644
index 0000000..11a981f
--- /dev/null
+++ b/tests/fixtures/registry/modules/applications/beta/home.nix
@@ -0,0 +1,3 @@
+{
+  test.homeValues = [ "beta-home" ];
+}
diff --git a/tests/fixtures/registry/modules/namespace/helper.nix b/tests/fixtures/registry/modules/namespace/helper.nix
new file mode 100644
index 0000000..c0edb1a
--- /dev/null
+++ b/tests/fixtures/registry/modules/namespace/helper.nix
@@ -0,0 +1,3 @@
+{
+  thisFileMustNotBeDiscovered = true;
+}
diff --git a/tests/fixtures/registry/modules/profiles/interface/test/meta.nix b/tests/fixtures/registry/modules/profiles/interface/test/meta.nix
new file mode 100644
index 0000000..6f2061d
--- /dev/null
+++ b/tests/fixtures/registry/modules/profiles/interface/test/meta.nix
@@ -0,0 +1,6 @@
+{
+  includes = [
+    "applications.alpha"
+    "applications.beta"
+  ];
+}
diff --git a/tests/fixtures/registry/modules/services/nested/meta.nix b/tests/fixtures/registry/modules/services/nested/meta.nix
new file mode 100644
index 0000000..c851c93
--- /dev/null
+++ b/tests/fixtures/registry/modules/services/nested/meta.nix
@@ -0,0 +1,11 @@
+{ lib, ... }:
+{
+  imports.nixos = [
+    {
+      options.test.nested = lib.mkOption {
+        type = lib.types.bool;
+        default = false;
+      };
+    }
+  ];
+}
diff --git a/tests/fixtures/registry/modules/services/nested/nixos.nix b/tests/fixtures/registry/modules/services/nested/nixos.nix
new file mode 100644
index 0000000..9cf542c
--- /dev/null
+++ b/tests/fixtures/registry/modules/services/nested/nixos.nix
@@ -0,0 +1,3 @@
+{
+  test.nested = true;
+}
diff --git a/tests/fixtures/registry/modules/users/test/home.nix b/tests/fixtures/registry/modules/users/test/home.nix
new file mode 100644
index 0000000..4ce2994
--- /dev/null
+++ b/tests/fixtures/registry/modules/users/test/home.nix
@@ -0,0 +1,8 @@
+{ pkgs, ... }:
+{
+  home = {
+    username = "test";
+    homeDirectory = if pkgs.stdenv.hostPlatform.isDarwin then "/Users/test" else "/home/test";
+    stateVersion = "26.05";
+  };
+}
diff --git a/tests/registry.nix b/tests/registry.nix
new file mode 100644
index 0000000..2cc21fb
--- /dev/null
+++ b/tests/registry.nix
@@ -0,0 +1,134 @@
+{
+  inputs,
+  lib,
+  pkgs,
+}:
+let
+  registry = import ../libs/registry.nix {
+    inherit inputs lib;
+    modulesRoot = ./fixtures/registry/modules;
+  };
+
+  hostLib = import ../libs/hosts.nix {
+    inherit inputs lib registry;
+  };
+
+  baseModule = {
+    options = {
+      home = {
+        username = lib.mkOption { type = lib.types.str; };
+        homeDirectory = lib.mkOption { type = lib.types.str; };
+        stateVersion = lib.mkOption { type = lib.types.str; };
+      };
+    };
+  };
+
+  eval =
+    class: selected:
+    lib.evalModules {
+      specialArgs = {
+        inherit pkgs;
+        marker = "special-arg";
+      };
+      modules = [
+        baseModule
+        (registry.mkModule { inherit class; })
+        (registry.mkSelectionModule selected)
+      ];
+    };
+
+  nixos = eval "nixos" [ "profiles.interface.test" ];
+  darwin = eval "darwin" [ "applications.alpha" ];
+  home = eval "home" [ "profiles.interface.test" ];
+
+  nixosHost = hostLib.mkNixos "registry-test" {
+    system = "x86_64-linux";
+    user = "test";
+    path = ./fixtures/registry/hosts;
+    profiles = [ "interface.test" ];
+  };
+
+  darwinHost = hostLib.mkDarwin "registry-test-darwin" {
+    system = "aarch64-darwin";
+    user = "test";
+    path = ./fixtures/registry/hosts;
+    applications = [ "alpha" ];
+  };
+
+  missingUnit = builtins.tryEval (
+    builtins.deepSeq (registry.validateUnitIds [ "applications.missing" ]) true
+  );
+
+  invalidDependencyRegistry = import ../libs/registry.nix {
+    inherit inputs lib;
+    modulesRoot = ./fixtures/registry-invalid-dependency/modules;
+  };
+  missingDependency = builtins.tryEval (builtins.deepSeq invalidDependencyRegistry.unitIds true);
+
+  invalidFragmentRegistry = import ../libs/registry.nix {
+    inherit inputs lib;
+    modulesRoot = ./fixtures/registry-invalid-fragment/modules;
+  };
+  invalidFragmentEvaluation = lib.evalModules {
+    modules = [
+      (invalidFragmentRegistry.mkModule { class = "home"; })
+      (invalidFragmentRegistry.mkSelectionModule [ "applications.broken" ])
+    ];
+  };
+  invalidFragment = builtins.tryEval (builtins.deepSeq invalidFragmentEvaluation.config true);
+
+  assertions =
+    assert
+      registry.unitIds == [
+        "applications.alpha"
+        "applications.beta"
+        "profiles.interface.test"
+        "services.nested"
+        "users.test"
+      ];
+    assert !(builtins.elem "namespace" registry.unitIds);
+    assert nixos.config.my.profiles.interface.test.enable;
+    assert nixos.config.my.applications.alpha.enable;
+    assert nixos.config.my.applications.beta.enable;
+    assert nixos.config.my.services.nested.enable;
+    assert nixos.config.test.nested;
+    assert
+      nixos.config.test.systemValues == [
+        "common:special-arg"
+        "nixos"
+      ];
+    assert nixos.config.test.external == "set-by-nixos-fragment";
+    assert
+      darwin.config.test.systemValues == [
+        "common:special-arg"
+        "darwin"
+      ];
+    assert
+      home.config.test.homeValues == [
+        "home"
+        "beta-home"
+      ];
+    assert nixosHost.config.my.profiles.interface.test.enable;
+    assert nixosHost.config.home-manager.users.test.my.applications.alpha.enable;
+    assert
+      nixosHost.config.home-manager.users.test.test.homeValues == [
+        "home"
+        "beta-home"
+      ];
+    assert darwinHost.config.my.applications.alpha.enable;
+    assert darwinHost.config.home-manager.users.test.my.applications.alpha.enable;
+    assert darwinHost.config.home-manager.users.test.test.homeValues == [ "home" ];
+    assert
+      darwinHost.config.test.systemValues == [
+        "common:host"
+        "darwin"
+      ];
+    assert !missingUnit.success;
+    assert !missingDependency.success;
+    assert !invalidFragment.success;
+    true;
+in
+assert assertions;
+pkgs.runCommand "unit-registry-evaluation-tests" { } ''
+  touch "$out"
+''