diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 1f248798..bcc4f4c9 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -84,7 +84,7 @@ jobs:
     # max-parallel: 2
     strategy:
       matrix:
-        template: [bogus, minimal, microvm, nvf-standalone]
+        template: [bogus, minimal, microvm, nvf-standalone, flake-parts-modules]
         os: [ubuntu-latest]
     name: Check template ${{matrix.template}} ${{matrix.os}}
     runs-on: ${{matrix.os}}
diff --git a/nix/default.nix b/nix/default.nix
index f8448f27..145a432a 100644
--- a/nix/default.nix
+++ b/nix/default.nix
@@ -19,6 +19,8 @@ let
     microvm.description = "MicroVM example";
     nvf-standalone.path = ../templates/nvf-standalone;
     nvf-standalone.description = "Standalone NVF";
+    flake-parts-modules.path = ../templates/flake-parts-modules;
+    flake-parts-modules.description = "flake-parts classes";
     ci.path = ../templates/ci;
     ci.description = "Feature Tests";
     bogus.path = ../templates/bogus;
diff --git a/nix/lib/forward.nix b/nix/lib/forward.nix
index e681f0f2..e6ffe387 100644
--- a/nix/lib/forward.nix
+++ b/nix/lib/forward.nix
@@ -33,7 +33,7 @@ let
         };
 
       freeformMod = {
-        config._module.freeformType = lib.types.lazyAttrsOf lib.types.anything;
+        config._module.freeformType = lib.types.lazyAttrsOf lib.types.unspecified;
       };
 
       adapterMods = [
diff --git a/templates/flake-parts-modules/README.md b/templates/flake-parts-modules/README.md
new file mode 100644
index 00000000..0c364658
--- /dev/null
+++ b/templates/flake-parts-modules/README.md
@@ -0,0 +1,7 @@
+# Example Den custom classes for flake-parts modules and perSystem outputs.
+
+This example showcases Den forward classes for third-party flake-parts modules.
+
+This README is generated by `nix run .#write-files`.
+
+See den.nix, custom-classes.nix and perSystem-forward.nix
diff --git a/templates/flake-parts-modules/flake.lock b/templates/flake-parts-modules/flake.lock
new file mode 100644
index 00000000..ee08026a
--- /dev/null
+++ b/templates/flake-parts-modules/flake.lock
@@ -0,0 +1,206 @@
+{
+  "nodes": {
+    "den": {
+      "locked": {
+        "lastModified": 1774981725,
+        "narHash": "sha256-eaNpxQLoNpALVyiolZQEIoi6eH529jcXeQ7LfuwtHIY=",
+        "owner": "vic",
+        "repo": "den",
+        "rev": "66677bf5f235c5f5fe742e3839f6c184d66065df",
+        "type": "github"
+      },
+      "original": {
+        "owner": "vic",
+        "repo": "den",
+        "type": "github"
+      }
+    },
+    "devshell": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1768818222,
+        "narHash": "sha256-460jc0+CZfyaO8+w8JNtlClB2n4ui1RbHfPTLkpwhU8=",
+        "owner": "numtide",
+        "repo": "devshell",
+        "rev": "255a2b1725a20d060f566e4755dbf571bbbb5f76",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "devshell",
+        "type": "github"
+      }
+    },
+    "files": {
+      "locked": {
+        "lastModified": 1762767848,
+        "narHash": "sha256-trF2WXzH57uSkA1zeI8FTz8tU6vPYrklkTjh/7DZy/M=",
+        "owner": "mightyiam",
+        "repo": "files",
+        "rev": "2d9d4937e94006ad8f165dfab9dff87cfab7eb2b",
+        "type": "github"
+      },
+      "original": {
+        "owner": "mightyiam",
+        "repo": "files",
+        "type": "github"
+      }
+    },
+    "flake-parts": {
+      "inputs": {
+        "nixpkgs-lib": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1772408722,
+        "narHash": "sha256-rHuJtdcOjK7rAHpHphUb1iCvgkU3GpfvicLMwwnfMT0=",
+        "owner": "hercules-ci",
+        "repo": "flake-parts",
+        "rev": "f20dc5d9b8027381c474144ecabc9034d6a839a3",
+        "type": "github"
+      },
+      "original": {
+        "owner": "hercules-ci",
+        "repo": "flake-parts",
+        "type": "github"
+      }
+    },
+    "import-tree": {
+      "locked": {
+        "lastModified": 1773693634,
+        "narHash": "sha256-BtZ2dtkBdSUnFPPFc+n0kcMbgaTxzFNPv2iaO326Ffg=",
+        "owner": "vic",
+        "repo": "import-tree",
+        "rev": "c41e7d58045f9057880b0d85e1152d6a4430dbf1",
+        "type": "github"
+      },
+      "original": {
+        "owner": "vic",
+        "repo": "import-tree",
+        "type": "github"
+      }
+    },
+    "nix-topology": {
+      "inputs": {
+        "flake-parts": [
+          "flake-parts"
+        ],
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1774824790,
+        "narHash": "sha256-3R2aoykbutdJ7YQaZiU7uO8w4O8b6RjztTPNo8isLTI=",
+        "owner": "oddlama",
+        "repo": "nix-topology",
+        "rev": "5765ce41be8a4fb5471a57671c2b740a350c5da0",
+        "type": "github"
+      },
+      "original": {
+        "owner": "oddlama",
+        "repo": "nix-topology",
+        "type": "github"
+      }
+    },
+    "nix-unit": {
+      "inputs": {
+        "flake-parts": [
+          "flake-parts"
+        ],
+        "nix-github-actions": [],
+        "nixpkgs": [
+          "nixpkgs"
+        ],
+        "treefmt-nix": [
+          "treefmt-nix"
+        ]
+      },
+      "locked": {
+        "lastModified": 1762774186,
+        "narHash": "sha256-hRADkHjNt41+JUHw2EiSkMaL4owL83g5ZppjYUdF/Dc=",
+        "owner": "nix-community",
+        "repo": "nix-unit",
+        "rev": "1c9ab50554eed0b768f9e5b6f646d63c9673f0f7",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-community",
+        "repo": "nix-unit",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1774855581,
+        "narHash": "sha256-YkreHeMgTCYvJ5fESV0YyqQK49bHGe2B51tH6claUh4=",
+        "owner": "nixos",
+        "repo": "nixpkgs",
+        "rev": "15c6719d8c604779cf59e03c245ea61d3d7ab69b",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nixos",
+        "ref": "nixpkgs-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "pkgs-by-name-for-flake-parts": {
+      "locked": {
+        "lastModified": 1769286518,
+        "narHash": "sha256-F7VlUICvYp2wJKyQqM6d46BNEZCzMQ6dMH9zbmjuybc=",
+        "owner": "drupol",
+        "repo": "pkgs-by-name-for-flake-parts",
+        "rev": "7ba1cd4a9a72c9c6c272018a63f090f2c912a171",
+        "type": "github"
+      },
+      "original": {
+        "owner": "drupol",
+        "repo": "pkgs-by-name-for-flake-parts",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "den": "den",
+        "devshell": "devshell",
+        "files": "files",
+        "flake-parts": "flake-parts",
+        "import-tree": "import-tree",
+        "nix-topology": "nix-topology",
+        "nix-unit": "nix-unit",
+        "nixpkgs": "nixpkgs",
+        "pkgs-by-name-for-flake-parts": "pkgs-by-name-for-flake-parts",
+        "treefmt-nix": "treefmt-nix"
+      }
+    },
+    "treefmt-nix": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1773297127,
+        "narHash": "sha256-6E/yhXP7Oy/NbXtf1ktzmU8SdVqJQ09HC/48ebEGBpk=",
+        "owner": "numtide",
+        "repo": "treefmt-nix",
+        "rev": "71b125cd05fbfd78cab3e070b73544abe24c5016",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "treefmt-nix",
+        "type": "github"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}
diff --git a/templates/flake-parts-modules/flake.nix b/templates/flake-parts-modules/flake.nix
new file mode 100644
index 00000000..b1b363f6
--- /dev/null
+++ b/templates/flake-parts-modules/flake.nix
@@ -0,0 +1,38 @@
+{
+  description = "Using flake-parts modules from Den";
+
+  outputs = inputs: inputs.flake-parts.lib.mkFlake { inherit inputs; } (inputs.import-tree ./modules);
+
+  inputs = {
+    den.url = "github:vic/den";
+    import-tree.url = "github:vic/import-tree";
+    nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
+    flake-parts.url = "github:hercules-ci/flake-parts";
+    flake-parts.inputs.nixpkgs-lib.follows = "nixpkgs";
+
+    pkgs-by-name-for-flake-parts.url = "github:drupol/pkgs-by-name-for-flake-parts";
+
+    files.url = "github:mightyiam/files";
+
+    devshell.url = "github:numtide/devshell";
+    devshell.inputs.nixpkgs.follows = "nixpkgs";
+
+    treefmt-nix.url = "github:numtide/treefmt-nix";
+    treefmt-nix.inputs.nixpkgs.follows = "nixpkgs";
+
+    nix-unit.url = "github:nix-community/nix-unit";
+    nix-unit.inputs = {
+      nixpkgs.follows = "nixpkgs";
+      flake-parts.follows = "flake-parts";
+      nix-github-actions.follows = "";
+      treefmt-nix.follows = "treefmt-nix";
+    };
+
+    nix-topology.url = "github:oddlama/nix-topology";
+    nix-topology.inputs = {
+      nixpkgs.follows = "nixpkgs";
+      flake-parts.follows = "flake-parts";
+    };
+
+  };
+}
diff --git a/templates/flake-parts-modules/modules/custom-classes.nix b/templates/flake-parts-modules/modules/custom-classes.nix
new file mode 100644
index 00000000..76655a57
--- /dev/null
+++ b/templates/flake-parts-modules/modules/custom-classes.nix
@@ -0,0 +1,94 @@
+{
+  den,
+  lib,
+  inputs,
+  config,
+  ...
+}:
+{
+  systems = builtins.attrNames den.hosts;
+
+  # Some third-party flake-parts modules for demo purposes.
+  # Read their documentation at https://flake.parts for usage.
+  imports = [
+    inputs.devshell.flakeModule
+    inputs.files.flakeModules.default
+    inputs.nix-topology.flakeModule
+    inputs.nix-unit.modules.flake.default
+    inputs.pkgs-by-name-for-flake-parts.flakeModule
+    inputs.treefmt-nix.flakeModule
+  ];
+
+  # some globals
+  perSystem = {
+    pkgsDirectory = ../packages;
+    nix-unit = {
+      allowNetwork = true;
+      inputs = inputs;
+    };
+  };
+
+  # den.ctx.flake-parts transformations define which
+  # custom classes exist and where to read them.
+  den.ctx.flake-parts.into = _: {
+
+    # Read flake-parts classes from hosts and their includes
+    host = map (host: { inherit host; }) (
+      builtins.concatMap builtins.attrValues (builtins.attrValues den.hosts)
+    );
+
+    # Our custom flake-parts perSystem classes.
+    # These are partial params for `den._.forward`.
+    # See ./perSystem-forward.nix
+    flake-parts-system = [
+
+      # A class for flake-parts' perSystem.packages
+      # NOTE: this is different from Den's flake-packages class.
+      {
+        fromClass = _: "packages";
+        intoPath = _: [ "packages" ];
+      }
+
+      {
+        fromClass = _: "files";
+        intoPath = _: [
+          "files"
+        ];
+        adapterModule = { };
+      }
+
+      # a default `devshell` class
+      {
+        fromClass = _: "devshell";
+        intoPath = _: [
+          "devshells"
+          "default"
+        ];
+      }
+
+      {
+        fromClass = _: "treefmt";
+        intoPath = _: [ "treefmt" ];
+      }
+
+      {
+        fromClass = _: "tests";
+        intoPath = _: [
+          "nix-unit"
+          "tests"
+        ];
+        # test helpers
+        adaptArgs =
+          { pkgs, ... }:
+          let
+            igloo = config.flake.nixosConfigurations.igloo.config;
+            tux = igloo.users.users.tux;
+          in
+          {
+            inherit pkgs igloo tux;
+          };
+      }
+    ];
+  };
+
+}
diff --git a/templates/flake-parts-modules/modules/den.nix b/templates/flake-parts-modules/modules/den.nix
new file mode 100644
index 00000000..8a10ba3c
--- /dev/null
+++ b/templates/flake-parts-modules/modules/den.nix
@@ -0,0 +1,98 @@
+{
+  den,
+  lib,
+  inputs,
+  config,
+  ...
+}:
+{
+  imports = [ inputs.den.flakeModule ];
+
+  den.hosts.x86_64-linux.igloo.users.tux = { };
+
+  den.aspects.igloo = {
+    # User TODO: Remove for real host
+    nixos.fileSystems."/".device = "/dev/no-boot";
+    nixos.boot.loader.grub.enable = false;
+  };
+
+  den.aspects.tux = {
+    includes = [ den._.define-user ];
+    user.description = "Bird";
+
+    tests =
+      { tux, ... }:
+      {
+        test-tux-is-bird = {
+          expr = tux.description;
+          expected = "Bird";
+        };
+      };
+  };
+
+  # Read flake-parts classes from foo aspect and its includes
+  den.ctx.flake-parts.includes = [ den.aspects.foo ];
+
+  den.aspects.foo = {
+    includes = [ den.aspects.bar ];
+
+    treefmt.settings.on-unmatched = "warn";
+
+    devshell.commands = [ { package = "cowsay"; } ];
+
+    packages =
+      { pkgs, ... }:
+      {
+        topology = config.flake.topology.${pkgs.stdenv.targetPlatform.system}.config.output;
+      };
+  };
+
+  den.aspects.bar = {
+    treefmt.programs.nixfmt.enable = true;
+
+    tests.test-math-works = {
+      expr = 22 * 2;
+      expected = 44;
+    };
+
+    packages =
+      { config, ... }:
+      {
+        write-files = config.files.writer.drv;
+      };
+
+    files =
+      { pkgs, ... }:
+      {
+        files = [
+          {
+            path_ = "README.md";
+            drv = pkgs.writeText "README.md" ''
+              # Example Den custom classes for flake-parts modules and perSystem outputs.
+
+              This example showcases Den forward classes for third-party flake-parts modules.
+
+              This README is generated by `nix run .#write-files`.
+
+              See den.nix, custom-classes.nix and perSystem-forward.nix
+            '';
+          }
+        ];
+      };
+
+    devshell =
+      { pkgs, self', ... }:
+      {
+        commands = [
+          {
+            package = self'.packages.write-files;
+          }
+        ];
+        packages = [
+          pkgs.hello
+        ];
+      };
+
+  };
+
+}
diff --git a/templates/flake-parts-modules/modules/perSystem-forward.nix b/templates/flake-parts-modules/modules/perSystem-forward.nix
new file mode 100644
index 00000000..bfb0cbef
--- /dev/null
+++ b/templates/flake-parts-modules/modules/perSystem-forward.nix
@@ -0,0 +1,24 @@
+{ den, lib, ... }:
+let
+
+  perSystemFwd =
+    forwardArgs:
+    { class, aspect-chain }:
+    den._.forward (
+      {
+        each = lib.optional (class == "flake-parts") forwardArgs;
+        intoClass = _: "flake-parts";
+        fromAspect = _: lib.head aspect-chain;
+        adaptArgs = { config, ... }: config.allModuleArgs;
+      }
+      // forwardArgs
+    );
+
+  ctx.flake-parts = { };
+  ctx.flake-parts-system.provides.flake-parts-system = perSystemFwd;
+  perSystemModule = den.lib.aspects.resolve "flake-parts" (den.ctx.flake-parts { });
+in
+{
+  den.ctx = ctx;
+  perSystem.imports = [ perSystemModule ];
+}
diff --git a/templates/flake-parts-modules/packages/hola.nix b/templates/flake-parts-modules/packages/hola.nix
new file mode 100644
index 00000000..ac18240a
--- /dev/null
+++ b/templates/flake-parts-modules/packages/hola.nix
@@ -0,0 +1 @@
+{ hello }: hello