diff --git a/nixos/modules/misc/nixpkgs.nix b/nixos/modules/misc/nixpkgs.nix
index 02bb76cce6c5..a9e49a2a26cd 100644
--- a/nixos/modules/misc/nixpkgs.nix
+++ b/nixos/modules/misc/nixpkgs.nix
@@ -34,6 +34,8 @@ let
     ++ lib.optional (opt.localSystem.highestPrio < (lib.mkOptionDefault { }).priority) opt.localSystem
     ++ lib.optional (opt.crossSystem.highestPrio < (lib.mkOptionDefault { }).priority) opt.crossSystem;
 
+  _configDefinitions = opt.config.definitionsWithLocations;
+
   defaultPkgs =
     if opt.hostPlatform.isDefined then
       let
@@ -51,14 +53,15 @@ let
       in
       import ../../.. (
         {
-          inherit (cfg) config overlays;
+          inherit _configDefinitions;
+          inherit (cfg) overlays;
         }
         // systemArgs
       )
     else
       import ../../.. {
+        inherit _configDefinitions;
         inherit (cfg)
-          config
           overlays
           localSystem
           crossSystem
@@ -126,13 +129,15 @@ in
       example = lib.literalExpression ''
         { allowBroken = true; allowUnfree = true; }
       '';
-      type = lib.types.submoduleWith {
-        modules = [ ../../../pkgs/top-level/config.nix ];
-        specialArgs = {
-          docPrefix = "https://nixos.org/manual/nixpkgs/unstable/";
-        };
-        shorthandOnlyDefinesConfig = true;
+      type = lib.types.deferredModuleWith {
+        staticModules = [
+          { _module.args.docPrefix = "https://nixos.org/manual/nixpkgs/unstable/"; }
+          ../../../pkgs/top-level/config.nix
+        ];
       };
+      # Returns pkgs.config instead of nixpkgs.config
+      # This shadows the deferredModule to make it look like a submodule
+      apply = _: finalPkgs.config;
       description = ''
         Global configuration for Nixpkgs.
         The complete list of [Nixpkgs configuration options](https://nixos.org/manual/nixpkgs/unstable/#sec-config-options-reference) is in the [Nixpkgs manual section on global configuration](https://nixos.org/manual/nixpkgs/unstable/#chap-packageconfig).
diff --git a/pkgs/test/config-nix-unit.nix b/pkgs/test/config-nix-unit.nix
new file mode 100644
index 000000000000..cd16d5ca1c43
--- /dev/null
+++ b/pkgs/test/config-nix-unit.nix
@@ -0,0 +1,117 @@
+# Tests for nixpkgs config forwarding from NixOS modules.
+#
+# Run with:
+#   nix-unit pkgs/test/config-nix-unit.nix
+# or
+#   nix-build -A tests.config-nix-unit
+#
+{
+  nixpkgsPath ? ../..,
+  pkgs ? import nixpkgsPath { },
+}:
+let
+  lib = pkgs.lib;
+
+  # Test helper
+  evalNixos =
+    modules:
+    import (nixpkgsPath + "/nixos/lib/eval-config.nix") {
+      modules = [ { nixpkgs.hostPlatform = "x86_64-linux"; } ] ++ modules;
+    };
+in
+{
+  # Basic: a single config option is forwarded correctly.
+  testSingleConfigOption = {
+    expr = (evalNixos [ { nixpkgs.config.allowUnfree = true; } ]).config.nixpkgs.config.allowUnfree;
+    expected = true;
+  };
+
+  # Multiple config definitions from separate modules are merged.
+  testMultipleModulesMerge = {
+    expr =
+      let
+        eval = evalNixos [
+          { nixpkgs.config.allowUnfree = true; }
+          { nixpkgs.config.allowBroken = true; }
+        ];
+      in
+      {
+        inherit (eval.config.nixpkgs.config) allowUnfree allowBroken;
+      };
+    expected = {
+      allowUnfree = true;
+      allowBroken = true;
+    };
+  };
+
+  # mkForce works. Also covers other properties
+  testMkForce = {
+    expr =
+      (evalNixos [
+        { nixpkgs.config.allowUnfree = true; }
+        { nixpkgs.config.allowUnfree = lib.mkForce false; }
+      ]).config.nixpkgs.config.allowUnfree;
+    expected = false;
+  };
+
+  testDefaults = {
+    expr = (evalNixos [ ]).config.nixpkgs.config.allowUnfree;
+    expected = false;
+  };
+
+  # Standalone nixpkgs (i.e. import <nixpkgs> { ... })
+  testStandaloneConfig = {
+    expr = (import nixpkgsPath { config.allowUnfree = true; }).config.allowUnfree;
+    expected = true;
+  };
+
+  # Standalone nixpkgs with a function (i.e. import <nixpkgs> ({pkgs, lib, ...}: { ... })
+  testStandaloneConfigFunctionPkgs = {
+    expr =
+      (import nixpkgsPath {
+        config =
+          { pkgs, lib, ... }:
+          {
+            allowUnfree = lib.isAttrs pkgs;
+          };
+      }).config.allowUnfree;
+    expected = true;
+  };
+
+  # NixOS module sets nixpkgs.config as a function
+  testNixosConfigFunction = {
+    expr =
+      (evalNixos [
+        {
+          nixpkgs.config =
+            { lib, ... }:
+            {
+              allowUnfree = lib.isFunction lib.id;
+            };
+        }
+      ]).config.nixpkgs.config.allowUnfree;
+    expected = true;
+  };
+
+  # Passing both config and _configDefinitions is not allowed
+  testConfigAndDefinitionsMutuallyExclusive = {
+    expr =
+      (import nixpkgsPath {
+        config = {
+          allowUnfree = true;
+        };
+        _configDefinitions = [
+          {
+            file = "test";
+            value = {
+              allowBroken = true;
+            };
+          }
+        ];
+      }).config.allowUnfree;
+    expectedError = {
+      type = "ThrownError";
+      msg = ".*_configDefinitions.*internal.*must not be combined.*";
+    };
+  };
+}
diff --git a/pkgs/test/default.nix b/pkgs/test/default.nix
index 927078483580..cd7a1436ebf1 100644
--- a/pkgs/test/default.nix
+++ b/pkgs/test/default.nix
@@ -128,6 +128,21 @@ in
 
   config = callPackage ./config.nix { };
 
+  # Technically nix-unit binds to a fixed nix version
+  # We have tests in lib to test the module system itself against different nix-versions
+  # Based on this assumption (transitivity of correctness) this test should therefore also cover all tested nix-versions
+  config-nix-unit =
+    pkgs.runCommand "config-nix-unit"
+      {
+        nativeBuildInputs = [ pkgs.nix-unit ];
+      }
+      ''
+        export HOME=$TMPDIR
+        nix-unit --eval-store "$HOME" ${./config-nix-unit.nix} \
+          --arg nixpkgsPath "${../..}"
+        mkdir $out
+      '';
+
   top-level = callPackage ./top-level { };
 
   haskell = callPackage ./haskell { };
diff --git a/pkgs/top-level/config.nix b/pkgs/top-level/config.nix
index 39f32645e45a..8c4438ae154d 100644
--- a/pkgs/top-level/config.nix
+++ b/pkgs/top-level/config.nix
@@ -9,11 +9,11 @@
 {
   config,
   lib,
+  docPrefix ? "",
   ...
-}@args:
+}:
 
 let
-  docPrefix = args.docPrefix or "";
   inherit (lib)
     literalExpression
     mapAttrsToList
@@ -471,7 +471,7 @@ let
         Silence the warning for the upcoming deprecation of the
         `x86_64-darwin` platform in Nixpkgs 26.11.
 
-        See the [release notes](#x86_64-darwin-26.05) for more
+        See the [release notes](${docPrefix}#x86_64-darwin-26.05) for more
         information.
       '';
     };
diff --git a/pkgs/top-level/default.nix b/pkgs/top-level/default.nix
index 521745d67ec9..12fdbed08197 100644
--- a/pkgs/top-level/default.nix
+++ b/pkgs/top-level/default.nix
@@ -64,6 +64,11 @@ in
   # list it returns.
   stdenvStages ? import ../stdenv,
 
+  # Temporary parameter to unify nixpkgs/pkgs evaluation
+  # Internal, do not use this manually!
+  # Will be removed again within the next releases
+  _configDefinitions ? null,
+
   # Ignore unexpected args.
   ...
 }@args:
@@ -109,7 +114,13 @@ let
         then
           x86_64DarwinDeprecationWarning
         else
-          x: x
+          x:
+          x throwIfNot (lib.all lib.isFunction crossOverlays)
+            "All crossOverlays passed to nixpkgs must be functions."
+      )
+      (
+        throwIfNot (_configDefinitions == null || config0 == { })
+          "The `_configDefinitions` argument is an internal interface and must not be combined with `config`."
       );
 
   localSystem = lib.systems.elaborate args.localSystem;
@@ -134,20 +145,24 @@ let
 
   # Allow both:
   # { /* the config */ } and
-  # { pkgs, ... } : { /* the config */ }
+  # { lib, pkgs, ... } : { /* the config */ }
   config1 = if lib.isFunction config0 then config0 { inherit lib pkgs; } else config0;
 
   configEval = lib.evalModules {
     modules = [
       ./config.nix
-      (
-        { options, ... }:
-        {
-          _file = "nixpkgs.config";
-          config = config1;
-        }
-      )
-    ];
+    ]
+    ++ (
+      if _configDefinitions != null then
+        map (def: lib.modules.setDefaultModuleLocation def.file def.value) _configDefinitions
+      else
+        [
+          {
+            _file = "nixpkgs.config";
+            config = config1;
+          }
+        ]
+    );
     class = "nixpkgsConfig";
   };