diff --git a/nixos/modules/misc/nixpkgs.nix b/nixos/modules/misc/nixpkgs.nix
index a9e49a2a26cd..dcb05e349197 100644
--- a/nixos/modules/misc/nixpkgs.nix
+++ b/nixos/modules/misc/nixpkgs.nix
@@ -2,12 +2,51 @@
   config,
   options,
   lib,
+  pkgs,
   ...
 }:
 let
   cfg = config.nixpkgs;
   opt = options.nixpkgs;
 
+  isConfig = x: builtins.isAttrs x || lib.isFunction x;
+
+  optCall = f: x: if lib.isFunction f then f x else f;
+
+  mergeConfig =
+    lhs_: rhs_:
+    let
+      lhs = optCall lhs_ { inherit lib pkgs; };
+      rhs = optCall rhs_ { inherit lib pkgs; };
+    in
+    lib.recursiveUpdate lhs rhs
+    // lib.optionalAttrs (lhs ? allowUnfreePackages) {
+      allowUnfreePackages = lhs.allowUnfreePackages ++ (lib.attrByPath [ "allowUnfreePackages" ] [ ] rhs);
+    }
+    // lib.optionalAttrs (lhs ? packageOverrides) {
+      packageOverrides =
+        pkgs:
+        optCall lhs.packageOverrides pkgs // optCall (lib.attrByPath [ "packageOverrides" ] { } rhs) pkgs;
+    }
+    // lib.optionalAttrs (lhs ? perlPackageOverrides) {
+      perlPackageOverrides =
+        pkgs:
+        optCall lhs.perlPackageOverrides pkgs
+        // optCall (lib.attrByPath [ "perlPackageOverrides" ] { } rhs) pkgs;
+    };
+
+  configType = lib.mkOptionType {
+    name = "nixpkgs-config";
+    description = "nixpkgs config";
+    check =
+      x:
+      let
+        traceXIfNot = c: if c x then true else lib.traceSeqN 1 x false;
+      in
+      traceXIfNot isConfig;
+    merge = args: lib.foldr (def: mergeConfig def.value) { };
+  };
+
   overlayType = lib.mkOptionType {
     name = "nixpkgs-overlay";
     description = "nixpkgs overlay";
@@ -34,8 +73,6 @@ 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
@@ -53,15 +90,14 @@ let
       in
       import ../../.. (
         {
-          inherit _configDefinitions;
-          inherit (cfg) overlays;
+          inherit (cfg) config overlays;
         }
         // systemArgs
       )
     else
       import ../../.. {
-        inherit _configDefinitions;
         inherit (cfg)
+          config
           overlays
           localSystem
           crossSystem
@@ -129,15 +165,7 @@ in
       example = lib.literalExpression ''
         { allowBroken = true; allowUnfree = 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;
+      type = configType;
       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).
@@ -378,7 +406,7 @@ in
           '';
         }
         {
-          assertion = opt.pkgs.isDefined -> opt.config.highestPrio == (lib.mkOptionDefault null).priority;
+          assertion = opt.pkgs.isDefined -> cfg.config == { };
           message = ''
             Your system configures nixpkgs with an externally created instance.
             `nixpkgs.config` options should be passed when creating the instance instead.
diff --git a/pkgs/test/config-nix-unit.nix b/pkgs/test/config-nix-unit.nix
deleted file mode 100644
index cd16d5ca1c43..000000000000
--- a/pkgs/test/config-nix-unit.nix
+++ /dev/null
@@ -1,117 +0,0 @@
-# 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 50a5cf2527ed..63e8e2b32aae 100644
--- a/pkgs/test/default.nix
+++ b/pkgs/test/default.nix
@@ -128,26 +128,6 @@ 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" ${./config-nix-unit.nix} \
-          --arg nixpkgsPath "${
-            builtins.path {
-              path = pkgs.path;
-              name = "source";
-            }
-          }"
-        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 a92dd200ccf6..526dd4182702 100644
--- a/pkgs/top-level/config.nix
+++ b/pkgs/top-level/config.nix
@@ -6,12 +6,7 @@
 #     nix-build -A tests.config
 #
 
-{
-  config,
-  lib,
-  docPrefix,
-  ...
-}:
+{ config, lib, ... }:
 
 let
   inherit (lib)
@@ -120,13 +115,13 @@ let
     gitConfig = mkOption {
       type = types.attrsOf (types.attrsOf types.anything);
       description = ''
-        The default [git configuration](https://git-scm.com/docs/git-config#_variables) for all [`pkgs.fetchgit`](${docPrefix}#fetchgit) calls.
+        The default [git configuration](https://git-scm.com/docs/git-config#_variables) for all [`pkgs.fetchgit`](#fetchgit) calls.
 
         Among many other potential uses, this can be used to override URLs to point to local mirrors.
 
         Changing this will not cause any rebuilds because `pkgs.fetchgit` produces a [fixed-output derivation](https://nix.dev/manual/nix/stable/glossary.html?highlight=fixed-output%20derivation#gloss-fixed-output-derivation).
 
-        To set the configuration file directly, use the [`gitConfigFile`](${docPrefix}#opt-gitConfigFile) option instead.
+        To set the configuration file directly, use the [`gitConfigFile`](#opt-gitConfigFile) option instead.
 
         To set the configuration file for individual calls, use `fetchgit { gitConfigFile = "..."; }`.
       '';
@@ -140,9 +135,9 @@ let
     gitConfigFile = mkOption {
       type = types.nullOr types.path;
       description = ''
-        A path to a [git configuration](https://git-scm.com/docs/git-config#_variables) file, to be used for all [`pkgs.fetchgit`](${docPrefix}#fetchgit) calls.
+        A path to a [git configuration](https://git-scm.com/docs/git-config#_variables) file, to be used for all [`pkgs.fetchgit`](#fetchgit) calls.
 
-        This overrides the [`gitConfig`](${docPrefix}#opt-gitConfig) option, see its documentation for more details.
+        This overrides the [`gitConfig`](#opt-gitConfig) option, see its documentation for more details.
       '';
       default =
         if config.gitConfig != { } then
@@ -160,7 +155,7 @@ let
 
         For example, an override like `"registry.npmjs.org" = "my-mirror.local/registry.npmjs.org"` will replace a URL like `https://registry.npmjs.org/foo.tar.gz` with `https://my-mirror.local/registry.npmjs.org/foo.tar.gz`.
 
-        To set the string directly, see [`npmRegistryOverridesString`](${docPrefix}#opt-npmRegistryOverridesString).
+        To set the string directly, see [`npmRegistryOverridesString`](#opt-npmRegistryOverridesString).
       '';
       default = { };
       example = {
@@ -179,7 +174,7 @@ let
       description = ''
         A string containing a string with a JSON representation of npm registry overrides for `fetchNpmDeps`.
 
-        This overrides the [`npmRegistryOverrides`](${docPrefix}#opt-npmRegistryOverrides) option, see its documentation for more details.
+        This overrides the [`npmRegistryOverrides`](#opt-npmRegistryOverrides) option, see its documentation for more details.
       '';
       default = builtins.toJSON config.npmRegistryOverrides;
     };
@@ -417,7 +412,7 @@ let
       type = types.listOf types.str;
       default = [ "https://tarballs.nixos.org" ];
       description = ''
-        The set of content-addressed/hashed mirror URLs used by [`pkgs.fetchurl`](${docPrefix}#sec-pkgs-fetchers-fetchurl).
+        The set of content-addressed/hashed mirror URLs used by [`pkgs.fetchurl`](#sec-pkgs-fetchers-fetchurl).
         In case `pkgs.fetchurl` can't download from the given URLs,
         it will try the hashed mirrors based on the expected output hash.
 
@@ -471,27 +466,11 @@ let
         Silence the warning for the upcoming deprecation of the
         `x86_64-darwin` platform in Nixpkgs 26.11.
 
-        See the [release notes](${docPrefix}#x86_64-darwin-26.05) for more
+        See the [release notes](#x86_64-darwin-26.05) for more
         information.
       '';
     };
 
-    packageOverrides = mkOption {
-      type = types.functionTo types.attrs;
-      default = pkgs: { };
-      description = ''
-        A function to replace or add packages in `pkgs` expects an attrset to be returned when called.
-      '';
-    };
-
-    perlPackageOverrides = mkOption {
-      type = types.functionTo types.attrs;
-      default = pkgs: { };
-      description = ''
-        The same as `packageOverrides` but for packages in the perl package set.
-      '';
-    };
-
     problems = (import ../stdenv/generic/problems.nix { inherit lib; }).configOptions;
   };
 
@@ -515,7 +494,6 @@ in
   inherit options;
 
   config = {
-    _module.args.docPrefix = lib.mkDefault "";
     warnings =
       optionals config.warnUndeclaredOptions (
         mapAttrsToList (k: v: "undeclared Nixpkgs option set: config.${k}") config._undeclared or { }
diff --git a/pkgs/top-level/default.nix b/pkgs/top-level/default.nix
index 12fdbed08197..521745d67ec9 100644
--- a/pkgs/top-level/default.nix
+++ b/pkgs/top-level/default.nix
@@ -64,11 +64,6 @@ 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:
@@ -114,13 +109,7 @@ let
         then
           x86_64DarwinDeprecationWarning
         else
-          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`."
+          x: x
       );
 
   localSystem = lib.systems.elaborate args.localSystem;
@@ -145,24 +134,20 @@ let
 
   # Allow both:
   # { /* the config */ } and
-  # { lib, pkgs, ... } : { /* the config */ }
+  # { pkgs, ... } : { /* the config */ }
   config1 = if lib.isFunction config0 then config0 { inherit lib pkgs; } else config0;
 
   configEval = lib.evalModules {
     modules = [
       ./config.nix
-    ]
-    ++ (
-      if _configDefinitions != null then
-        map (def: lib.modules.setDefaultModuleLocation def.file def.value) _configDefinitions
-      else
-        [
-          {
-            _file = "nixpkgs.config";
-            config = config1;
-          }
-        ]
-    );
+      (
+        { options, ... }:
+        {
+          _file = "nixpkgs.config";
+          config = config1;
+        }
+      )
+    ];
     class = "nixpkgsConfig";
   };