diff --git a/mkdocs/config/config_options.py b/mkdocs/config/config_options.py
index bc75ec85..23722a11 100644
--- a/mkdocs/config/config_options.py
+++ b/mkdocs/config/config_options.py
@@ -821,14 +821,10 @@ class Nav(OptionallyRequired):
             return f"a {type(value).__name__}: {value!r}"
 
 
-class Private(BaseConfigOption):
-    """
-    Private Config Option
+class Private(Generic[T], BaseConfigOption[T]):
+    """A config option that can only be populated programmatically. Raises an error if set by the user."""
 
-    A config option only for internal use. Raises an error if set by the user.
-    """
-
-    def run_validation(self, value: object):
+    def run_validation(self, value: object) -> None:
         if value is not None:
             raise ValidationError('For internal use only.')
 
@@ -855,7 +851,7 @@ class MarkdownExtensions(OptionallyRequired[List[str]]):
         self.builtins = builtins or []
         self.configkey = configkey
 
-    def validate_ext_cfg(self, ext, cfg):
+    def validate_ext_cfg(self, ext: object, cfg: object) -> None:
         if not isinstance(ext, str):
             raise ValidationError(f"'{ext}' is not a valid Markdown Extension name.")
         if not cfg:
@@ -864,7 +860,7 @@ class MarkdownExtensions(OptionallyRequired[List[str]]):
             raise ValidationError(f"Invalid config options for Markdown Extension '{ext}'.")
         self.configdata[ext] = cfg
 
-    def run_validation(self, value: object):
+    def run_validation(self, value: object) -> list[str]:
         self.configdata: dict[str, dict] = {}
         if not isinstance(value, (list, tuple, dict)):
             raise ValidationError('Invalid Markdown Extensions configuration')
diff --git a/mkdocs/config/defaults.py b/mkdocs/config/defaults.py
index 9590ace7..ba6681d3 100644
--- a/mkdocs/config/defaults.py
+++ b/mkdocs/config/defaults.py
@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+from typing import Dict
+
 from mkdocs.config import base
 from mkdocs.config import config_options as c
 
@@ -97,8 +99,8 @@ class MkDocsConfig(base.Config):
     )
     """PyMarkdown extension names."""
 
-    mdx_configs = c.Private()
-    """PyMarkdown Extension Configs. For internal use only."""
+    mdx_configs = c.Private[Dict[str, dict]]()
+    """PyMarkdown extension configs. Populated from `markdown_extensions`."""
 
     strict = c.Type(bool, default=False)
     """Enabling strict mode causes MkDocs to stop the build when a problem is
diff --git a/mkdocs/tests/config/config_options_tests.py b/mkdocs/tests/config/config_options_tests.py
index d6eb3cc3..952f4ce9 100644
--- a/mkdocs/tests/config/config_options_tests.py
+++ b/mkdocs/tests/config/config_options_tests.py
@@ -8,7 +8,7 @@ import re
 import sys
 import textwrap
 import unittest
-from typing import TYPE_CHECKING, Any, List, Optional, TypeVar
+from typing import TYPE_CHECKING, Any, Dict, List, Optional, TypeVar
 from unittest.mock import patch
 
 if TYPE_CHECKING:
@@ -1213,7 +1213,7 @@ class NavTest(TestCase):
 class PrivateTest(TestCase):
     def test_defined(self) -> None:
         class Schema(Config):
-            option = c.Private()
+            option = c.Private[Any]()
 
         with self.expect_error(option="For internal use only."):
             self.get_config(Schema, {'option': 'somevalue'})
@@ -1380,13 +1380,14 @@ class MarkdownExtensionsTest(TestCase):
     def test_simple_list(self, mock_md) -> None:
         class Schema(Config):
             markdown_extensions = c.MarkdownExtensions()
-            mdx_configs = c.Private()
+            mdx_configs = c.Private[Dict[str, dict]]()
 
         config = {
             'markdown_extensions': ['foo', 'bar'],
         }
         conf = self.get_config(Schema, config)
         assert_type(conf.markdown_extensions, List[str])
+        assert_type(conf.mdx_configs, Dict[str, dict])
         self.assertEqual(conf.markdown_extensions, ['foo', 'bar'])
         self.assertEqual(conf.mdx_configs, {})
 
@@ -1394,7 +1395,7 @@ class MarkdownExtensionsTest(TestCase):
     def test_list_dicts(self, mock_md) -> None:
         class Schema(Config):
             markdown_extensions = c.MarkdownExtensions()
-            mdx_configs = c.Private()
+            mdx_configs = c.Private[Dict[str, dict]]()
 
         config = {
             'markdown_extensions': [
@@ -1417,7 +1418,7 @@ class MarkdownExtensionsTest(TestCase):
     def test_mixed_list(self, mock_md) -> None:
         class Schema(Config):
             markdown_extensions = c.MarkdownExtensions()
-            mdx_configs = c.Private()
+            mdx_configs = c.Private[Dict[str, dict]]()
 
         config = {
             'markdown_extensions': [
@@ -1438,7 +1439,7 @@ class MarkdownExtensionsTest(TestCase):
     def test_dict_of_dicts(self, mock_md) -> None:
         class Schema(Config):
             markdown_extensions = c.MarkdownExtensions()
-            mdx_configs = c.Private()
+            mdx_configs = c.Private[Dict[str, dict]]()
 
         config = {
             'markdown_extensions': {
@@ -1461,7 +1462,7 @@ class MarkdownExtensionsTest(TestCase):
     def test_builtins(self, mock_md) -> None:
         class Schema(Config):
             markdown_extensions = c.MarkdownExtensions(builtins=['meta', 'toc'])
-            mdx_configs = c.Private()
+            mdx_configs = c.Private[Dict[str, dict]]()
 
         config = {
             'markdown_extensions': ['foo', 'bar'],
@@ -1473,7 +1474,7 @@ class MarkdownExtensionsTest(TestCase):
     def test_duplicates(self) -> None:
         class Schema(Config):
             markdown_extensions = c.MarkdownExtensions(builtins=['meta', 'toc'])
-            mdx_configs = c.Private()
+            mdx_configs = c.Private[Dict[str, dict]]()
 
         config = {
             'markdown_extensions': ['meta', 'toc'],
@@ -1485,7 +1486,7 @@ class MarkdownExtensionsTest(TestCase):
     def test_builtins_config(self) -> None:
         class Schema(Config):
             markdown_extensions = c.MarkdownExtensions(builtins=['meta', 'toc'])
-            mdx_configs = c.Private()
+            mdx_configs = c.Private[Dict[str, dict]]()
 
         config = {
             'markdown_extensions': [
@@ -1500,7 +1501,7 @@ class MarkdownExtensionsTest(TestCase):
     def test_configkey(self, mock_md) -> None:
         class Schema(Config):
             markdown_extensions = c.MarkdownExtensions(configkey='bar')
-            bar = c.Private()
+            bar = c.Private[Dict[str, dict]]()
 
         config = {
             'markdown_extensions': [
@@ -1519,7 +1520,7 @@ class MarkdownExtensionsTest(TestCase):
     def test_missing_default(self) -> None:
         class Schema(Config):
             markdown_extensions = c.MarkdownExtensions()
-            mdx_configs = c.Private()
+            mdx_configs = c.Private[Dict[str, dict]]()
 
         conf = self.get_config(Schema, {})
         self.assertEqual(conf.markdown_extensions, [])
@@ -1528,7 +1529,7 @@ class MarkdownExtensionsTest(TestCase):
     def test_none(self) -> None:
         class Schema(Config):
             markdown_extensions = c.MarkdownExtensions(default=[])
-            mdx_configs = c.Private()
+            mdx_configs = c.Private[Dict[str, dict]]()
 
         config = {
             'markdown_extensions': None,
@@ -1603,7 +1604,7 @@ class MarkdownExtensionsTest(TestCase):
         # config instances that didn't specify extensions.
         class Schema(Config):
             markdown_extensions = c.MarkdownExtensions()
-            mdx_configs = c.Private()
+            mdx_configs = c.Private[Dict[str, dict]]()
 
         conf = self.get_config(
             Schema,