diff --git a/tests/_test_utils/test_multidict.py b/tests/_test_utils/test_multidict.py
index 05d604390..c4942bc9a 100644
--- a/tests/_test_utils/test_multidict.py
+++ b/tests/_test_utils/test_multidict.py
@@ -1,6 +1,5 @@
 # -*- coding: utf-8 -*-
 """
-
     tests._test_utils.test_multidict
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -8,26 +7,14 @@
 
     This file is part of MSS.
 
-
     :copyright: Copyright 2016 Reimar Bauer
     :copyright: Copyright 2016-2026 by the MSS team, see AUTHORS.
     :license: APACHE-2.0, see LICENSE for details.
-
-    Licensed under the Apache License, Version 2.0 (the "License");
-    you may not use this file except in compliance with the License.
-    You may obtain a copy of the License at
-
-       http://www.apache.org/licenses/LICENSE-2.0
-
-    Unless required by applicable law or agreed to in writing, software
-    distributed under the License is distributed on an "AS IS" BASIS,
-    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-    See the License for the specific language governing permissions and
-    limitations under the License.
 """
 import logging
 import multidict
 import werkzeug
+import pytest
 
 LOGGER = logging.getLogger(__name__)
 
@@ -40,9 +27,6 @@ class CaseInsensitiveMultiDict(werkzeug.datastructures.ImmutableMultiDict):
 
         The only overridden method is __getitem__(), which converts string keys
         to lower case before carrying out comparisons.
-
-        See ../paste/util/multidict.py as well as
-          http://stackoverflow.com/questions/2082152/case-insensitive-dictionary
         """
 
         def __getitem__(self, key):
@@ -55,9 +39,80 @@ def __getitem__(self, key):
                     return v
             raise KeyError(repr(key))
 
-    def test_multidict(self):
+    def test_multidict_basic_case_insensitivity(self):
+        """Basic behavior: keys should be looked up case-insensitively."""
         test_dict = TestCIMultiDict.CaseInsensitiveMultiDict([('title', 'MSS')])
         dict_multidict = multidict.CIMultiDict([('title', 'MSS')])
+
+        # membership checks should be case-insensitive
         assert 'title' in dict_multidict
         assert 'tiTLE' in dict_multidict
+
+        # direct indexing should honor case-insensitive lookup
         assert dict_multidict['Title'] == test_dict['tITLE']
+
+    def test_multidict_missing_key_raises_keyerror(self):
+        """Accessing a missing key should raise KeyError for both implementations."""
+        dict_multidict = multidict.CIMultiDict([('a', '1')])
+        test_dict = TestCIMultiDict.CaseInsensitiveMultiDict([('a', '1')])
+
+        with pytest.raises(KeyError):
+            _ = dict_multidict['missing_key']  # multidict raises KeyError
+
+        with pytest.raises(KeyError):
+            _ = test_dict['another_missing']
+
+    def test_multidict_non_string_keys(self):
+        """Non-string keys (e.g., ints) should still work and not be lowered."""
+        dict_multidict = multidict.CIMultiDict([(1, 'one'), ('Two', '2')])
+        test_dict = TestCIMultiDict.CaseInsensitiveMultiDict([(1, 'one'), ('Two', '2')])
+
+        # integer key should be retrievable as-is
+        assert dict_multidict[1] == 'one'
+        assert test_dict[1] == 'one'
+
+        # string key should still be case-insensitive
+        assert dict_multidict['two'] == '2'
+        assert test_dict['tWo'] == '2'
+
+    def test_multidict_contains_and_iteration(self):
+        """Ensure 'in' checks and .items() iteration behave sensibly."""
+        dict_multidict = multidict.CIMultiDict([('X-Header', 'val'), ('Content', 'c')])
+        test_dict = TestCIMultiDict.CaseInsensitiveMultiDict([('X-Header', 'val'), ('Content', 'c')])
+
+        # membership by different case
+        assert 'x-header' in dict_multidict
+        assert 'content' in dict_multidict
+        assert 'X-HEADER' in dict_multidict
+
+        assert 'X-HEADER' in test_dict
+        assert 'content' in test_dict
+
+        # iteration should expose the original stored keys and values
+        items_dict = dict(dict_multidict.items())
+        assert items_dict.get('X-Header') == 'val'
+        assert items_dict.get('Content') == 'c'
+
+        items_test = dict(test_dict.items())
+        assert items_test.get('X-Header') == 'val'
+        assert items_test.get('Content') == 'c'
+
+    def test_multidict_multiple_entries_same_key(self):
+        """
+        Check behavior when multiple entries for same key exist (different cases).
+        We don't assert a specific rule about which value is returned across
+        implementations — only that lookups find one of the stored values
+        and membership is case-insensitive.
+        """
+        md = multidict.CIMultiDict([('Title', 'first'), ('title', 'second')])
+        cim = TestCIMultiDict.CaseInsensitiveMultiDict([('Title', 'first'), ('title', 'second')])
+
+        # membership checks
+        assert 'title' in md
+        assert 'TITLE' in md
+        assert 'title' in cim
+        assert 'TITLE' in cim
+
+        # retrieving should return one of the inserted values (string)
+        assert md['title'] in ('first', 'second')
+        assert cim['title'] in ('first', 'second')
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