also override del and in methdos

src/pymatgen/io/vasp/inputs.py

@@ -758,11 +758,11 @@ def __getitem__(self, key: str) -> Any:
         """
         return super().__getitem__(key.strip().upper())
 
-    def get(self, key: str, default: Any = None) -> Any:
-        """
-        Get a value for a case-insensitive key, return default if not found.
-        """
-        return super().get(key.strip().upper(), default)
+    def __delitem__(self, key: str) -> None:
+        super().__delitem__(key.strip().upper())
+
+    def __contains__(self, key: str) -> bool:
+        return super().__contains__(key.upper().strip())
 
     def __str__(self) -> str:
         return self.get_str(sort_keys=True, pretty=False)
@@ -779,6 +779,12 @@ def __add__(self, other: Self) -> Self:
             params[key] = val
         return type(self)(params)
 
+    def get(self, key: str, default: Any = None) -> Any:
+        """
+        Get a value for a case-insensitive key, return default if not found.
+        """
+        return super().get(key.strip().upper(), default)
+
     def as_dict(self) -> dict:
         """MSONable dict."""
         dct = dict(self)

tests/io/vasp/test_inputs.py

@@ -572,6 +572,14 @@ def test_key_case_insensitive(self):
         incar[test_tag.upper()] = 500
         assert incar[test_tag.lower()] == 500
 
+        # Test delete
+        del incar["algo"]
+        assert "ALGO" not in incar
+
+        # Test membership check
+        assert test_tag.upper() in incar
+        assert test_tag.lower() in incar
+
         # Test update
         incar.update({test_tag.lower(): 510})
         assert incar[test_tag] == 510