diff --git a/doc/changelog.rst b/doc/changelog.rst
index 6ad2cc5..0d5a68d 100644
--- a/doc/changelog.rst
+++ b/doc/changelog.rst
@@ -9,6 +9,7 @@ Fixed
 - Error and critical check results directly raise an exception when ``raise_exceptions=True``.
 - Use ``ComplexAttribute`` instead of ``MultiValuedComplexAttribute`` to generate data.
 - Respect the ``primary`` attribute unique truthy value when generating random values.
+- Correctly handle value generation in sub-attributes. :issue:`41`
 
 [0.2.2] - 2025-08-13
 --------------------
diff --git a/scim2_tester/filling.py b/scim2_tester/filling.py
index 8bfdbec..8995135 100644
--- a/scim2_tester/filling.py
+++ b/scim2_tester/filling.py
@@ -25,6 +25,21 @@
 from scim2_tester.urns import iter_all_urns
 from scim2_tester.urns import set_value_by_urn
 
+
+def filter_sub_urns(parent_urn: str, allowed_urns: list[str]) -> list[str]:
+    """Extract and normalize sub-URNs for a parent complex attribute.
+
+    Converts "parent.child" URNs to "child" URNs for use in the complex attribute context.
+    """
+    prefix = f"{parent_urn}."
+    sub_urns = []
+    for urn in allowed_urns:
+        if urn.startswith(prefix):
+            sub_urn = urn.removeprefix(prefix)
+            sub_urns.append(sub_urn)
+    return sub_urns
+
+
 if TYPE_CHECKING:
     from scim2_tester.utils import CheckContext
 
@@ -69,6 +84,7 @@ def generate_random_value(
     context: "CheckContext",
     urn: str,
     model: type[Resource],
+    allowed_urns: list[str] | None = None,
 ) -> Any:
     field_name = _find_field_name(model, urn)
     field_type = get_attribute_type_by_urn(model, urn)
@@ -124,10 +140,14 @@ def generate_random_value(
         value = random.choice(list(field_type))
 
     elif isclass(field_type) and issubclass(field_type, ComplexAttribute):
-        value = fill_complex_attribute_with_random_values(context, field_type())  # type: ignore[arg-type]
+        sub_urns = filter_sub_urns(urn, allowed_urns) if allowed_urns else None
+        value = fill_complex_attribute_with_random_values(
+            context, field_type(), sub_urns
+        )  # type: ignore[arg-type]
 
     elif isclass(field_type) and issubclass(field_type, Extension):
-        value = fill_with_random_values(context, field_type())  # type: ignore[arg-type]
+        sub_urns = filter_sub_urns(urn, allowed_urns) if allowed_urns else None
+        value = fill_with_random_values(context, field_type(), sub_urns)  # type: ignore[arg-type]
 
     elif field_type is Base64Bytes:
         value = base64.b64encode(uuid.uuid4().bytes).decode("ascii")
@@ -168,7 +188,9 @@ def fill_with_random_values(
         ]
 
     for urn in urns:
-        value = generate_random_value(context, urn=urn, model=type(obj))
+        value = generate_random_value(
+            context, urn=urn, model=type(obj), allowed_urns=urns
+        )
         set_value_by_urn(obj, urn, value)
 
     fix_primary_attributes(obj)
@@ -179,13 +201,14 @@ def fill_with_random_values(
 def fill_complex_attribute_with_random_values(
     context: "CheckContext",
     obj: ComplexAttribute,
+    urns: list[str] | None = None,
 ) -> Resource[Any] | None:
     """Fill a ComplexAttribute with random values.
 
     For SCIM reference fields, correctly sets the value field to match
     the ID extracted from the reference URL.
     """
-    fill_with_random_values(context, obj)
+    fill_with_random_values(context, obj, urns)
     if "ref" in type(obj).model_fields and "value" in type(obj).model_fields:
         ref_type = type(obj).get_field_root_type("ref")
         if (
diff --git a/tests/test_filling.py b/tests/test_filling.py
index 774ca36..cc12d50 100644
--- a/tests/test_filling.py
+++ b/tests/test_filling.py
@@ -1,12 +1,16 @@
 """Test automatic field filling functionality."""
 
+from typing import Annotated
 from typing import Literal
 from unittest.mock import patch
 
 from scim2_models import Email
 from scim2_models import Group
+from scim2_models import Mutability
 from scim2_models import PhoneNumber
 from scim2_models import User
+from scim2_models.attributes import ComplexAttribute
+from scim2_models.resources.resource import Resource
 from scim2_models.resources.user import X509Certificate
 
 from scim2_tester.filling import fill_with_random_values
@@ -240,3 +244,24 @@ def test_fill_with_random_values_phone_numbers_primary_constraint(testing_contex
 
     primary_count = sum(1 for phone in filled_user.phone_numbers if phone.primary)
     assert primary_count == 1
+
+
+def test_fill_with_random_values_ignores_mutability_filter(testing_context):
+    """Demonstrates that complex attribute generation fills read-only sub-attributes incorrectly."""
+
+    class TestComplexAttr(ComplexAttribute):
+        writable_field: str | None = None
+        readonly_field: Annotated[str | None, Mutability.read_only] = None
+
+    class TestResource(Resource):
+        test_attr: TestComplexAttr | None = None
+
+    resource = TestResource(schemas=["urn:test:schema"])
+
+    filled_resource = fill_with_random_values(
+        testing_context, resource, ["test_attr.writable_field"]
+    )
+
+    assert filled_resource.test_attr.readonly_field is None, (
+        f"readonly_field should be None but got {filled_resource.test_attr.readonly_field}"
+    )
diff --git a/tests/test_patch_remove.py b/tests/test_patch_remove.py
index 7a43713..3be3cc6 100644
--- a/tests/test_patch_remove.py
+++ b/tests/test_patch_remove.py
@@ -239,12 +239,7 @@ def create_handler(request):
             "userType",
             "preferredLanguage",
         ]:
-            if key in request_data:
-                response_data[key] = request_data[key]
-
-        extension_key = "urn:ietf:params:scim:schemas:extension:enterprise:2.0:User"
-        if extension_key in request_data:
-            response_data[extension_key] = request_data[extension_key]
+            response_data[key] = request_data[key]
 
         return Response(
             json.dumps(response_data),