Reconstruct concrete nested subtypes using their actual dataclass owner

src/objectstate/object_state.py

@@ -2600,20 +2600,11 @@ def _extract_all_parameters_flat(
             # First try from type annotation, then fall back to checking actual value
             nested_type = self._get_nested_dataclass_type(info.param_type)
 
-            # A registered lazy runtime value owns its raw reconstruction type.
-            # The enclosing annotation commonly names the concrete base config,
-            # but reconstructing unresolved ``None`` sentinels through that base
-            # would run concrete validation before inheritance resolution.
-            if current_value is not None:
-                value_type = type(current_value)
-                if is_dataclass(value_type):
-                    from objectstate.lazy_factory import get_base_type_for_lazy
-
-                    if (
-                        nested_type is None
-                        or get_base_type_for_lazy(value_type) is not None
-                    ):
-                        nested_type = value_type
+            # The actual dataclass owns both its flattened fields and their
+            # reconstruction, including concrete nominal subtypes and lazy
+            # overlays. An annotation supplies the owner only without a value.
+            if current_value is not None and is_dataclass(type(current_value)):
+                nested_type = type(current_value)
 
             if nested_type is not None and current_value is not None:
                 # Store the nested config type reference at this path

tests/test_object_state_update_instance.py

@@ -70,6 +70,45 @@ class ConfigWithNestedTopology:
     nested: NestedTopology = field(default_factory=NestedTopology)
 
 
+@dataclass(frozen=True)
+class SpatialConfig:
+    width: int = 12
+
+
+@dataclass(frozen=True)
+class VolumeConfig(SpatialConfig):
+    depth: int = 60
+
+
+@dataclass
+class ConfigWithSpatialDomain:
+    spatial: SpatialConfig = field(default_factory=SpatialConfig)
+
+
+def test_nested_nominal_subtype_survives_raw_live_and_saved_reconstruction():
+    state = ObjectState(ConfigWithSpatialDomain(VolumeConfig()), scope_id="volume")
+    assert state.to_object().spatial == VolumeConfig()
+    assert state.to_saved_resolved_object().spatial == VolumeConfig()
+    assert state.to_resolved_object().spatial == VolumeConfig()
+
+    state.update_parameter("spatial.depth", 30)
+    assert state.to_object().spatial == VolumeConfig(depth=30)
+    assert state.to_resolved_object().spatial == VolumeConfig(depth=30)
+    assert state.to_saved_resolved_object().spatial == VolumeConfig(depth=60)
+
+
+def test_nested_nominal_owner_changes_rebuild_parameter_topology():
+    state = ObjectState(ConfigWithSpatialDomain(), scope_id="spatial")
+    state.update_object_instance(ConfigWithSpatialDomain(VolumeConfig(width=24)))
+    assert state.to_saved_resolved_object().spatial == VolumeConfig(width=24)
+    assert "spatial.depth" in state.parameters
+
+    state.update_object_instance(ConfigWithSpatialDomain(SpatialConfig(width=18)))
+    assert state.to_object().spatial == SpatialConfig(width=18)
+    assert state.to_saved_resolved_object().spatial == SpatialConfig(width=18)
+    assert "spatial.depth" not in state.parameters
+
+
 class DelegatedHost:
     __objectstate_delegate__ = "config"