diff --git a/sdk/prototype_state.py b/sdk/prototype_state.py
index 78243e4..3423701 100644
--- a/sdk/prototype_state.py
+++ b/sdk/prototype_state.py
@@ -44,7 +44,7 @@ class PrototypeStateStore:
         if existing is not None:
             stored = existing.model_copy(update={"is_new": False})
             self._users[key] = stored
-            return stored
+            return stored.model_copy()
 
         user = User(
             user_id=f"usr-{platform}-{external_id}",
@@ -55,7 +55,7 @@ class PrototypeStateStore:
             is_new=True,
         )
         self._users[key] = user
-        return user
+        return user.model_copy()
 
     async def get_settings(self, user_id: str) -> UserSettings:
         stored = self._settings.get(user_id, {})
diff --git a/tests/platform/test_prototype_state.py b/tests/platform/test_prototype_state.py
index 3c2c25a..c1a2d73 100644
--- a/tests/platform/test_prototype_state.py
+++ b/tests/platform/test_prototype_state.py
@@ -21,9 +21,14 @@ async def test_get_or_create_user_is_stable_per_surface_identity():
 
     assert first.user_id == "usr-matrix-@alice:example.org"
     assert first.is_new is True
+
+    first.display_name = "Mallory"
+    first.is_new = False
+
     assert second.user_id == first.user_id
     assert second.is_new is False
     assert second.display_name == "Alice"
+    assert store._users["matrix:@alice:example.org"].display_name == "Alice"
     assert store._users["matrix:@alice:example.org"].is_new is False