diff --git a/sdk/prototype_state.py b/sdk/prototype_state.py
index 4682bd9..78243e4 100644
--- a/sdk/prototype_state.py
+++ b/sdk/prototype_state.py
@@ -42,7 +42,9 @@ class PrototypeStateStore:
         key = f"{platform}:{external_id}"
         existing = self._users.get(key)
         if existing is not None:
-            return existing.model_copy(update={"is_new": False})
+            stored = existing.model_copy(update={"is_new": False})
+            self._users[key] = stored
+            return stored
 
         user = User(
             user_id=f"usr-{platform}-{external_id}",
diff --git a/tests/platform/test_prototype_state.py b/tests/platform/test_prototype_state.py
index 2a49375..3c2c25a 100644
--- a/tests/platform/test_prototype_state.py
+++ b/tests/platform/test_prototype_state.py
@@ -24,6 +24,7 @@ async def test_get_or_create_user_is_stable_per_surface_identity():
     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"].is_new is False
 
 
 @pytest.mark.asyncio