diff --git a/ccflow/base.py b/ccflow/base.py
index 7a831eb..e59a01a 100644
--- a/ccflow/base.py
+++ b/ccflow/base.py
@@ -642,6 +642,10 @@ def __contains__(self, item: object) -> bool:
             return isinstance(registry, ModelRegistry) and name in registry
         return item in self._models
 
+    def __setitem__(self, name: str, model: BaseModel) -> None:
+        """Reject item assignment in favor of the registry's controlled API."""
+        raise TypeError("ModelRegistry does not support item assignment; use the 'add' method instead.")
+
     def __iter__(self):
         for key, model in self._models.items():
             yield key
diff --git a/ccflow/tests/test_base_registry.py b/ccflow/tests/test_base_registry.py
index c92059d..0d20b39 100644
--- a/ccflow/tests/test_base_registry.py
+++ b/ccflow/tests/test_base_registry.py
@@ -226,6 +226,15 @@ def test_add_twice(self):
         self.assertListEqual(m.get_registrations(), [(r, "foo"), (r, "bar"), (r2, "foo2"), (r2, "bar2")])
         self.assertListEqual(m.get_registered_names(), ["/foo", "/bar"])
 
+    def test_setitem_directs_users_to_add(self):
+        registry = ModelRegistry(name="test")
+        model = MyTestModel(a="test", b=0.0)
+
+        with self.assertRaisesRegex(TypeError, "use the 'add' method"):
+            registry["foo"] = model
+
+        self.assertEqual(registry.models, {})
+
     def test_add_two_places(self):
         m = MyTestModel(a="test", b=0.0)
         r1 = ModelRegistry(name="test")