diff --git a/swh/model/model.py b/swh/model/model.py
--- a/swh/model/model.py
+++ b/swh/model/model.py
@@ -567,7 +567,7 @@
     """Usually one of the values of `swh.model.from_disk.DentryPerms`."""
 
 
-@attr.s(frozen=True)
+@attr.s(frozen=True, slots=True)
 class Directory(HashableObject, BaseModel):
     object_type: Final = "directory"