On reload, the namespaces are safe, won't be reloaded. The loader only reloads what it manages, which in this case is the adapter itself.

I assume that the ActiveStorage::Attachment class gets reloaded dynamically and that the extensions are lost as they are only added during initialization. You’re correct. Use Rails.configuration.to_prepare to mix your module in after application boot and every time code is reloaded: Rails.configuration.to_prepare do ActiveStorage::Attachment.send :include, ::ActiveStorageAttachmentExtension end