An inline "answered by a backup model" notice — the failed primary and the model that answered as a chip chain, reason-class copy, and a one-shot retry-with-primary button that suppresses itself when a retry cannot help.
npx shadcn@latest add @zyeon/fallback-noticeZyeon UI limits how often it can be asked, and Fallback Notice hasn't had its turn. Its source will show here once it has.