It's not an unknown error; Opus 4 and Opus 4.1 were removed today from the Claude app and web app, with no prior notice to users. You can export chats and continue them on different platforms, ask Claudes to write you exporters, etc. https://t.co/SEACJfEjgl
cited on: claude-opus-4-1
Reproduced against link rot, credited and linked to its original. Yours and you’d rather it weren’t here? Open an issue.