It asks you to create new chats (Claude Design) and Claude Code nudges you to start over too, I think `/compact` is optional though?
That used to be one of the major issues why clearing or compacting context when the context nears 150 to 170k is usually preferred.
It really depends on the session, and what you are doing how far you can push it.
I don't think I've ever pushed past 500k though, it may well fall apart at the higher end of what is officially supported.
^ this is literally what the op did. Don’t flag facts.