I have been refactoring a project using Opus 4.7/4.8 for the past few weeks or so. I just decided to switch to Fable 5 max today. It stopped half way through and it just blocked me and switched back to Opus 4.8 automatically. "This model has specific safety measures that flagged something in this message. This sometimes happens with safe, normal conversations. Send feedback or learn more." It would not identify what the problem was. I left feedback saying that their heuristics are too sensitive. For now I will not be using Fable 5.

[0] https://support.claude.com/en/articles/15363606-why-claude-s...

I suspect this will be a significant problem blocking long-horizon tasks in practice, basically the more turns there are, the larger the chance the classifier produces a false positive. The disappointment of the user will also scale with the length of the task, as you're in the middle of some complex thing and now gets derailed, after already have paid for many tokens.