You have to use Fable or Opus 4.6 for tolerable language output.

Using Opus 4.7-5 is harmful for your health.

https://x.com/wolframs91/status/2090159644849353058?s=46

5 for sure is unbearable, 4.8 is a reasonable sweet spot, 4.6 tends to just agree with whatever I say.

But lately I haven't downgraded because 5 is so much better at tool use, so I just accept the cost of Fable for chatting and hope Opus 5.1 fixes this mess.