I had fable running in a loop overnight last night, finding bugs. It found a heap overflow. That triggered its safety guards, which converted the thing to opus, leaving opus to run the rest of the night, wasting my precious time with Fable.
Oh well, it was pretty funny, all things considered.
That sucks - I've thankfully only had that happen once. Similar thing - I was testing my new X11 server, and it turns out that broken X11 packets can make Firefox crash, and I got a refusal on a sub-agent request Fable prompted to have an agent narrow down why.
There's a setting that stops fallback to Opus 4.8 if you would like to avoid that behavior.
Config -> Switch models when a message is flagged -> false
That should stop it entirely when a message is flagged and then you can come back without Opus having potentially made a mess.