Anthropic claims the only difference is the draconian bans on cybersecurity and biology queries.

The Sol benchmarks show Fable has slightly lower performance compared to Mythos.

https://openai.com/index/previewing-gpt-5-6-sol/

I assume they did something to the model itself.

Either way, I do hope they lift those draconian bans. Using the model was a terrible experience because of the constant downgrades. I didn't manage to harden my own projects before Fable got banned.