Agreed: GPT-5.6 Pro (which is only available via the chatbot) is the smartest model out there. GPT-5.6 Ultra (the best model in Codex) is not near as powerful on hard tasks.
For really hard problems: GPT-5.6 Sol Pro>Fable 5 Ultracode > GPT-5.6 Sol Ultra. (Yes, this is confusing)