Fable/mythos are the first models from anthropic that hide 100% of reasoning tokens. So it seems to me like we're about to get a lot more data about to what extent Chinese model progress has been a consequence of distillation techniques.
The thinking traces disappeared because Anthropic changed them to be hidden by default. The rationale for hiding it was that most people don't look at the thinking traces https://news.ycombinator.com/item?id=47664442 . You can reenable thinking traces in Claude code settings with the flag showThinkingSummaries: true.
>On Claude 4 models, the first few lines of thinking output are more verbose, providing detailed reasoning that's particularly helpful for prompt engineering purposes. Claude Mythos Preview summarizes from the first token, so its thinking blocks do not show this verbose preamble. https://platform.claude.com/docs/en/build-with-claude/extend...