Qualified it with "100%" because claude4 models show the first few lines of the chain of thought:

>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...