As said elsewhere:

Chain of thought does not exist in the output of Claude, they disabled true thinking due to distillation risk. What you see when thinking summaries are enabled are just that, summaries of thinking into Claude-isms, therefore you cannot make any inferences on what the model is doing unless you literally work at Anthropic and can see the true thinking traces.

I remember enjoying watching Fable think during the original limited preview. It was full CoT for sure. They must have removed that feature recently.

I use open models for non work stuff and sometimes I cancel the output because the CoT is all I needed to read.