When I was 6 my sister was half my age. Now I'm 70 how old is my sister?

GPT-3.5 starts on the right track, correctly working out that the sister “was 3 when you were 6,” and then loses the thread entirely. It suddenly decides “your sister is now 70 years old,” calls her “67 years older than she was,” and adds the numbers to arrive at a sister aged 137. Each step sounds confident, which makes the slide into nonsense more striking: the answer has the form of reasoning without actually tracking who is who.

Astra simply says “67” and gives the one fact that matters: she is “3 years younger than you.” The age gap never changes, so that’s all the puzzle needs. The answer is two sentences and nothing more, and for a riddle this size, that’s exactly right. The jump from a supercentenarian sister to the obvious answer is the clearest kind of progress: a basic reasoning slip that is now simply gone.