I have a PhD, in mathematics, from a top university. If you give me, say, 100 10-digit numbers to add up and tell me to do the job in my head then I will probably get the answer wrong.

Of course, if you give me 100 10-digit numbers to add up and let me use a calculator, or pencil and paper, then I will probably get it right.

Same for, say, two 100-digit numbers. (I can probably get that one right without tools if you obligingly print them monospaced and put one of them immediately above the other where I can look at them.)

Anyway, the premise here seems to be simply false. I just gave ChatGPT and Claude (free versions of both; ChatGPT5, whatever specific model it routed my query to, and Sonnet 4) a list of 100 random 10-digit numbers to add up, with a prompt encouraging them to be careful about it but nothing beyond that (e.g., no specific strategies or tools to use), and both of them got the right total. Then I did the same with two 100-digit numbers and both of them got that right too.