I just used ChatGPT only, twice. Web interface in a Firefox private window, and in a Chrome incognito window. I asked them both the identical question "How many names of the days of the week contain the letter D?"
In Firefox I got 6. In Chrome I got 7. LLMs are not even self-consistent.
How many days of the week contain the letter d?
The answer I get with ChatGPT, and Grok is 3 and 6 with Claude.