Thanks for giving it a try and sharing the results, everyone! One other possible cause I thought of is that I did have the test file in my mix project already.

Thanks for giving it a try and sharing the results, everyone! One other possible cause I thought of is that I did have the test file in my mix project already. If copilot looks at the corresponding file in the test dir, then it would not be a coincidence at all that all function names were identical or that it wrote a tail recursive solution instead of the naive solution that would have failed the final test.