Considering there's no (explicit) instruction forbidding or discouraging it, I'd consider the REPL solution to be perfectly valid. In fact some interview tests specifically look for this kind of problem solving.
I get it still, I'd expect some valuable signal from this test. Candidates who execute this code are likely to do so because they really want to avoid running the code in their head, not just because it's more straightforward, and that's probably a bad sign. And pasting that into an LLM instead of a REPL would be a massive red flag.
I just don't think answering "-11" here is a signal strong enough to disqualify candidates on its own.
OR:
I could run the code in the interpreter and be 100% certain.
I know what attitude I would prefer out of my developers.
I agree many developers do blindly copy and paste things off the Internet, but I don't think that's something to desire or celebrate.
How many cases have you faced in your real job where this has happened?
I'm perfectly happy to use Stack Overflow and other resources/tutorials, blog posts etc... to find solutions to problems, but just instinctively I would never think to copy and paste a solution from these sites and incorporate it into my codebase and I sure as heck wouldn't think to execute code from some untrusted site I happened to come across.
But this may also be a consequence of the domain I work in where we take security very seriously.
Like, there's no way you're going to copy a 20 line algorithm from stack overflow on balancing a red-black tree and have it encrypt your harddrive.
Obviously you still need to test the code to make sure it works and understand what it's doing, but there is very little security risk here. Just look up the functions youre using and understand the code and you're fine.
So for them, yes, it would clearly be faster to run the code than to work through it manually.
What you're doing here is selecting for candidates who are less comfortable with using the tools that they'd be expected to use every day in the role you're hiring for. It's likely to provide a negative signal.