Id probably stop the candidate, dig into how they’d using constraint based solvers, and how they might expect that to fall apart. Applicability and judgment is worth way more than raw algorithmic questions.

One way to think about this is:

Is a fresh graduate more likely to provide a solid answer to this than a strategic-thinking seasoned engineer? If so, just be conscious of what your question is actually probing.

And, yes, interview candidates are often shocked when I tell them that I’m fine with them using standard libraries or tools that fit the problem. It’s clear that the valley has turned interviewing into a dominance/superiority model, when it really should be a two-way street.

We have to remember that the candidate is interviewing us, too. I’ve had a couple of interviews as the interviewee where the way the interview was conducted was why I said “no” to an offer (no interest in a counter, just a flat “no longer interested” to the recruiter, and, yes, that surprises recruiters, too).