There's quite the distance between FizzBuzz and LeetCode medium/hard questions. If you want to filter out candidates that barely can't code, just have them walk

There's quite the distance between FizzBuzz and LeetCode medium/hard questions. If you want to filter out candidates that barely can't code, just have them walk you through a small code-base containing bugs. What does the code do? Does it work? How would you get it working? How do you test it? What is the complexity? etc.