We were hiring for senior devs java/spring boot so I coded a tiny "Todo app" rest API, basically a single crud controller, and filled the code with many mistakes. Some obvious, some less obvious. Missing authentication, no tests, logging with System.out etc.. then we ask the candidate to review the code (we provide a laptop with a few IDEs but no AI tools). When they point out errors, I get them to explain why they are errors by playing the junior engineer to see how they explain technical issues to someone that might not have the same understanding I am not looking for them to find all issues but if they can't find at least some it's a red flag. It takes about 20-25 minutes and gives really good insights. The other 30 minutes we asked some questions about past projects and some technical questions

So far it provided good signals