After a quick phone call with the recruiter, they presented me with a coding exercise. It is a project that you have to finish in one week. My project was a binary memcache protocol implementation (get and set commands only). The protocol is supported by famous memcached. I submitted my implementation on time, and after another week I got a call from the recruiter who informed me that my implementation doesn't meet some never unspecified criteria. I have never been contacted by a Slack engineer. After spending a week, I would expect some technical feedback, discussion, comments, questions. I find that kind of response somewhat unprofessional and disrespectful. If you'd rather not wasting a week, or the exercise isn't something that is very interesting for you, I would not bother with Slack.