At many tech startups, a human-resources chief arrives long after the ping-pong table and 100-employee milestone. Then there is CodeFights Inc., whose founders made a head of talent the first hire at their San Francisco headquarters.

The company, which sets up online coding challenges to help programmers hone skills and get noticed by tech firms, is bucking the startup world’s grow-at-all-costs ethos. Rather than scale hastily, CodeFights uses its blind coding battles to avoid hidden biases as it assesses prospective recruits....