This post documents how I built a cross-platform Elixir NIF in C to get on-demand up-to-date disk-usage stats without relying on os_mon and its disksup service. I had Grok 3 generate the initial C code and Makefile, then iterated through multiple code reviews by Gemini 2.5 Flash and GPT-5 to make it work on Linux, macOS, Windows, and the BSDs (except DragonFlyBSD).
Along the way, I ran into typical LLM hiccups that speak volumes about the breathless hyperbole often peddled by LLM vendors, compute providers, and over-enthusiastic consultants, middle managers and executives on LinkedIn. Nevertheless, the result is a working, cross-platform Elixir package on Hex.pm, plus a real-world case study in where LLMs shine, where they fail, and what “human-in-the-loop” can mean in practice.
Spoiler alert: the hype is exactly that; even so, we ended up with working code that is, at the very least, a solid starting point for further improvements by actual general intelligence.