Ilgyu worked on the planetarium/lib9c repository, focusing on backend development and build management using C# and XML. He delivered a more robust SwapRateSheet by ensuring the Pair key is always non-null and allowing the Rate to be nullable, which accurately represents unavailable rates. To improve error handling, he enhanced exception reporting with concrete arguments, making debugging more straightforward. Ilgyu also managed release hygiene by updating version prefixes across project files for the 1.22.1 release, ensuring consistent and reproducible builds. His work contributed to more reliable rate handling and smoother deployments, reducing downtime and supporting faster issue resolution for the team.