Worked on stabilizing the core build tooling for the facebook/buck2 and facebook/buck2-prelude repositories by rolling back recent migrations from Java immutables to Kotlin data classes. Addressed regressions and compatibility issues by restoring Java-based immutable implementations and reverting the IsolatedStep model from an interface back to a class-based architecture. This approach ensured reliable builds and continuous integration stability while maintaining interoperability between Java and Kotlin. The work involved careful code reversion, build system expertise, and a focus on long-term maintainability, setting up a safer path for future migrations without disrupting existing Java APIs or developer workflows.