During March 2025, Egan Mendonca stabilized the core build tooling for the facebook/buck2 and facebook/buck2-prelude repositories by rolling back recent Kotlin immutables migrations. He restored Java-based immutable implementations and reverted the IsolatedStep model from an interface back to a class-based architecture, addressing regressions and ensuring compatibility with existing Java APIs. This work involved careful code reversion and refactoring using both Java and Kotlin, with a focus on build system reliability and maintainability. By coordinating changes across multiple repositories, Egan resolved critical bugs, improved CI stability, and established a safer foundation for future, more controlled migration efforts.