Guilherme Diasantos focused on backend stability and concurrency improvements across the apache/amoro and apache/gravitino repositories using Java and advanced concurrency techniques. In apache/amoro, he addressed a runtime ConcurrentModificationException by replacing a synchronized ArrayList with a CopyOnWriteArrayList, ensuring thread-safe quota management under concurrent workloads. In apache/gravitino, he refactored exception handling logic to catch Exception instead of Throwable, preventing critical JVM errors from being inadvertently masked and improving system robustness. His work demonstrated a strong grasp of Java collections, exception handling, and concurrency, delivering targeted bug fixes that enhanced runtime reliability and maintainability without introducing new features during the review period.