Sergej Zubarew worked on the bettercodepaul/swt2-bsa-backend repository, focusing on enhancing the reliability of session runtime logic through improved unit testing. He expanded the SessionRuntimeTest suite by adding comprehensive assertions and handling edge cases, which reduced test flakiness and strengthened regression protection for backend session handling. Sergej updated the business-logic module to incorporate AssertJ, enabling more expressive and robust test assertions, and utilized Java and Mockito to reinforce test coverage. His work addressed the need for higher test quality and reliability, demonstrating depth in both technical implementation and understanding of backend testing requirements within a short project period.