Worked on the typelevel/cats-effect repository to enhance API encapsulation and maintainability, focusing on the IO.onError functionality. The main contribution involved restricting access to the deprecated IO.onError by making it package-private, thereby tightening API boundaries and reducing potential misuse. To further ensure API integrity, a compile-time test was introduced to verify that IO.onError with a total function in a PartialFunction compiles as expected. This work emphasized careful API design and functional programming principles, utilizing Scala as the primary language. The changes contributed to improved code quality and maintainability, with no major bugs reported during the development period.