Connor Smith enhanced API encapsulation in the typelevel/cats-effect repository by restricting access to the deprecated IO.onError method, making it package-private to reinforce API boundaries. He introduced a compile-time test to verify that IO.onError, when used with a total function in a PartialFunction, compiles as intended, thereby improving API integrity and maintainability. Working exclusively in Scala, Connor applied principles of API design and functional programming to ensure the codebase remained robust and clear. His work focused on proactive maintenance and quality improvements, addressing potential integration issues before they could arise, and contributing to the long-term stability of the project.