Geir Fiksdal enhanced F# discriminated union serialization in the dotnet/orleans repository, focusing on expanding support for complex F# data structures. He implemented serialization improvements to handle private constructors, named fields, and mutually recursive discriminated unions, addressing nuanced F# language features. Using F# and C#, Geir broadened test coverage by introducing comprehensive tests for struct discriminated unions and diverse union shapes, ensuring robust and reliable serialization behavior. His work in code generation and serialization improved compatibility and reduced the risk of regressions, enabling safer interoperability with F# data. The depth of testing reflects a thorough and maintainable engineering approach.