Worked on improving error handling and stability for union types in constructors across the symfony/symfony and symfony/serializer repositories. Focused on backend development using PHP, with an emphasis on serialization, deserialization, and XML denormalization. Addressed bugs where union types in constructors led to unclear or incorrect error messages during deserialization, refining the reporting to enumerate all possible types and reduce debugging time. Enhanced test coverage by introducing dedicated fixtures and regression tests, ensuring reliability and compatibility. The work strengthened type handling and object normalization, resulting in more maintainable code and a better developer experience when working with complex data structures.