Alexander Vanderbellen contributed to the maths/moodle-qtype_stack repository by focusing on backend reliability and maintainability. Over two months, he refactored PHPUnit test infrastructure, renaming mock classes in API controller tests to prevent naming conflicts and improve CI stability without altering production code. He also addressed two bugs, resolving an undefined array key in healthcheck.php using PHP’s null coalescing operator and adding cache definitions to support unit test reliability. His work demonstrated strong skills in PHP development, caching, and Moodle plugin patterns, resulting in more robust health checks, streamlined debugging, and a maintainable codebase for ongoing plugin development.