Alexander van der Bellen contributed to the maths/moodle-qtype_stack repository by focusing on backend reliability and maintainability. He refactored API controller test infrastructure, renaming mock classes in PHPUnit-based tests to prevent naming conflicts and improve CI stability without altering production code. In addition, he resolved two bugs: one addressed an undefined array key in healthcheck.php using PHP’s null coalescing operator, ensuring correct health check output, while the other added cache definitions to support unit test reliability for the STACK plugin. His work demonstrated strong skills in PHP development, caching, and Moodle plugin patterns, resulting in more robust automated testing workflows.