Worked on reliability improvements for the Find Duplicates and Merge Contacts workflow in the civicrm-core repository, focusing on backend stability rather than new feature development. Addressed a bug in the AJAX callback by ensuring that successful database updates in toggleDedupeSelect() return a valid response body, which eliminated misleading error dialogs and improved the user experience during deduplication. Utilized PHP and AJAX within the CiviCRM core to debug and resolve the issue, applying careful regression planning and code review to prevent related problems. The work resulted in a more stable deduplication process and reduced support overhead for end users.