Worked on enhancing the ads-client within the mozilla/application-services repository by implementing round-trip handling of the cap_key parameter in sponsored content impression callbacks. This involved ensuring that cap_key is included in impression request URLs and properly removed during processing, addressing requirements for accurate impression tracking. Developed and integrated comprehensive tests to validate cap_key handling throughout the sponsored content impression flow, which improved the reliability and data integrity of the ads-client. Leveraged Rust for backend development, focusing on robust data processing and test coverage. These changes supported more accurate revenue analytics and user targeting by maintaining precise impression data throughout the workflow.