Worked on the alphagov/pay-connector repository to enhance Adyen payment processing by upgrading the integration to Adyen Checkout API version 72. Focused on backend development using Java and YAML, the work introduced idempotency key mapping across authorisation, capture, cancel, and refund operations to prevent duplicate transactions and improve payment reliability. Refactored and expanded unit and integration tests to validate new API headers and ensure compatibility with the updated Adyen platform. These changes reduced the risk of duplicate payments, stabilized end-to-end payment flows, and increased test coverage, aligning the system with the latest Adyen API standards and configuration requirements.