Worked on the getlago/lago-api repository to enhance billing accuracy and API usability. Addressed a recurring 1-cent rounding discrepancy in invoice calculations by introducing a reconciliation step within the AppliedPrepaidCreditsService, ensuring precise alignment between prepaid credits and invoice totals. Additionally, implemented a GraphQL API enhancement that allows filtering of credit notes by type, improving data accessibility for finance and user experience teams. The work demonstrated expertise in Ruby, Ruby on Rails, and GraphQL, with a focus on precision arithmetic, Rails service patterns, and comprehensive RSpec testing to maintain consistency and reliability across critical backend billing components.