Raffi contributed to the getlago/lago-openapi repository by correcting the API schema for billing periods, focusing on aligning the schema definition with the underlying data model. He identified and fixed a typo, updating the field from billing_period to billing_periods to accurately represent an array, which improved consistency and reliability for API clients. Working primarily with YAML and applying skills in API design and schema definition, Raffi ensured the schema served as a single source of truth. The work addressed a specific bug rather than introducing new features, reflecting a targeted and detail-oriented approach to maintaining API correctness and client trust.