Arthur contributed to the neynarxyz/OAS repository by refining the OpenAPI specification, focusing on standardizing reaction type enum names to improve clarity and downstream compatibility. He updated the enum values from uppercase identifiers to more readable forms, such as changing REACTION_TYPE_LIKE to 'Like' and REACTION_TYPE_RECAST to 'Recast', and incremented the V2 spec version to reflect these changes. Working primarily with YAML and leveraging his skills in API design and OpenAPI Specification, Arthur’s work enhanced consistency across the API, reduced integration friction for clients, and laid groundwork for future versioned evolution, though the scope was limited to a single feature.