Developed secure, standards-based OAuth 2.0 authentication for mCSD query and admin clients within the nuts-foundation/nuts-knooppunt repository, focusing on robust access control and enterprise security alignment. Leveraged Go for backend development, implementing centralized OAuth2 configuration and an authenticated HTTP client to facilitate FHIR requests. The work included comprehensive automated tests to validate the OAuth2 authentication flow, reducing the risk of misconfiguration and ensuring consistent security policies across endpoints. This feature laid the foundation for scalable authentication across multiple client types, emphasizing maintainability and reliability in API development while supporting future extensibility and compliance with security best practices.