Feature flags are essential for agile and robust modern application development, enabling rapid experimentation and controlled feature rollouts. However, conventional feature flag management platforms can pose privacy risks by exposing sensitive user data. In this paper, we introduce ZKFlag, an open-source platform that delivers feature flags management as a service while preserving user privacy. At the core of ZKFlag is the use of zero-knowledge proofs, which ensure that sensitive data remains confidential without compromising functionality. We detail the platform’s architecture, key functionalities, and security properties. We demonstrate that ZKFlag offers equivalent integration ease to popular solutions while outsourcing platform management while providing a high level of security. By leveraging advanced cryptographic techniques, ZKFlag creates an agile and secure environment for modern software development.