To support a rich ecosystem of third-party applications around a personally controlled health record (PCHR), we have redesigned Indivo, the original PCHR, as a web-based platform with feature-level substitutability. Core to this new release is the Indivo X Application Programming Interface (API), the contract between the PCHR platform and the end-user apps. Using rapid iterative development to build a minimal feature set from real-world requirements, the resulting Indivo X API, now in public stable beta, is enabling developers, including third-party contributors, to quickly create and integrate novel features into patients' online records, ultimately building a fully customizable experience for diverse patient needs.