In my opinion, the most important feature in a healthcare interoperability API (HL7/FHIR) is secure and standardized data exchange, because the main purpose of interoperability is to ensure that healthcare systems can accurately and safely share patient information across hospitals, clinics, laboratories, and other providers. Standardized formats like HL7 and FHIR enable consistent communication between different systems, while strong security measures such as encryption, authentication, and access controls help protect sensitive patient data and maintain compliance with healthcare regulations. Although scalability, real-time access, and integration capabilities are also important, they are most effective when data can be exchanged securely and consistently. Therefore, secure and standardized interoperability directly improves care coordination, reduces medical errors, and supports better patient outcomes through timely access to accurate health information.