The infrastructure for indoor spatial data remains fragmented compared to its outdoor counterpart, lacking the interoperable and developer-friendly tools that have made outdoor mapping so prevalent. This paper presents API-IndoorFeatures, an open-source RESTful API crafted to bridge this gap by delivering IndoorGML 2.0 data in the IndoorJSON format through standardized HTTPS endpoints. This framework grants developers access to OGC indoor spatial standards without requiring in-depth knowledge of the underlying data models.<br />Built on pygeoAPI, the system enhances its architecture with a specialized backend for indoor spatial data that complies with the OGC API – Features standard. Spatial data is managed in PostgreSQL using PostGIS, enabling efficient geometric queries, while pgRouting supports indoor navigation. A notable contribution of this work is the adoption of IndoorJSON, a lightweight, web-native JSON serialization of the IndoorGML information model, which is currently in the process of standardization by OGC. This format replaces the parsing complexities associated with XML-based encodings and integrates seamlessly into contemporary development environments.<br />API-IndoorFeatures offers hierarchical data access across both the primal and dual space layers of IndoorGML 2.0, providing comprehensive Create, Read, Update, and Delete (CRUD) support, as well as geometric bounding box queries that enable progressive floor-by-floor rendering and routing from origin to destination. Distinct among open-source indoor spatial systems, it enforces IndoorGML 2.0 schema validation and referential integrity on all write operations. To our knowledge, this may be one of the first systems to operationalize the revised IndoorGML 2.0 conceptual model introduced in August 2025. This work expands previous contributions beyond mere data construction to encompass serving, validation, and consumption, thereby offering a reusable open-source platform for the next generation of indoor location services.