The Open Procurement Two-stage tendering procedure is a plugin to Open Procurement API software.
REST-ful interface to plugin is in line with the core software design principles.
The project is in active development.
The source repository for this project is on GitHub: https://github.com/openprocurement/openprocurement.tender.twostage
API is highly unstable, and while API endpoints are expected to remain relatively stable the data exchange formats are expected to be changed a lot. The changes in the API are communicated via Open Procurement API maillist.