OpenDraft

Open Draft - Free, open-source screenwriting software — a modern alternative to Final Draft. Professional screenplay formatting, real-time collaboration, beat boards, and version control. Desktop (macOS/Windows/Linux) & browser. Maintained by Proteus-Technologies-Private-Limited on GitHub, where it has earned 36 stars from the community.

It's actively developed around collaborative-writing, fastapi, fountain, and is a solid reference for anyone building with these tools.