Qt is a cross-platform application and UI framework - some tools available for no charge from Valve utilize Qt.
Valve ships a modified version of Qt, primarily to allow interoperation with the rest of Source2:
- Valve-Modified Qt 5.15.2 for Win64 (Source2 workshop tools since March 2021)
- Valve-Modified Qt 4.8.3 for Win64 (Source2 workshop tools released before 2021)
To confirm the version of Qt being used, you can select "About Qt" from the Help menu in the Asset Browser or other tool windows.