In QGIS 2.10 we mentioned that there is a new geometry architecture for QGIS but that not all features were supported in the digitising tools. With QGIS 2.12 we now have editing support for the creation of curves / 'circular strings`. Note again that you need to be using a data provider (e.g. PostGIS, GML or WFS) that supports curves. These improvements to the digitising tools were also added in QGIS 2.12:
Additionally, more of the geometry editing and modification tools were updated to work correctly with layers containing z or m dimensions.
This feature was developed by: Marco Hugentobler at Sourcepole AG This feature was funded by: Canton of Solothurn