@JeremyECrawford
A interrupts with Counterspell an ongoing spellcasting process of B (requires longer casting time), B losts his spell slot?— Draconis (@DerynDraconis) November 6, 2017
A successful counterspell causes a spell to fail, regardless of that spell's casting time. The thwarted spell's slot, if any, is spent. #DnD https://t.co/KVVVU59D77
— Jeremy Crawford (@JeremyECrawford) November 7, 2017