Date: Ta'anis Esther 5573
From: Rabbi Moshe Sofer
To: Unknown recipient
The Noda BiYehuda rules that opening a parasol on Shabbos constitutes a Torah-level prohibition of making an ohel (tent). The Chasam Sofer disagrees, offering three reasons it cannot be a Torah violation: it is temporary construction, has no Mishkan precedent for a mobile tent, and its suspended sides do not constitute proper walls.
Source: Shu"t Chasam Sofer OC §72