[He feels slightly guilty that his date with Magnus is being sidetracked but the festival is for a few days. They could spend an hour or two with Jace, couldn't they?

Would Magnus be okay with it? He'd been jealous of Jace before, hating even the smallest gesture of affection that Alec shared with his parabatai. They seemed to be getting along now that they were rooming together, but Alec doesn't want to push and have that good will ruined. It doesn't help that Jace is sending mixed messages, saying he should leave while clinging to Alec like he expects him to disappear forever if he lets go. For months, he barely existed to Jace and now this. As Magnus pretends to look at one of the displays, Alec knows what his husband is doing and gives him a grateful smile as he walks by.]

Magnus has been around parabatai before. He knows that where one goes, the others follows. He knew we're a package deal when he started dating me.

If you're really worried about it, keep an eye on Max later so we can sneak out. [Maybe if he added that 'incentive' for their date to be postponed, Jace won't feel like he's intruding.]