So below is, the original version of the scene that begins on page 137 in City of Glass, Jace looked at Alec steadily. Whats between you and Magnus Bane? In the original version, Jace actually does kiss Alec, more to make a point than anything else, but the resultant scene made me laugh and made my editor laugh, hysterically. It didnt work  made the emotional impact of Jace telling off Alec less. I warn you, it is not romantic.
 - Cassie 

Jace looked at Alec steadily. Then he said, Whats between you and Magnus Bane?
Alecs head jerked to the side, as if Jace had slapped him or pushed him. I dont  theres nothing 
I know better, Jace said, forestalling him. Im not stupid. Tell me the truth.
There isnt anything between us, Alec said  and then, catching the look
on Jaces face, added with great reluctance, any more. Theres nothing between us any more. Okay?
And why is that? Magnus really liked you.
Drop it, Jace, Alec said in a warning tone.
Jace was having none of being warned. Magnus says its because youre hung
up on me. Is that true?
There was a moment of utter silence. Then Alec gave a despairing howl of
horror and put his hands up to cover his face. I am going to kill Magnus. Kill him dead.
Dont. He cares about you. He really does. I believe that, Jace said,
managing to sound only a little bit awkward. Look. I dont want to push you into anything, but do you maybe want to 
Call Magnus? Look, thats a dead end, I know youre trying to be helpful,
but 
kiss me? Jace finished.
Alec looked as if he were about to fall off his chair. WHAT? What? What?
Once what would do. Jace did his best to look as if this were the sort of
suggestion one made all the time. I think it might help.
Alec looked at him with something like horror. You dont mean that.
Why wouldnt I mean it?
Because youre the straightest person I know. Possibly the straightest person in the universe.
Exactly, Jace said, and leaned forward, and kissed Alec on the mouth.
The kiss lasted approximately four seconds before Alec pulled forcefully away, throwing his hands up as if to ward Jace off from coming at him again.
He looked as if he were about to throw up. By the Angel, he said. Dont ever do that again.
Oh yeah? Jace grinned, and almost meant it. That bad?
Like kissing my brother, said Alec, with a look of horror in his eyes.
I thought you might feel that way. Jace crossed his arms over his chest.
Also, Im hoping we can just gloss over all the irony in what you just said.
We can gloss over whatever you want to, Alec said fervently. Just dont kiss me again.
Im not going to. I have other business to take care of. Jace stood up,
kicking his chair back. If anyone asks where I am, tell them I went for a walk.
Where are you actually going? Alec asked, watching him walk to the door.
To see Clary?
No. Jace shook his head. Im going to the Gard. Im going to break Simon out of jail.
