He doesn't know whether or not to trust her agreement. It's a heavy spot to throw her in: both her father and father figures, brought back into her life quite suddenly. And - him, he supposes. Cassian doesn't put much stock in how heavily he might be effecting her, considering she left in the first place, but the triple threat of it can't be easy.

"Jedha will be a small mission. No more than four people, yourself and me included. We are not looking to for a fight."

Cassian pauses, still holding himself strictly as a Captain. It's not enough to just tell her that her father is alive; he needs to give her more reason to stay with the mission and not fuck off the second she sees a chance. She could have the chance to see him again, and he absolutely uses that. "The ultimate end goal is to find the location of Galen Erso. The defector pilot holds the key to that."

The real part of finding Galen Erso is related to the superweapon, but it's better to avoid phrasing it that way to Jyn. She may figure it out on her own, but he probably has a little bit of time while she's likely more focused on her father for now.