As long as it's "I'll keep my mouth shut" I don't care how you say it.

[She doesn't seem at ease just yet, even as the car pulls in to park.]

And don't think I didn't catch the sarcasm there. Are you swearing to keep it to yourself or not?

[She's not stupid, she knows how to get around promises and agreements very well. Saying you agree and then asking if that's what you're supposed to say is exactly that kind of thing. Because it's not actually agreeing, it's just asking if that's what she wants to hear.]