What to text back when she says she's flattered but wants to keep it casual

The flattered line is a polite cushion. The part that matters comes after it, where she names what she wants, which is something light. She isn't asking you to read between the lines. If you go looking for a hidden yes, you'll find one, because people do that to themselves all the time, but it won't be in her message.

Read the whole thing as a clear answer with a kind opening. Your job is to match her clarity. People who say casual and then send a follow-up asking if she's sure usually aren't hearing the answer they got. They're hoping it changes if they keep the topic alive. It almost never does, and it makes casual feel like homework for her.

A casual reply should look like a casual reply. A line or two, a normal tone, and nothing that tries to reopen the question of your feelings. Heavy emotion in your response tells her the boundary is costing you something, which puts her in the position of managing your reaction. That's exactly the dynamic she's trying to avoid.

It helps to make your next messages look like the conversation you were having before. If you were talking about food, a show, or weekend plans, go back to that. You're showing her casual can be casual, and that you're not turning it into an audition.

Reply at the time of day you'd normally text her, not in the first ten minutes while you're still reacting. A few hours is usually enough. Replying instantly suggests you were waiting for it, and going silent for several days suggests you took it badly. She'll read both.

If she doesn't reply to your casual message, leave it alone. Don't send a second message asking if she's still there, and don't send one a week later that tries to restart the thread with a big question. If you talk again, it'll probably be because she starts something, usually small. If she doesn't, you have your answer for now, and the most useful thing you can do is get on with your week.