What's a flirty response to someone texting me "I'm tired"?

A single "I'm tired" tells you almost nothing without context. Look at how the conversation has been going before it. If the last few exchanges were easy and she was asking you things, the tiredness is probably just a real-life thing like a long day or a late hour. If her replies have been getting shorter, taking longer, or she's been the one to end conversations, then tiredness may be her soft way of pulling back. Also check the clock. A message at 11pm reads very differently from one at 3pm. Most of the time people send this when they're about to stop, not when they want you to fix their mood. You want your reply to fit whichever of those is true, which is why guessing wrong in one direction costs you the chat and in the other direction makes you look like you're not paying attention.

Two kinds of reply tend to work here. The first is a short warm acknowledgement with a playful edge, something that shows you noticed her and are comfortable letting the conversation sit. Its job is to make her feel seen without asking for anything. A draft is working if she could reply with one word and still feel the exchange was nice. The second kind is a small, low-stakes teaser that gives her something to answer if she wants to, like a tiny shared joke or a light question with an easy answer. Its job is to leave a door open. You can tell it's right if it doesn't require her to explain her day or make a decision. Leave out anything that sounds like a plan you need her to commit to, any question that asks her to share feelings, and any line that tries too hard to be clever in one go.

Send your reply within a few minutes if she's still active, because a tired message late at night is often the last thing she'll read before stopping. If she doesn't reply, that is usually the answer, not a cue to send a second text. Let it sit until the next time you'd normally be talking, which could be the next morning or the next day depending on how often you two text. A follow-up is fine if it's a different topic that clearly doesn't need a reply, like something you saw that made you think of her. If she goes quiet for several days after you've been talking regularly, the tiredness was probably a soft exit, and you can stop asking for more without being rude. Pushing past that point turns a pleasant thread into something she has to manage.