I spent months on this exact problem building an AI texting keyboard. What actually works:

**1. "Make it casual" does nothing.** Models default to full sentences and proper punctuation because that's their training data. Use hard constraints instead: *"under 12 words, lowercase, contractions, no em-dashes."*

**2. Show it your own texts.** Two or three examples of how you actually text beats any instruction. Models copy style; they ignore descriptions of style.

**3. Feed it the whole conversation.** A reply to one isolated line always misses. The callback to something said earlier is what makes it sound human.

**4. Label who said what.** Paste a chat unlabeled and the model frequently replies from the *other* person's perspective. Marking "them:" and "me:" fixed more bad outputs than anything else I tried.

**5. Emojis need a number.** "Use some emojis" = three dumped at the end. "Exactly one emoji, mid-sentence" = looks human.

Disclosure: I'm the dev behind Txtor Ai, so obvious bias. But all of this works in plain ChatGPT too — 2 and 4 alone kill most of the work-email problem.

If anyone wants it automated: I built this into an iOS keyboard — pick a tone, length and emoji count, it reads the whole chat, works inside any app. Free tier is 5 replies/month, no account.

Why AI texts always sound like work emails — 5 things that actually fix it (I built one of these tools)