What should I reply to a girl who texts "good boy"?

Look at what came right before. If you did something she asked for, like picking the place, sending the thing she wanted, or getting the plan sorted, the phrase is praise and a bit of a tease about how easily you obeyed. If the topic was unrelated, she's enjoying a playful dynamic and checking whether you'll play along. Either way she is testing the tone. She wants to see whether you can be light and a little cheeky without getting awkward.

It also tells you something about her. A woman who says this is comfortable with a teasing register and likely wants to see you hold your own inside it. Pay attention to whether she follows up with more of the same, which means she's invested in the bit, or drops it, which means she used it once and is moving on. Her next message will tell you more than the phrase did.

A reply that works here usually does one of three jobs. It acknowledges the tease with a light, self-aware line that shows you got the joke. It turns the tease back on her in a small way, which shows you're not only receiving. Or it takes the playful frame and moves the conversation forward, so the tease becomes a bridge to a plan or a question. The first kind is safe, the second shows confidence, and the third is what you use when you want the chat to go somewhere.

A draft is in the right shape when it's short, matches her energy, and leaves her something easy to answer. Leave out anything that reads as begging for approval, anything that explains the joke, and anything that jumps to sexual territory unless she has already gone there herself.

If she sent it in the middle of a live conversation, reply within the same back and forth, usually within a few minutes if you're both active. Waiting an hour after a playful line makes the moment cold. If it came late at night and you're both winding down, a short reply is fine and you can pick up the next day.

If she doesn't answer your reply, don't send a second joke to restart it. Wait until the next natural point, like the next day or when you have something real to suggest. If she replies warmly, keep the energy and move toward a plan. If the conversation dies after one or two exchanges, let it go and try again later with something new rather than digging for the same tone.