You've spent weeks now helping Undyne with her surprise for Alphys, as have Sans, Papyrus, and Mettaton. It's all about to pay off, if the Core temperature readings would just behave for five minutes.
(As it turns out, Mettaton's been working on a surprise for you, too - if he ever gets the chance to give it to you.)
[Reader has a vulva, but no gendered pronouns are used. There's also no specific chest stuff (mentions of bras, etc.), in case that's a concern of yours.]