“Thanks for the invitation” is a good start, but not a complete response on its own. The host needs to know: will you be coming? Who’s coming with you? And sometimes: what’s your meal preference or arrival time? A good RSVP can be friendly without being overly long; it can also be polite without being overly formal.
An effective RSVP puts the most important information right up front. Yes, you’re coming: “Thank you for the invitation. I’d love to be there on Saturday.” No, you’re not coming: “Thank you for inviting me. I won’t be able to come, but I hope you have a lovely time.” The tone is kind, but the host doesn’t have to guess what you mean.
A lot of novice writers mess up their RSVPs by trying too hard to be polite. They pile on explanations or apologizing, or save their main point until the very end. That can leave the host needing to follow up just to get an answer. Good manners involves making it easier for other people to manage their plans. A helpful response provides that answer right off, supplies any details the host asked for, and adds a thank-you to wrap things up.
Try a quick exercise in your head: take a blunt “I can’t make it” and make it nicer but keep it equally brief. “Can’t come” becomes “Thank you for the invitation. I’m sorry I won’t be able to come, but I really appreciate the invitation.” “I’ll be there” becomes “Thanks for inviting me. I’d be glad to attend and look forward to seeing you.” The revision isn’t fancy. It just has a little gratitude, a little less drama, and a more relaxed tone.
Timing, of course, matters. A late response makes it hard for the host to make arrangements for food, seating, and so on, particularly when a meal is being served or an event is being held. Try to reply by the reply-by date shown on your invitation. When no such date appears, reply as soon as it makes sense. When you need to wait on an answer, it’s better to write, “Thanks so much for the invitation. I’m checking one detail and will let you know by tomorrow,” than to not say anything at all to the host.
Take care of whatever else was asked of you in the invitation, too. When the host asks for any details like dietary restrictions, the names of other invited guests, or how they’d like to hear from you, give them those details along with your answer. Don’t ask a question when you could easily find out for yourself; don’t make the reply process more complicated unless it really impacts your ability to attend. A great RSVP acknowledges and respects the planning process that the host has already put in, in addition to the event itself.
Take a quick read over your response before you send it. Is your “yes” or your “no” clear? Is your tone kind? Do you know the answer to the questions they’re asking? Have you given them any details they specifically requested? Is your response a lot shorter than the anxiety you were feeling when you first read the invitation? If your reply can be read and understood in one go, it’s doing its job.