What do we do if SETI is successful?

Isn't that what the movie Contact was about?

In all seriousness, I think if we did receive something, it would be classified immediately, and the government, or governments, will move very swiftly with a heavy hand to silence the discovery. At the very least until they know exactly what it is, what it is conveying, and how to respond.

That said, I think that if it got out, a lot of people would absolutely lose their snot. Completely. It would be chaos in some places.

Ha, or, perhaps for a 2025 variant: it would quickly be shared publicly by government scientists (who are not as secretive or good at keeping secrets as the public seems to think!), the evidence all shared publicly, subject to international peer review and consensus. And then 70% of people would believe it's made-up. The US-sphere would believe China made it up as a plot (or "globalists") and the developing world and BRICS would believe the US made it up as a plot. Western countries would repeatedly sign and then remove themselves from international treaties to prepare for contact.

Bit too on the nose, maybe, but a heck of a lot more likely than a coverup by government scientists.

So... Like Contact?