If you confess a crime to anyone, that's an admission against party interest. And it can usually come in under that hearsay exception. It's pretty common for cops to testify to what the defendant said in custody, and also for jailhouse snitches to testify to what the defendant said in custody (i.e. this is not a special cop hearsay exception).

And the kicker is that this doesn't help you, the defendant. Anything you say will be used against you, but it can't ever be used for you. Because as soon as you're trying to introduce your own prior statements, it's inadmissable hearsay and not an admission against interest any more.

You're right though that it's going to be an argument on this every time.