A SPY! ?
Wouldn’t that be a bit obvious?
Just because you’re paranoid doesn’t mean they aren’t out to get you.
Besides, under current jurisprudence, the jailhouse informant never has to actually hear anything incriminating, they just have to tell the court they did so they can be rewarded with an instant release. It would suck to be an innocent man sharing a cell with one of these craven creeps, but, as far as the law (and half the random strangers you pass on the street) are concerned, you wouldn’t be in a cell if you weren’t guilty in the first place...