Dean Winchester: Cas is chumming it up with Crowley.
Sam Winchester: Hmm.
Dean Winchester: They’re hunting Lucifer together. That’s right one’s an angel, one’s a demon, and apparently, they solve crimes.
From Supernatural – Season 12 Episode 4: ‘American Nightmare’ (12×04) | TV series produced by The CW
Cas and Crowley are posing as agents while lookin for Lucifer, same way as Sam and Dean do, when they’re hunting.