[Haskell-cafe] HLS in VS Code is identifying problems that don't when I run stack on the command line

I'm trying to get HLS set up with a stack project on VS Code. I'm getting messages like this: Could not load module ‘Control.Monad.Except’ It is a member of the hidden package ‘mtl-2.2.2’. You can run ‘:set -package mtl’ to expose it. (Note: this unloads all the modules in the current scope.) But these are in the "Problems" tab of VS Code (and show as red underlines). When I run stack clean, stack build I don't see any error like this at all. And yes, I made sure mtl was added everywhere in cac.cabal. (I'm using cac.cabal not stack.yaml... is that my problem?) I tried stack clean; stack build; restart HLS several times.