diff --git a/hie-bios.cabal b/hie-bios.cabal
index e65855c1c..5ccd8e565 100644
--- a/hie-bios.cabal
+++ b/hie-bios.cabal
@@ -144,6 +144,7 @@ Library
                         extra                >= 1.6.14 && < 1.8,
                         process              >= 1.6.1 && < 1.7,
                         ghc                  >= 8.4.1 && < 8.11,
+                        ghc-paths            >= 0.1 && < 0.2,
                         transformers         >= 0.5.2 && < 0.6,
                         temporary            >= 1.2 && < 1.4,
                         text                 >= 1.2.3 && < 1.3,
diff --git a/src/HIE/Bios/Cradle.hs b/src/HIE/Bios/Cradle.hs
index c248c605d..08d890a99 100644
--- a/src/HIE/Bios/Cradle.hs
+++ b/src/HIE/Bios/Cradle.hs
@@ -37,9 +37,10 @@ import Control.Monad
 import System.Info.Extra
 import Control.Monad.IO.Class
 import System.Environment
-import Control.Applicative ((<|>))
+import Control.Applicative ((<|>), optional)
 import System.IO.Temp
 import System.IO.Error (isPermissionError)
+import Data.Char
 import Data.List
 import Data.Ord (Down(..))
 
@@ -212,7 +213,9 @@ defaultCradle cur_dir =
     { cradleRootDir = cur_dir
     , cradleOptsProg = CradleAction
         { actionName = Types.Default
-        , runCradle = \_ _ -> return (CradleSuccess (ComponentOptions [] cur_dir []))
+        , runCradle = \_ _ ->
+            return (CradleSuccess (ComponentOptions [] cur_dir []))
+        , getGhcPath = findExecutable "ghc"
         }
     }
 
@@ -226,6 +229,7 @@ noneCradle cur_dir =
     , cradleOptsProg = CradleAction
         { actionName = Types.None
         , runCradle = \_ _ -> return CradleNone
+        , getGhcPath = pure Nothing
         }
     }
 
@@ -239,6 +243,14 @@ multiCradle buildCustomCradle cur_dir cs =
     , cradleOptsProg = CradleAction
         { actionName = multiActionName
         , runCradle  = \l fp -> canonicalizePath fp >>= multiAction buildCustomCradle cur_dir cs l
+        , getGhcPath =
+            -- We're being lazy here and just returning the ghc path for the
+            -- first non-none cradle. This shouldn't matter in practice: all
+            -- sub cradles should be using the same ghc version!
+            case filter (not . isNoneCradleConfig) $ map snd cs of
+              [] -> return Nothing
+              (cfg:_) -> getGhcPath $ cradleOptsProg $
+                getCradle buildCustomCradle (cfg, cur_dir)
         }
     }
   where
@@ -305,12 +317,14 @@ multiAction buildCustomCradle cur_dir cs l cur_fp =
 -------------------------------------------------------------------------
 
 directCradle :: FilePath -> [String] -> Cradle a
-directCradle wdir args  =
+directCradle wdir args =
   Cradle
     { cradleRootDir = wdir
     , cradleOptsProg = CradleAction
         { actionName = Types.Direct
-        , runCradle = \_ _ -> return (CradleSuccess (ComponentOptions args wdir []))
+        , runCradle = \_ _ ->
+            return (CradleSuccess (ComponentOptions args wdir []))
+        , getGhcPath = findExecutable "ghc"
         }
     }
 
@@ -326,6 +340,7 @@ biosCradle wdir biosCall biosDepsCall =
     , cradleOptsProg   = CradleAction
         { actionName = Types.Bios
         , runCradle = biosAction wdir biosCall biosDepsCall
+        , getGhcPath = findExecutable "ghc"
         }
     }
 
@@ -378,6 +393,9 @@ cabalCradle wdir mc =
     , cradleOptsProg   = CradleAction
         { actionName = Types.Cabal
         , runCradle = cabalAction wdir mc
+        , getGhcPath = optional $ fmap trim $
+            -- Need to pass -v0 otherwise we get "resolving dependencies..."
+            readProcess "cabal" ["v2-exec", "which", "-v0", "--", "ghc"] ""
         }
     }
 
@@ -495,6 +513,8 @@ stackCradle wdir mc =
     , cradleOptsProg   = CradleAction
         { actionName = Types.Stack
         , runCradle = stackAction wdir mc
+        , getGhcPath = optional $ fmap trim $
+            readProcess "stack" ["exec", "which", "--silent", "--", "ghc"] ""
         }
     }
 
@@ -697,3 +717,7 @@ makeCradleResult (ex, err, componentDir, gopts) deps =
     _ ->
         let compOpts = ComponentOptions gopts componentDir deps
         in CradleSuccess compOpts
+
+-- Used for clipping the trailing newlines on some commands
+trim :: String -> String
+trim = dropWhileEnd isSpace
diff --git a/src/HIE/Bios/Environment.hs b/src/HIE/Bios/Environment.hs
index f93a19abe..323adb476 100644
--- a/src/HIE/Bios/Environment.hs
+++ b/src/HIE/Bios/Environment.hs
@@ -1,14 +1,17 @@
 {-# LANGUAGE RecordWildCards, CPP #-}
-module HIE.Bios.Environment (initSession, getSystemLibDir, makeDynFlagsAbsolute, getCacheDir, addCmdOpts) where
+module HIE.Bios.Environment (initSession, getGhcLibDir, makeDynFlagsAbsolute, getCacheDir, addCmdOpts) where
 
 import CoreMonad (liftIO)
-import GHC (DynFlags(..), GhcLink(..), HscTarget(..), GhcMonad)
+import GHC (GhcMonad)
 import qualified GHC as G
 import qualified DriverPhases as G
 import qualified Util as G
 import DynFlags
+import qualified GHC.Paths as Paths
 
+import Control.Applicative
 import Control.Monad (void)
+import Control.Monad.Trans.Maybe
 
 import System.Process (readProcess)
 import System.Directory
@@ -20,8 +23,8 @@ import qualified Data.ByteString.Char8 as B
 import Data.ByteString.Base16
 import Data.List
 import Data.Char (isSpace)
-import Control.Applicative ((<|>))
-import Text.ParserCombinators.ReadP
+import Data.Maybe
+import Text.ParserCombinators.ReadP hiding (optional)
 import HIE.Bios.Types
 import HIE.Bios.Ghc.Gap
 
@@ -53,13 +56,22 @@ initSession  ComponentOptions {..} = do
 
 ----------------------------------------------------------------
 
--- | Obtain the directory for system libraries.
-getSystemLibDir :: IO (Maybe FilePath)
-getSystemLibDir = do
-    res <- readProcess "ghc" ["--print-libdir"] []
-    return $ case res of
-        ""   -> Nothing
-        dirn -> Just (init dirn)
+-- | Obtain the directory for the ghc library directory.
+-- It tries this specific order of paths:
+-- 1. the NIX_GHC_LIBDIR if it is set
+-- 2. calling --print-libdir with the cradle's ghc
+-- 3. using ghc-paths
+-- Note that 3 will only work if we are running on the same machine that
+-- hie-bios compiled on, so we avoid using it unless we have to!
+getGhcLibDir :: Cradle a -> IO FilePath
+getGhcLibDir cradle =
+  fmap (fromMaybe Paths.libdir) $ runMaybeT $ fromNix <|> fromCradle
+  where
+    fromNix = MaybeT $ lookupEnv "NIX_GHC_LIBDIR"
+    fromCradle = do
+      ghc <- MaybeT $ getGhcPath (cradleOptsProg cradle)
+      libDir <- MaybeT $ optional $ readProcess ghc ["--print-libdir"] []
+      return (dropWhileEnd isSpace libDir)
 
 ----------------------------------------------------------------
 
@@ -123,12 +135,25 @@ setHiDir f d = d { hiDir      = Just f}
 addCmdOpts :: (GhcMonad m)
            => [String] -> DynFlags -> m (DynFlags, [G.Target])
 addCmdOpts cmdOpts df1 = do
-  (df2, leftovers', _warns) <- G.parseDynamicFlags df1 (map G.noLoc cmdOpts)
+  -- liftIO $ print cmdOpts
+  (df2, leftovers'', _warns) <- G.parseDynamicFlags df1 (map G.noLoc cmdOpts)
+
+  -- remove any RTS flags from the leftovers
+  let leftovers' = go False (G.unLoc <$> leftovers'')
+      go _     []          = []
+      go _     ("+RTS":xs) = go True xs
+      go True  ("-RTS":xs) = go False xs
+      go True  (_:xs)      = go True xs
+      go False (x:xs)      = x : go False xs
+
+  -- liftIO $ putStrLn "leftovers'':" >> print (G.unLoc <$> leftovers'')
+  -- liftIO $ putStrLn "leftovers':" >> print leftovers'
+
   -- parse targets from ghci-scripts. Only extract targets that have been ":add"'ed.
   additionalTargets <- concat <$> mapM (liftIO . getTargetsFromGhciScript) (ghciScripts df2)
 
   -- leftovers contains all Targets from the command line
-  let leftovers = leftovers' ++ map G.noLoc additionalTargets
+  let leftovers = leftovers' ++ additionalTargets
 
   let
      -- To simplify the handling of filepaths, we normalise all filepaths right
@@ -148,7 +173,7 @@ addCmdOpts cmdOpts df1 = do
 #endif
           cur_dir = '.' : [pathSeparator]
           nfp = normalise fp
-    normal_fileish_paths = map (normalise_hyp . G.unLoc) leftovers
+    normal_fileish_paths = map normalise_hyp leftovers
   let
    (srcs, objs) = partition_args normal_fileish_paths [] []
    df3 = df2 { ldInputs = map (FileOption "") objs ++ ldInputs df2 }
diff --git a/src/HIE/Bios/Ghc/Api.hs b/src/HIE/Bios/Ghc/Api.hs
index e7d2afcb8..6191512df 100644
--- a/src/HIE/Bios/Ghc/Api.hs
+++ b/src/HIE/Bios/Ghc/Api.hs
@@ -6,9 +6,8 @@ module HIE.Bios.Ghc.Api (
   , G.SuccessFlag(..)
   -- * Utility functions for running the GHC monad and implementing internal utilities
   , withGHC
-  , withGHC'
   , withGhcT
-  , getSystemLibDir
+  , getGhcLibDir
   , withDynFlags
   ) where
 
@@ -32,10 +31,13 @@ import HIE.Bios.Flags
 ----------------------------------------------------------------
 
 -- | Converting the 'Ghc' monad to the 'IO' monad. All exceptions are ignored and logged.
-withGHC :: FilePath  -- ^ A target file displayed in an error message.
+withGHC :: Cradle c  -- ^ The cradle to use for resolving the lib dir
+        -> FilePath  -- ^ A target file displayed in an error message.
         -> Ghc a -- ^ 'Ghc' actions created by the Ghc utilities.
         -> IO a
-withGHC file body = ghandle ignore $ withGHC' body
+withGHC cradle file body = do
+    libDir <- getGhcLibDir cradle
+    ghandle ignore (G.runGhc (Just libDir) body)
   where
     ignore :: SomeException -> IO a
     ignore e = do
@@ -43,18 +45,11 @@ withGHC file body = ghandle ignore $ withGHC' body
         Log.logm (show e)
         exitSuccess
 
--- | Run a Ghc monad computation with an automatically discovered libdir.
--- It calculates the lib dir by calling ghc with the `--print-libdir` flag.
-withGHC' :: Ghc a -> IO a
-withGHC' body = do
-    -- TODO: Why is this not using ghc-paths?
-    mlibdir <- getSystemLibDir
-    G.runGhc mlibdir body
-
-withGhcT :: (Exception.ExceptionMonad m, G.MonadIO m, Monad m) => GhcT m a -> m a
-withGhcT body = do
-  mlibdir <- G.liftIO $ getSystemLibDir
-  G.runGhcT mlibdir body
+withGhcT :: (Exception.ExceptionMonad m, G.MonadIO m, Monad m)
+         => Cradle c -> GhcT m a -> m a
+withGhcT cradle body = do
+  libDir <- liftIO $ getGhcLibDir cradle
+  G.runGhcT (Just libDir) body
 
 ----------------------------------------------------------------
 
diff --git a/src/HIE/Bios/Ghc/Check.hs b/src/HIE/Bios/Ghc/Check.hs
index bc5863210..2f16726aa 100644
--- a/src/HIE/Bios/Ghc/Check.hs
+++ b/src/HIE/Bios/Ghc/Check.hs
@@ -16,6 +16,7 @@ import Control.Monad.IO.Class
 import System.IO.Unsafe (unsafePerformIO)
 import qualified HIE.Bios.Ghc.Gap as Gap
 
+import qualified DynFlags as G
 import qualified GHC as G
 import HIE.Bios.Environment
 
@@ -28,7 +29,7 @@ checkSyntax :: Show a
             -> [FilePath]  -- ^ The target files.
             -> IO String
 checkSyntax _      []    = return ""
-checkSyntax cradle files = withGhcT $ do
+checkSyntax cradle files = withGhcT cradle $ do
     Log.debugm $ "Cradle: " ++ show cradle
     res <- initializeFlagsWithCradle (head files) cradle
     case res of
@@ -53,7 +54,9 @@ checkSyntax cradle files = withGhcT $ do
 check :: (GhcMonad m)
       => [FilePath]  -- ^ The target files.
       -> m (Either String String)
-check fileNames = withLogger setAllWarningFlags $ setTargetFiles (map dup fileNames)
+check fileNames = do
+  topDir <- G.topDir <$> G.getDynFlags
+  withLogger (setAllWarningFlags topDir) $ setTargetFiles (map dup fileNames)
 
 dup :: a -> (a, a)
 dup x = (x, x)
@@ -61,14 +64,13 @@ dup x = (x, x)
 ----------------------------------------------------------------
 
 -- | Set 'DynFlags' equivalent to "-Wall".
-setAllWarningFlags :: DynFlags -> DynFlags
-setAllWarningFlags df = df { warningFlags = allWarningFlags }
+setAllWarningFlags :: FilePath -> DynFlags -> DynFlags
+setAllWarningFlags topDir df = df { warningFlags = allWarningFlags topDir }
 
 {-# NOINLINE allWarningFlags #-}
-allWarningFlags :: Gap.WarnFlags
-allWarningFlags = unsafePerformIO $ do
-    mlibdir <- getSystemLibDir
-    G.runGhcT mlibdir $ do
+allWarningFlags :: FilePath -> Gap.WarnFlags
+allWarningFlags topDir = unsafePerformIO $
+    G.runGhcT (Just topDir) $ do
         df <- G.getSessionDynFlags
         (df', _) <- addCmdOpts ["-Wall"] df
-        return $ G.warningFlags df'
\ No newline at end of file
+        return $ G.warningFlags df'
diff --git a/src/HIE/Bios/Ghc/Logger.hs b/src/HIE/Bios/Ghc/Logger.hs
index d6dfa0ce6..d55ac292e 100644
--- a/src/HIE/Bios/Ghc/Logger.hs
+++ b/src/HIE/Bios/Ghc/Logger.hs
@@ -10,7 +10,7 @@ import DynFlags (LogAction, dopt, DumpFlag(Opt_D_dump_splices))
 import ErrUtils
 import Exception (ghandle)
 import FastString (unpackFS)
-import GHC (DynFlags(..), SrcSpan(..), Severity(SevError), GhcMonad)
+import GHC (DynFlags(..), SrcSpan(..), GhcMonad)
 import qualified GHC as G
 import HscTypes (SourceError, srcErrorMessages)
 import Outputable (PprStyle, SDoc)
diff --git a/src/HIE/Bios/Internal/Debug.hs b/src/HIE/Bios/Internal/Debug.hs
index 1bc114d65..824de6a93 100644
--- a/src/HIE/Bios/Internal/Debug.hs
+++ b/src/HIE/Bios/Internal/Debug.hs
@@ -1,12 +1,10 @@
 {-# LANGUAGE LambdaCase #-}
 module HIE.Bios.Internal.Debug (debugInfo, rootInfo, configInfo, cradleInfo) where
 
-import Control.Monad.IO.Class (liftIO)
 import Control.Monad
 import Data.Void
 
 import qualified Data.Char as Char
-import Data.Maybe (fromMaybe)
 
 import HIE.Bios.Ghc.Api
 import HIE.Bios.Cradle
@@ -34,17 +32,19 @@ debugInfo fp cradle = unlines <$> do
     canonFp <- canonicalizePath fp
     conf <- findConfig canonFp
     crdl <- findCradle' canonFp
+    ghcPath <- getGhcPath (cradleOptsProg cradle)
+    ghcLibDir <- getGhcLibDir cradle
     case res of
       CradleSuccess (ComponentOptions gopts croot deps) -> do
-        mglibdir <- liftIO getSystemLibDir
         return [
-            "Root directory:      " ++ rootDir
-          , "Component directory: " ++ croot
-          , "GHC options:         " ++ unwords (map quoteIfNeeded gopts)
-          , "System libraries:    " ++ fromMaybe "" mglibdir
-          , "Config Location:     " ++ conf
-          , "Cradle:              " ++ crdl
-          , "Dependencies:        " ++ unwords deps
+            "Root directory:        " ++ rootDir
+          , "Component directory:   " ++ croot
+          , "GHC options:           " ++ unwords (map quoteIfNeeded gopts)
+          , "GHC path:              " ++ show ghcPath
+          , "GHC library directory: " ++ ghcLibDir
+          , "Config Location:       " ++ conf
+          , "Cradle:                " ++ crdl
+          , "Dependencies:          " ++ unwords deps
           ]
       CradleFail (CradleError deps ext stderr) ->
         return ["Cradle failed to load"
diff --git a/src/HIE/Bios/Types.hs b/src/HIE/Bios/Types.hs
index 8f699f1f9..22891bc09 100644
--- a/src/HIE/Bios/Types.hs
+++ b/src/HIE/Bios/Types.hs
@@ -53,10 +53,13 @@ data ActionName a
   deriving (Show, Eq, Ord, Functor)
 
 data CradleAction a = CradleAction {
-                      actionName :: ActionName a
+                        actionName :: ActionName a
                       -- ^ Name of the action.
                       , runCradle :: LoggingFunction -> FilePath -> IO (CradleLoadResult ComponentOptions)
                       -- ^ Options to compile the given file with.
+                      , getGhcPath :: IO (Maybe FilePath)
+                      -- ^ Try to find the path to the GHC that this cradle uses
+                      -- to compile stuff normally.
                       }
   deriving (Functor)
 
@@ -84,6 +87,9 @@ data CradleError = CradleError
   -- the loading error.
   }
   deriving (Show, Eq)
+-- data CradleError = CradleFileError FilePath (Maybe (Int, Int)) String
+--                  | CradleGeneralError ExitCode [String]
+--   deriving (Show)
 
 instance Exception CradleError where
 ----------------------------------------------------------------
diff --git a/tests/BiosTests.hs b/tests/BiosTests.hs
index f31505767..284a200e2 100644
--- a/tests/BiosTests.hs
+++ b/tests/BiosTests.hs
@@ -4,7 +4,6 @@
 module Main where
 
 import Test.Tasty
-import Test.Tasty.ExpectedFailure
 import Test.Tasty.HUnit
 import Test.Hspec.Expectations
 import HIE.Bios
@@ -125,7 +124,7 @@ testLoadFile :: Cradle a -> FilePath -> (String -> IO ()) -> IO ()
 testLoadFile crd fp step = do
   a_fp <- canonicalizePath fp
   withCurrentDirectory (cradleRootDir crd) $
-    withGHC' $ do
+    withGHC crd fp $ do
       let relFp = makeRelative (cradleRootDir crd) a_fp
       res <- initializeFlagsWithCradleWithMessage (Just (\_ n _ _ -> step (show n))) relFp crd
       case res of
@@ -143,7 +142,7 @@ testLoadFileCradleFail :: Cradle a -> FilePath -> (CradleError -> Expectation) -
 testLoadFileCradleFail crd fp cradleErrorExpectation step = do
   a_fp <- canonicalizePath fp
   withCurrentDirectory (cradleRootDir crd) $
-    withGHC' $ do
+    withGHC crd fp $ do
       let relFp = makeRelative (cradleRootDir crd) a_fp
       res <- initializeFlagsWithCradleWithMessage (Just (\_ n _ _ -> step (show n))) relFp crd
       case res of