diff --git a/effectful-core/effectful-core.cabal b/effectful-core/effectful-core.cabal
index 6dac602..3ab32d0 100644
--- a/effectful-core/effectful-core.cabal
+++ b/effectful-core/effectful-core.cabal
@@ -82,6 +82,8 @@ library
     exposed-modules: Effectful
                      Effectful.Dispatch.Dynamic
                      Effectful.Dispatch.Static
+                     Effectful.Dispatch.Static.Primitive
+                     Effectful.Dispatch.Static.Unsafe
                      Effectful.Error.Dynamic
                      Effectful.Error.Static
                      Effectful.Fail
diff --git a/effectful-core/src/Effectful/Dispatch/Dynamic.hs b/effectful-core/src/Effectful/Dispatch/Dynamic.hs
index e8fabc9..98b4d52 100644
--- a/effectful-core/src/Effectful/Dispatch/Dynamic.hs
+++ b/effectful-core/src/Effectful/Dispatch/Dynamic.hs
@@ -356,7 +356,7 @@ localUnliftIO (LocalEnv les) strategy k = case strategy of
 --
 -- @'Eff' localEs a -> 'Eff' localEs b@
 --
--- /Note:/ the computation must not run its argument in a separate thread,
+-- /Note:/ the computation must not run its argument in a different thread,
 -- attempting to do so will result in a runtime error.
 withLiftMap
   :: (HasCallStack, SuffixOf es handlerEs)
@@ -380,7 +380,7 @@ withLiftMap !_ k = unsafeEff $ \es -> do
 --
 -- @'Eff' localEs a -> 'Eff' localEs b@
 --
--- /Note:/ the computation must not run its argument in a separate thread,
+-- /Note:/ the computation must not run its argument in a different thread,
 -- attempting to do so will result in a runtime error.
 --
 -- Useful e.g. for lifting the unmasking function in
diff --git a/effectful-core/src/Effectful/Dispatch/Static.hs b/effectful-core/src/Effectful/Dispatch/Static.hs
index 024b2da..b61d298 100644
--- a/effectful-core/src/Effectful/Dispatch/Static.hs
+++ b/effectful-core/src/Effectful/Dispatch/Static.hs
@@ -24,41 +24,18 @@ module Effectful.Dispatch.Static
     -- ** Unlifts
   , seqUnliftIO
   , concUnliftIO
+  , unsafeSeqUnliftIO
+  , unsafeConcUnliftIO
 
     -- ** Utils
   , unEff
   , unsafeEff
   , unsafeEff_
   , unsafeLiftMapIO
-  , unsafeUnliftIO
-
-    -- * Primitive API
-  , Env
-  , Relinker(..)
-  , dummyRelinker
-
-    -- ** Representation of effects
-  , EffectRep
-
-    -- ** Operations
-  , emptyEnv
-  , cloneEnv
-  , forkEnv
-  , sizeEnv
-  , checkSizeEnv
-
-    -- ** Extending and shrinking
-  , unsafeConsEnv
-  , unsafeTailEnv
-
-    -- ** Data retrieval and update
-  , getEnv
-  , putEnv
-  , stateEnv
-  , modifyEnv
   ) where
 
-import Effectful.Internal.Env
+import GHC.Stack (HasCallStack)
+
 import Effectful.Internal.Monad
 
 -- $intro
@@ -180,27 +157,40 @@ import Effectful.Internal.Monad
 --
 -- @'Eff' es a -> 'Eff' es b@
 --
--- This function is __unsafe__ because:
+-- /Note:/ the computation must not run its argument in a separate thread,
+-- attempting to do so will result in a runtime error.
 --
--- - It can be used to introduce arbitrary 'IO' actions into pure 'Eff'
---   computations.
---
--- - The 'IO' computation must not run its argument in a separate thread, but
---   it's not checked anywhere.
-unsafeLiftMapIO :: (IO a -> IO b) -> Eff es a -> Eff es b
-unsafeLiftMapIO f m = unsafeEff $ \es -> f (unEff m es)
+-- This function is __unsafe__ because it can be used to introduce arbitrary
+-- 'IO' actions into pure 'Eff' computations.
+unsafeLiftMapIO :: HasCallStack => (IO a -> IO b) -> Eff es a -> Eff es b
+unsafeLiftMapIO f m = unsafeEff $ \es -> do
+  seqUnliftIO es $ \unlift -> f (unlift m)
 
--- | Utility for running 'Eff' computations locally in the 'IO' monad.
+-- | Create an unlifting function with the 'SeqUnlift' strategy.
 --
--- This function is __unsafe__ because:
+-- This function is __unsafe__ because it can be used to introduce arbitrary
+-- 'IO' actions into pure 'Eff' computations.
+unsafeSeqUnliftIO
+  :: HasCallStack
+  => ((forall r. Eff es r -> IO r) -> IO a)
+  -- ^ Continuation with the unlifting function in scope.
+  -> Eff es a
+unsafeSeqUnliftIO k = unsafeEff $ \es -> do
+  seqUnliftIO es k
+
+-- | Create an unlifting function with the 'ConcUnlift' strategy.
 --
--- - It can be used to introduce arbitrary 'IO' actions into pure 'Eff'
---   computations.
---
--- - Unlifted 'Eff' computations must not be run in a thread distinct from the
---   caller of 'unsafeUnliftIO', but it's not checked anywhere.
-unsafeUnliftIO :: ((forall r. Eff es r -> IO r) -> IO a) -> Eff es a
-unsafeUnliftIO k = unsafeEff $ \es -> k (`unEff` es)
+-- This function is __unsafe__ because it can be used to introduce arbitrary
+-- 'IO' actions into pure 'Eff' computations.
+unsafeConcUnliftIO
+  :: HasCallStack
+  => Persistence
+  -> Limit
+  -> ((forall r. Eff es r -> IO r) -> IO a)
+  -- ^ Continuation with the unlifting function in scope.
+  -> Eff es a
+unsafeConcUnliftIO persistence limit k = unsafeEff $ \es -> do
+  concUnliftIO es persistence limit k
 
 -- $setup
 -- >>> import Effectful
diff --git a/effectful-core/src/Effectful/Dispatch/Static/Primitive.hs b/effectful-core/src/Effectful/Dispatch/Static/Primitive.hs
new file mode 100644
index 0000000..e8d3302
--- /dev/null
+++ b/effectful-core/src/Effectful/Dispatch/Static/Primitive.hs
@@ -0,0 +1,36 @@
+-- | Primitive API for statically dispatched effects.
+--
+-- /Warning:/ this module exposes internal implementation details of the 'Eff'
+-- monad. Most of the time functions from "Effectful.Dispatch.Static" are
+-- sufficient and safer to use.
+module Effectful.Dispatch.Static.Primitive
+  ( -- * The environment
+    Env
+
+    -- ** Relinker
+  , Relinker(..)
+  , dummyRelinker
+
+    -- ** Representation of effects
+  , EffectRep
+
+    -- ** Extending and shrinking
+  , unsafeConsEnv
+  , unsafeTailEnv
+
+    -- ** Data retrieval and update
+  , getEnv
+  , putEnv
+  , stateEnv
+  , modifyEnv
+
+    -- ** Utils
+  , emptyEnv
+  , cloneEnv
+  , forkEnv
+  , sizeEnv
+  , checkSizeEnv
+  ) where
+
+import Effectful.Internal.Env
+import Effectful.Internal.Monad
diff --git a/effectful-core/src/Effectful/Dispatch/Static/Unsafe.hs b/effectful-core/src/Effectful/Dispatch/Static/Unsafe.hs
new file mode 100644
index 0000000..78512c2
--- /dev/null
+++ b/effectful-core/src/Effectful/Dispatch/Static/Unsafe.hs
@@ -0,0 +1,40 @@
+module Effectful.Dispatch.Static.Unsafe
+  ( reallyUnsafeLiftMapIO
+  , reallyUnsafeUnliftIO
+  ) where
+
+import Effectful.Internal.Monad
+
+-- | Utility for lifting 'IO' computations of type
+--
+-- @'IO' a -> 'IO' b@
+--
+-- to
+--
+-- @'Eff' es a -> 'Eff' es b@
+--
+-- This function is __highly unsafe__ because:
+--
+-- - It can be used to introduce arbitrary 'IO' actions into pure 'Eff'
+--   computations.
+--
+-- - The 'IO' computation must not run its argument in a different thread, but
+--   it's not checked anywhere.
+--
+-- __If you disregard the second point, segmentation faults await.__
+reallyUnsafeLiftMapIO :: (IO a -> IO b) -> Eff es a -> Eff es b
+reallyUnsafeLiftMapIO f m = unsafeEff $ \es -> f (unEff m es)
+
+-- | Create an unlifting function.
+--
+-- This function is __highly unsafe__ because:
+--
+-- - It can be used to introduce arbitrary 'IO' actions into pure 'Eff'
+--   computations.
+--
+-- - Unlifted 'Eff' computations must not be run in a thread distinct from the
+--   caller of 'reallyUnsafeUnliftIO', but it's not checked anywhere.
+--
+-- __If you disregard the second point, segmentation faults await.__
+reallyUnsafeUnliftIO :: ((forall r. Eff es r -> IO r) -> IO a) -> Eff es a
+reallyUnsafeUnliftIO k = unsafeEff $ \es -> k (`unEff` es)
diff --git a/effectful-core/src/Effectful/Error/Static.hs b/effectful-core/src/Effectful/Error/Static.hs
index 2d3f70c..0b70ab2 100644
--- a/effectful-core/src/Effectful/Error/Static.hs
+++ b/effectful-core/src/Effectful/Error/Static.hs
@@ -102,6 +102,7 @@ import GHC.Stack
 
 import Effectful
 import Effectful.Dispatch.Static
+import Effectful.Dispatch.Static.Primitive
 import Effectful.Internal.Utils
 
 -- | Provide the ability to handle errors of type @e@.
diff --git a/effectful-core/src/Effectful/Internal/Env.hs b/effectful-core/src/Effectful/Internal/Env.hs
index f994967..6fb888e 100644
--- a/effectful-core/src/Effectful/Internal/Env.hs
+++ b/effectful-core/src/Effectful/Internal/Env.hs
@@ -29,7 +29,7 @@ module Effectful.Internal.Env
   , checkSizeEnv
 
     -- ** Extending and shrinking
-  , veryUnsafeConsEnv
+  , reallyUnsafeConsEnv
   , unsafeTailEnv
 
     -- ** Data retrieval and update
@@ -52,12 +52,17 @@ import Effectful.Internal.Utils
 
 type role Env nominal
 
--- | A strict (WHNF), thread local, mutable, extensible record indexed by types
+-- | A strict (WHNF), __thread local__, mutable, extensible record indexed by types
 -- of kind 'Effect'.
 --
 -- Supports forking, i.e. introduction of local branches for encapsulation of
 -- data specific to effect handlers.
 --
+-- __Warning: the environment is a mutable data structure and cannot be simultaneously used from multiple threads under any circumstances.__
+--
+-- In order to pass it to a different thread, you need to perform a deep copy
+-- with the 'cloneEnv' funtion.
+--
 -- Offers very good performance characteristics:
 --
 -- - Extending: /O(1)/ (amortized).
@@ -68,6 +73,7 @@ type role Env nominal
 --
 -- - Modification of a specific element: /O(1)/.
 --
+--
 -- Here's an example of how the environment might look:
 --
 -- @
@@ -182,6 +188,8 @@ emptyEnv :: IO (Env '[])
 emptyEnv = Env NoFork <$> emptyEnvRef <*> newForkIdGen
 
 -- | Clone the environment.
+--
+-- Mostly used to pass the environment to a different thread.
 cloneEnv :: Env es -> IO (Env es)
 cloneEnv (Env NoFork gref0 gen0) = do
   EnvRef n es0 fs0 <- readIORef gref0
@@ -361,10 +369,12 @@ checkSizeEnv k (Env (Forks _ baseIx lref _) _ _) = do
 -- - The @rep@ type variable is unrestricted, so it's possible to put in a
 --   different data type than the one retrieved later.
 --
--- - It renders the input 'Env' unusable until the corresponding 'unsafeTailEnv'
---   call is made, but it's not checked anywhere.
-veryUnsafeConsEnv :: rep e -> Relinker rep e -> Env es -> IO (Env (e : es))
-veryUnsafeConsEnv e f (Env fork gref gen) = case fork of
+-- - It renders the input 'Env' __unusable__ until the corresponding
+--   'unsafeTailEnv' call is made, but it's not checked anywhere.
+--
+-- __If you disregard the above, segmentation faults await.__
+reallyUnsafeConsEnv :: rep e -> Relinker rep e -> Env es -> IO (Env (e : es))
+reallyUnsafeConsEnv e f (Env fork gref gen) = case fork of
   NoFork -> do
     extendEnvRef gref
     pure $ Env NoFork gref gen
@@ -394,13 +404,15 @@ veryUnsafeConsEnv e f (Env fork gref gen) = case fork of
 
     doubleCapacity :: Int -> Int
     doubleCapacity n = max 1 n * 2
-{-# NOINLINE veryUnsafeConsEnv #-}
+{-# NOINLINE reallyUnsafeConsEnv #-}
 
 -- | Shrink the environment by one data type (in place). Makes sure the size of
 -- the environment is as expected.
 --
 -- This function is __highly unsafe__ because it renders the input 'Env'
--- unusable, but it's not checked anywhere.
+-- __unusable__, but it's not checked anywhere.
+--
+-- __If you disregard the above, segmentation faults await.__
 unsafeTailEnv :: Int -> Env (e : es) -> IO ()
 unsafeTailEnv len (Env fork gref _) = case fork of
   NoFork                -> shrinkEnvRef len gref
diff --git a/effectful-core/src/Effectful/Internal/Monad.hs b/effectful-core/src/Effectful/Internal/Monad.hs
index 8968f07..67fc043 100644
--- a/effectful-core/src/Effectful/Internal/Monad.hs
+++ b/effectful-core/src/Effectful/Internal/Monad.hs
@@ -475,13 +475,15 @@ localStaticRep f m = unsafeEff $ \es -> do
 -- This function is __highly unsafe__ because it renders the input 'Env'
 -- unusable until the corresponding 'unsafeTailEnv' call is made, but it's not
 -- checked anywhere.
+--
+-- __If you disregard the above, segmentation faults await.__
 unsafeConsEnv
   :: EffectRep (DispatchOf e) e
   -- ^ The representation of the effect.
   -> Relinker (EffectRep (DispatchOf e)) e
   -> Env es
   -> IO (Env (e : es))
-unsafeConsEnv = veryUnsafeConsEnv
+unsafeConsEnv = reallyUnsafeConsEnv
 
 -- | Extract a specific representation of the effect from the environment.
 getEnv :: e :> es => Env es -> IO (EffectRep (DispatchOf e) e)
diff --git a/effectful-core/src/Effectful/State/Static/Shared.hs b/effectful-core/src/Effectful/State/Static/Shared.hs
index d3fc7a5..74d8584 100644
--- a/effectful-core/src/Effectful/State/Static/Shared.hs
+++ b/effectful-core/src/Effectful/State/Static/Shared.hs
@@ -45,6 +45,7 @@ import Control.Concurrent.MVar
 
 import Effectful
 import Effectful.Dispatch.Static
+import Effectful.Dispatch.Static.Primitive
 
 -- | Provide access to a strict (WHNF), shared, mutable value of type @s@.
 data State s :: Effect
diff --git a/effectful-core/src/Effectful/Writer/Static/Local.hs b/effectful-core/src/Effectful/Writer/Static/Local.hs
index f43345b..a79ca5b 100644
--- a/effectful-core/src/Effectful/Writer/Static/Local.hs
+++ b/effectful-core/src/Effectful/Writer/Static/Local.hs
@@ -31,6 +31,7 @@ import Control.Exception (onException, mask)
 
 import Effectful
 import Effectful.Dispatch.Static
+import Effectful.Dispatch.Static.Primitive
 
 -- | Provide access to a strict (WHNF), thread local, write only value of type
 -- @w@.
diff --git a/effectful-core/src/Effectful/Writer/Static/Shared.hs b/effectful-core/src/Effectful/Writer/Static/Shared.hs
index 4916fec..1a46466 100644
--- a/effectful-core/src/Effectful/Writer/Static/Shared.hs
+++ b/effectful-core/src/Effectful/Writer/Static/Shared.hs
@@ -32,6 +32,7 @@ import Control.Exception (onException, uninterruptibleMask)
 
 import Effectful
 import Effectful.Dispatch.Static
+import Effectful.Dispatch.Static.Primitive
 
 -- | Provide access to a strict (WHNF), shared, write only value of type @w@.
 data Writer w :: Effect
diff --git a/effectful/src/Effectful/Concurrent.hs b/effectful/src/Effectful/Concurrent.hs
index 6df7476..f7e182a 100644
--- a/effectful/src/Effectful/Concurrent.hs
+++ b/effectful/src/Effectful/Concurrent.hs
@@ -70,6 +70,8 @@ import qualified GHC.Conc as GHC
 import Effectful
 import Effectful.Concurrent.Effect
 import Effectful.Dispatch.Static
+import Effectful.Dispatch.Static.Primitive
+import Effectful.Dispatch.Static.Unsafe
 
 ----------------------------------------
 -- Basic concurrency operations
@@ -99,9 +101,7 @@ forkIOWithUnmask
   :: Concurrent :> es
   => ((forall a. Eff es a -> Eff es a) -> Eff es ())
   -> Eff es C.ThreadId
-forkIOWithUnmask f = unsafeEff $ \es -> do
-  esF <- cloneEnv es
-  C.forkIOWithUnmask $ \unmask -> unEff (f $ unsafeLiftMapIO unmask) esF
+forkIOWithUnmask = liftForkWithUnmask C.forkIOWithUnmask
 
 -- | Lifted 'C.killThread'.
 killThread :: Concurrent :> es => C.ThreadId -> Eff es ()
@@ -126,9 +126,7 @@ forkOnWithUnmask
   => Int
   -> ((forall a. Eff es a -> Eff es a) -> Eff es ())
   -> Eff es C.ThreadId
-forkOnWithUnmask n f = unsafeEff $ \es -> do
-  esF <- cloneEnv es
-  C.forkOnWithUnmask n $ \unmask -> unEff (f $ unsafeLiftMapIO unmask) esF
+forkOnWithUnmask n = liftForkWithUnmask (C.forkOnWithUnmask n)
 
 -- | Lifted 'C.getNumCapabilities'.
 getNumCapabilities :: Concurrent :> es => Eff es Int
@@ -192,9 +190,7 @@ forkOSWithUnmask
   :: Concurrent :> es
   => ((forall a. Eff es a -> Eff es a) -> Eff es ())
   -> Eff es C.ThreadId
-forkOSWithUnmask f = unsafeEff $ \es -> do
-  esF <- cloneEnv es
-  C.forkOSWithUnmask $ \unmask -> unEff (f $ unsafeLiftMapIO unmask) esF
+forkOSWithUnmask = liftForkWithUnmask C.forkOSWithUnmask
 
 -- | Lifted 'C.isCurrentThreadBound'.
 isCurrentThreadBound :: Concurrent :> es => Eff es Bool
@@ -218,3 +214,15 @@ runInUnboundThread k = unsafeEff $ \es -> do
 -- | Lifted 'C.mkWeakThreadId'.
 mkWeakThreadId :: Concurrent :> es => C.ThreadId -> Eff es (Weak C.ThreadId)
 mkWeakThreadId = unsafeEff_ . C.mkWeakThreadId
+
+----------------------------------------
+-- Helpers
+
+liftForkWithUnmask
+  :: (((forall c. IO c -> IO c) -> IO a) -> IO C.ThreadId)
+  -> ((forall c. Eff es c -> Eff es c) -> Eff es a)
+  -> Eff es C.ThreadId
+liftForkWithUnmask fork action = unsafeEff $ \es -> do
+  esF <- cloneEnv es
+  -- Unmask never runs its argument in a different thread.
+  fork $ \unmask -> unEff (action $ reallyUnsafeLiftMapIO unmask) esF
diff --git a/effectful/src/Effectful/Concurrent/Async.hs b/effectful/src/Effectful/Concurrent/Async.hs
index c824b62..f77ed0e 100644
--- a/effectful/src/Effectful/Concurrent/Async.hs
+++ b/effectful/src/Effectful/Concurrent/Async.hs
@@ -87,6 +87,8 @@ import qualified UnliftIO.Internals.Async as I
 import Effectful
 import Effectful.Concurrent.Effect
 import Effectful.Dispatch.Static
+import Effectful.Dispatch.Static.Primitive
+import Effectful.Dispatch.Static.Unsafe
 
 -- | Lifted 'A.async'.
 async :: Concurrent :> es => Eff es a -> Eff es (Async a)
@@ -524,7 +526,8 @@ liftAsyncWithUnmask
   -> Eff es (Async a)
 liftAsyncWithUnmask fork action = unsafeEff $ \es -> do
   esA <- cloneEnv es
-  fork $ \unmask -> unEff (action $ unsafeLiftMapIO unmask) esA
+  -- Unmask never runs its argument in a different thread.
+  fork $ \unmask -> unEff (action $ reallyUnsafeLiftMapIO unmask) esA
 
 liftWithAsync
   :: (IO a -> (Async a -> IO b) -> IO b)
@@ -543,5 +546,6 @@ liftWithAsyncWithUnmask
   -> Eff es b
 liftWithAsyncWithUnmask withA action k = unsafeEff $ \es -> do
   esA <- cloneEnv es
-  withA (\unmask -> unEff (action $ unsafeLiftMapIO unmask) esA)
+  -- Unmask never runs its argument in a different thread.
+  withA (\unmask -> unEff (action $ reallyUnsafeLiftMapIO unmask) esA)
         (\a -> unEff (k a) es)
diff --git a/effectful/src/Effectful/Concurrent/MVar.hs b/effectful/src/Effectful/Concurrent/MVar.hs
index f61a9a4..debab45 100644
--- a/effectful/src/Effectful/Concurrent/MVar.hs
+++ b/effectful/src/Effectful/Concurrent/MVar.hs
@@ -34,6 +34,8 @@ import qualified Control.Concurrent.MVar as M
 import Effectful
 import Effectful.Concurrent.Effect
 import Effectful.Dispatch.Static
+import Effectful.Dispatch.Static.Primitive
+import Effectful.Dispatch.Static.Unsafe
 
 -- | Lifted 'M.newEmptyMVar'.
 newEmptyMVar :: Concurrent :> es => Eff es (MVar a)
@@ -77,35 +79,36 @@ tryReadMVar = unsafeEff_ . M.tryReadMVar
 
 -- | Lifted 'M.withMVar'.
 withMVar :: Concurrent :> es => MVar a -> (a -> Eff es b) -> Eff es b
-withMVar var f = unsafeUnliftIO $ \unlift -> do
+withMVar var f = reallyUnsafeUnliftIO $ \unlift -> do
   M.withMVar var $ unlift . f
 
 -- | Lifted 'M.withMVarMasked'.
 withMVarMasked :: Concurrent :> es => MVar a -> (a -> Eff es b) -> Eff es b
-withMVarMasked var f = unsafeUnliftIO $ \unlift -> do
+withMVarMasked var f = reallyUnsafeUnliftIO $ \unlift -> do
   M.withMVarMasked var $ unlift . f
 
 -- | Lifted 'M.modifyMVar_'.
 modifyMVar_ :: Concurrent :> es => MVar a -> (a -> Eff es a) -> Eff es ()
-modifyMVar_ var f = unsafeUnliftIO $ \unlift -> do
+modifyMVar_ var f = reallyUnsafeUnliftIO $ \unlift -> do
   M.modifyMVar_ var $ unlift . f
 
 -- | Lifted 'M.modifyMVar'.
 modifyMVar :: Concurrent :> es => MVar a -> (a -> Eff es (a, b)) -> Eff es b
-modifyMVar var f = unsafeUnliftIO $ \unlift -> do
+modifyMVar var f = reallyUnsafeUnliftIO $ \unlift -> do
   M.modifyMVar var $ unlift . f
 
 -- | Lifted 'M.modifyMVarMasked_'.
 modifyMVarMasked_ :: Concurrent :> es => MVar a -> (a -> Eff es a) -> Eff es ()
-modifyMVarMasked_ var f = unsafeUnliftIO $ \unlift -> do
+modifyMVarMasked_ var f = reallyUnsafeUnliftIO $ \unlift -> do
   M.modifyMVarMasked_ var $ unlift . f
 
 -- | Lifted 'M.modifyMVarMasked'.
 modifyMVarMasked :: Concurrent :> es => MVar a -> (a -> Eff es (a, b)) -> Eff es b
-modifyMVarMasked var f = unsafeUnliftIO $ \unlift -> do
+modifyMVarMasked var f = reallyUnsafeUnliftIO $ \unlift -> do
   M.modifyMVarMasked var $ unlift . f
 
 -- | Lifted 'M.mkWeakMVar'.
 mkWeakMVar :: Concurrent :> es => MVar a -> Eff es () -> Eff es (Weak (MVar a))
-mkWeakMVar var f = unsafeUnliftIO $ \unlift -> do
-  M.mkWeakMVar var $ unlift f
+mkWeakMVar var f = unsafeEff $ \es -> do
+  -- The finalizer can run at any point and in any thread.
+  M.mkWeakMVar var . unEff f =<< cloneEnv es
diff --git a/effectful/src/Effectful/Concurrent/STM.hs b/effectful/src/Effectful/Concurrent/STM.hs
index c761982..c70334e 100644
--- a/effectful/src/Effectful/Concurrent/STM.hs
+++ b/effectful/src/Effectful/Concurrent/STM.hs
@@ -97,53 +97,56 @@ import GHC.Natural (Natural)
 import Effectful
 import Effectful.Concurrent.Effect
 import Effectful.Dispatch.Static
+import Effectful.Dispatch.Static.Primitive
 
--- | Lifted version of 'STM.atomically'
+-- | Lifted version of 'STM.atomically'.
 atomically :: Concurrent :> es => STM a -> Eff es a
 atomically = unsafeEff_ . STM.atomically
 
--- | Lifted version of 'STM.newTVarIO'
+-- | Lifted version of 'STM.newTVarIO'.
 newTVarIO :: Concurrent :> es => a -> Eff es (TVar a)
 newTVarIO = unsafeEff_ . STM.newTVarIO
 
--- | Lifted version of 'STM.readTVarIO'
+-- | Lifted version of 'STM.readTVarIO'.
 readTVarIO :: Concurrent :> es => TVar a -> Eff es a
 readTVarIO = unsafeEff_ . STM.readTVarIO
 
--- | Lifted version of 'STM.registerDelay'
+-- | Lifted version of 'STM.registerDelay'.
 registerDelay :: Concurrent :> es => Int -> Eff es (TVar Bool)
 registerDelay = unsafeEff_ . STM.registerDelay
 
--- | Lifted version of 'STM.mkWeakTVar'
+-- | Lifted version of 'STM.mkWeakTVar'.
 mkWeakTVar :: Concurrent :> es => TVar a -> Eff es () -> Eff es (Weak (TVar a))
-mkWeakTVar var final = unsafeUnliftIO $ \unlift -> do
-  STM.mkWeakTVar var $ unlift final
+mkWeakTVar var f = unsafeEff $ \es -> do
+  -- The finalizer can run at any point and in any thread.
+  STM.mkWeakTVar var . unEff f =<< cloneEnv es
 
--- | Lifted version of 'STM.newTMVarIO'
+-- | Lifted version of 'STM.newTMVarIO'.
 newTMVarIO :: Concurrent :> es => a -> Eff es (TMVar a)
 newTMVarIO = unsafeEff_ . STM.newTMVarIO
 
--- | Lifted version of 'STM.newEmptyTMVarIO'
+-- | Lifted version of 'STM.newEmptyTMVarIO'.
 newEmptyTMVarIO :: Concurrent :> es => Eff es (TMVar a)
 newEmptyTMVarIO = unsafeEff_ STM.newEmptyTMVarIO
 
--- | Lifted version of 'STM.mkWeakTMVar'
+-- | Lifted version of 'STM.mkWeakTMVar'.
 mkWeakTMVar :: Concurrent :> es => TMVar a -> Eff es () -> Eff es (Weak (TMVar a))
-mkWeakTMVar var final = unsafeUnliftIO $ \unlift -> do
-  STM.mkWeakTMVar var $ unlift final
+mkWeakTMVar var f = unsafeEff $ \es -> do
+  -- The finalizer can run at any point and in any thread.
+  STM.mkWeakTMVar var . unEff f =<< cloneEnv es
 
--- | Lifted version of 'STM.newTChanIO'
+-- | Lifted version of 'STM.newTChanIO'.
 newTChanIO :: Concurrent :> es => Eff es (TChan a)
 newTChanIO = unsafeEff_ STM.newTChanIO
 
--- | Lifted version of 'STM.newBroadcastTChanIO'
+-- | Lifted version of 'STM.newBroadcastTChanIO'.
 newBroadcastTChanIO :: Concurrent :> es => Eff es (TChan a)
 newBroadcastTChanIO = unsafeEff_ STM.newBroadcastTChanIO
 
--- | Lifted version of 'STM.newTQueueIO'
+-- | Lifted version of 'STM.newTQueueIO'.
 newTQueueIO :: Concurrent :> es => Eff es (TQueue a)
 newTQueueIO = unsafeEff_ STM.newTQueueIO
 
--- | Lifted version of 'STM.newTBQueueIO'
+-- | Lifted version of 'STM.newTBQueueIO'.
 newTBQueueIO :: Concurrent :> es => Natural -> Eff es (TBQueue a)
 newTBQueueIO = unsafeEff_ . STM.newTBQueueIO
diff --git a/effectful/src/Effectful/FileSystem.hs b/effectful/src/Effectful/FileSystem.hs
index 8b36638..887d653 100644
--- a/effectful/src/Effectful/FileSystem.hs
+++ b/effectful/src/Effectful/FileSystem.hs
@@ -272,7 +272,7 @@ findFileWith
   -> [FilePath]
   -> String
   -> Eff es (Maybe FilePath)
-findFileWith p dirs n = unsafeUnliftIO $ \unlift -> do
+findFileWith p dirs n = unsafeSeqUnliftIO $ \unlift -> do
   D.findFileWith (unlift . p) dirs n
 
 -- | Lifted 'D.findFilesWith'.
@@ -282,7 +282,7 @@ findFilesWith
   -> [FilePath]
   -> String
   -> Eff es [FilePath]
-findFilesWith p dirs ns = unsafeUnliftIO $ \unlift -> do
+findFilesWith p dirs ns = unsafeSeqUnliftIO $ \unlift -> do
   D.findFilesWith (unlift . p) dirs ns
 
 ----------------------------------------
diff --git a/effectful/src/Effectful/FileSystem/IO.hs b/effectful/src/Effectful/FileSystem/IO.hs
index e6c9b8a..f8a6f79 100644
--- a/effectful/src/Effectful/FileSystem/IO.hs
+++ b/effectful/src/Effectful/FileSystem/IO.hs
@@ -52,7 +52,7 @@ withFile
   -> IOMode
   -> (Handle -> Eff es a)
   -> Eff es a
-withFile fp mode inner = unsafeUnliftIO $ \unlift -> do
+withFile fp mode inner = unsafeSeqUnliftIO $ \unlift -> do
   IO.withFile fp mode $ unlift . inner
 
 -- | Lifted version of 'IO.withBinaryFile'.
@@ -62,7 +62,7 @@ withBinaryFile
   -> IOMode
   -> (Handle -> Eff es a)
   -> Eff es a
-withBinaryFile fp mode inner = unsafeUnliftIO $ \unlift -> do
+withBinaryFile fp mode inner = unsafeSeqUnliftIO $ \unlift -> do
   IO.withBinaryFile fp mode $ unlift . inner
 
 -- | Lifted version of 'IO.openFile'
diff --git a/effectful/src/Effectful/Process.hs b/effectful/src/Effectful/Process.hs
index 697912c..8cbe05a 100644
--- a/effectful/src/Effectful/Process.hs
+++ b/effectful/src/Effectful/Process.hs
@@ -133,7 +133,7 @@ withCreateProcess
   => P.CreateProcess
   -> (Maybe Handle -> Maybe Handle -> Maybe Handle -> P.ProcessHandle -> Eff es a)
   -> Eff es a
-withCreateProcess cp cb = unsafeUnliftIO $ \unlift -> do
+withCreateProcess cp cb = unsafeSeqUnliftIO $ \unlift -> do
   P.withCreateProcess cp $ \inh outh errh ph -> unlift $ cb inh outh errh ph
 
 -- | Lifted 'P.cleanupProcess'.
diff --git a/effectful/src/Effectful/Resource.hs b/effectful/src/Effectful/Resource.hs
index 722c6ca..8472e6f 100644
--- a/effectful/src/Effectful/Resource.hs
+++ b/effectful/src/Effectful/Resource.hs
@@ -29,6 +29,7 @@ import qualified Control.Monad.Trans.Resource.Internal as RI
 
 import Effectful
 import Effectful.Dispatch.Static
+import Effectful.Dispatch.Static.Primitive
 
 -- | Data tag for a resource effect.
 data Resource :: Effect
diff --git a/effectful/src/Effectful/Temporary.hs b/effectful/src/Effectful/Temporary.hs
index 2d987f8..9438c53 100644
--- a/effectful/src/Effectful/Temporary.hs
+++ b/effectful/src/Effectful/Temporary.hs
@@ -36,7 +36,7 @@ withSystemTempFile
   -> (FilePath -> Handle -> Eff es a)
   -- ^ Callback that can use the file.
   -> Eff es a
-withSystemTempFile template action = unsafeUnliftIO $ \unlift -> do
+withSystemTempFile template action = unsafeSeqUnliftIO $ \unlift -> do
   T.withSystemTempFile template $ \fp handle -> unlift $ action fp handle
 
 -- | Lifted 'T.withSystemTempDirectory'.
@@ -47,7 +47,7 @@ withSystemTempDirectory
   -> (FilePath -> Eff es a)
   -- ^ Callback that can use the directory.
   -> Eff es a
-withSystemTempDirectory template action = unsafeUnliftIO $ \unlift -> do
+withSystemTempDirectory template action = unsafeSeqUnliftIO $ \unlift -> do
   T.withSystemTempDirectory template $ \fp -> unlift $ action fp
 
 -- | Lifted 'T.withTempFile'.
@@ -60,7 +60,7 @@ withTempFile
   -> (FilePath -> Handle -> Eff es a)
   -- ^ Callback that can use the file.
   -> Eff es a
-withTempFile tmpDir template action = unsafeUnliftIO $ \unlift -> do
+withTempFile tmpDir template action = unsafeSeqUnliftIO $ \unlift -> do
   T.withTempFile tmpDir template $ \fp handle -> unlift $ action fp handle
 
 -- | Lifted 'T.withTempDirectory'.
@@ -73,5 +73,5 @@ withTempDirectory
   -> (FilePath -> Eff es a)
   -- ^ Callback that can use the directory.
   -> Eff es a
-withTempDirectory tmpDir template action = unsafeUnliftIO $ \unlift -> do
+withTempDirectory tmpDir template action = unsafeSeqUnliftIO $ \unlift -> do
   T.withTempDirectory tmpDir template $ \fp -> unlift $ action fp