diff --git a/internal/terraform/hook.go b/internal/terraform/hook.go
index 2e457f2814..f7dbd6af04 100644
--- a/internal/terraform/hook.go
+++ b/internal/terraform/hook.go
@@ -114,9 +114,11 @@ type Hook interface {
 	// function is called.
 	Stopping()
 
-	// PostStateUpdate is called each time the state is updated. It receives
-	// a deep copy of the state, which it may therefore access freely without
-	// any need for locks to protect from concurrent writes from the caller.
+	// PostStateUpdate is called each time the state is updated. The caller must
+	// coordinate a lock for the state if necessary, such that the Hook may
+	// access it freely without any need for additional locks to protect from
+	// concurrent writes. Implementations which modify or retain the state after
+	// the call has returned must copy the state.
 	PostStateUpdate(new *states.State) (HookAction, error)
 }
 
diff --git a/internal/terraform/update_state_hook.go b/internal/terraform/update_state_hook.go
index 3aa551df50..35dc4d82cd 100644
--- a/internal/terraform/update_state_hook.go
+++ b/internal/terraform/update_state_hook.go
@@ -5,13 +5,10 @@ package terraform
 
 // updateStateHook calls the PostStateUpdate hook with the current state.
 func updateStateHook(ctx EvalContext) error {
-	// In principle we could grab the lock here just long enough to take a
-	// deep copy and then pass that to our hooks below, but we'll instead
-	// hold the hook for the duration to avoid the potential confusing
-	// situation of us racing to call PostStateUpdate concurrently with
-	// different state snapshots.
+	// PostStateUpdate requires that the state be locked and safe to read for
+	// the duration of the call.
 	stateSync := ctx.State()
-	state := stateSync.Lock().DeepCopy()
+	state := stateSync.Lock()
 	defer stateSync.Unlock()
 
 	// Call the hook