diff --git a/crates/roc_std/src/lib.rs b/crates/roc_std/src/lib.rs
index 47108e9c41..232b224baf 100644
--- a/crates/roc_std/src/lib.rs
+++ b/crates/roc_std/src/lib.rs
@@ -19,9 +19,9 @@ mod storage;
 
 pub use roc_box::RocBox;
 pub use roc_dict::RocDict;
-pub use roc_list::RocList;
+pub use roc_list::{RocList, SendSafeRocList};
 pub use roc_set::RocSet;
-pub use roc_str::{InteriorNulError, RocStr};
+pub use roc_str::{InteriorNulError, RocStr, SendSafeRocStr};
 pub use storage::Storage;
 
 // A list of C functions that are being imported
diff --git a/crates/roc_std/src/roc_list.rs b/crates/roc_std/src/roc_list.rs
index 050bdfe1bf..64e7280144 100644
--- a/crates/roc_std/src/roc_list.rs
+++ b/crates/roc_std/src/roc_list.rs
@@ -121,6 +121,15 @@ impl<T> RocList<T> {
         }
     }
 
+    // Marks a list as readonly. This means that it will be leaked.
+    // For constants passed in from platform to application, this may be reasonable.
+    // Marked unsafe because it should not be used lightly.
+    pub unsafe fn set_readonly(&self) {
+        if let Some((_, storage)) = self.elements_and_storage() {
+            storage.set(Storage::Readonly);
+        }
+    }
+
     /// Note that there is no way to convert directly to a Vec.
     ///
     /// This is because RocList values are not allocated using the system allocator, so
diff --git a/crates/roc_std/src/roc_str.rs b/crates/roc_std/src/roc_str.rs
index e8ce58771a..837ccb948e 100644
--- a/crates/roc_std/src/roc_str.rs
+++ b/crates/roc_std/src/roc_str.rs
@@ -123,6 +123,16 @@ impl RocStr {
         }
     }
 
+    // Marks a str as readonly. This means that it will be leaked.
+    // For constants passed in from platform to application, this may be reasonable.
+    // Marked unsafe because it should not be used lightly.
+    pub unsafe fn set_readonly(&self) {
+        match self.as_enum_ref() {
+            RocStrInnerRef::HeapAllocated(roc_list) => unsafe { roc_list.set_readonly() },
+            RocStrInnerRef::SmallString(_) => {}
+        }
+    }
+
     /// Note that there is no way to convert directly to a String.
     ///
     /// This is because RocStr values are not allocated using the system allocator, so
diff --git a/crates/roc_std/tests/test_roc_std.rs b/crates/roc_std/tests/test_roc_std.rs
index d81b93d896..4de4e543fe 100644
--- a/crates/roc_std/tests/test_roc_std.rs
+++ b/crates/roc_std/tests/test_roc_std.rs
@@ -59,7 +59,7 @@ pub unsafe extern "C" fn roc_memset(dst: *mut c_void, c: i32, n: usize) -> *mut
 
 #[cfg(test)]
 mod test_roc_std {
-    use roc_std::{RocBox, RocDec, RocList, RocResult, RocStr};
+    use roc_std::{RocBox, RocDec, RocList, RocResult, RocStr, SendSafeRocList, SendSafeRocStr};
 
     fn roc_str_byte_representation(string: &RocStr) -> [u8; RocStr::SIZE] {
         unsafe { core::mem::transmute_copy(string) }
@@ -126,7 +126,7 @@ mod test_roc_std {
 
         roc_str.reserve(42);
 
-        assert_gte!(roc_str.capacity(), 42);
+        assert_eq!(roc_str.capacity() >= 42, true);
     }
 
     #[test]
@@ -135,7 +135,7 @@ mod test_roc_std {
 
         roc_str.reserve(5000);
 
-        assert_gte!(roc_str.capacity(), 5000);
+        assert_eq!(roc_str.capacity() >= 5000, true);
     }
 
     #[test]
@@ -296,6 +296,80 @@ mod test_roc_std {
         let example = RocDec::from_str("1234.5678").unwrap();
         assert_eq!(format!("{}", example), "1234.5678");
     }
+
+    #[test]
+    fn safe_send_no_copy() {
+        let x = RocStr::from("This is a long string but still unique. Yay!!!");
+        assert_eq!(x.is_unique(), true);
+
+        let safe_x = SendSafeRocStr::from(x);
+        let new_x = RocStr::from(safe_x);
+        assert_eq!(new_x.is_unique(), true);
+        assert_eq!(
+            new_x.as_str(),
+            "This is a long string but still unique. Yay!!!"
+        );
+    }
+
+    #[test]
+    fn safe_send_requires_copy() {
+        let x = RocStr::from("This is a long string but still unique. Yay!!!");
+        let y = x.clone();
+        let z = y.clone();
+        assert_eq!(x.is_unique(), false);
+        assert_eq!(y.is_unique(), false);
+        assert_eq!(z.is_unique(), false);
+
+        let safe_x = SendSafeRocStr::from(x);
+        let new_x = RocStr::from(safe_x);
+        assert_eq!(new_x.is_unique(), true);
+        assert_eq!(y.is_unique(), false);
+        assert_eq!(z.is_unique(), false);
+        assert_eq!(
+            new_x.as_str(),
+            "This is a long string but still unique. Yay!!!"
+        );
+    }
+
+    #[test]
+    fn safe_send_small_str() {
+        let x = RocStr::from("short");
+        let y = x.clone();
+        let z = y.clone();
+        assert_eq!(x.is_unique(), true);
+        assert_eq!(y.is_unique(), true);
+        assert_eq!(z.is_unique(), true);
+
+        let safe_x = SendSafeRocStr::from(x);
+        let new_x = RocStr::from(safe_x);
+        assert_eq!(new_x.is_unique(), true);
+        assert_eq!(y.is_unique(), true);
+        assert_eq!(z.is_unique(), true);
+        assert_eq!(new_x.as_str(), "short");
+    }
+
+    #[test]
+    fn empty_list_is_unique() {
+        let roc_list = RocList::<RocStr>::empty();
+        assert_eq!(roc_list.is_unique(), true);
+    }
+
+    #[test]
+    fn readonly_list_is_sendsafe() {
+        let x = RocList::from_slice(&[1, 2, 3, 4, 5]);
+        unsafe { x.set_readonly() };
+        assert_eq!(x.is_readonly(), true);
+
+        let y = x.clone();
+        let z = y.clone();
+
+        let safe_x = SendSafeRocList::from(x);
+        let new_x = RocList::from(safe_x);
+        assert_eq!(new_x.is_readonly(), true);
+        assert_eq!(y.is_readonly(), true);
+        assert_eq!(z.is_readonly(), true);
+        assert_eq!(new_x.as_slice(), &[1, 2, 3, 4, 5]);
+    }
 }
 
 #[cfg(test)]