diff --git a/packages/cultcache-py/tests/test_union_parity.py b/packages/cultcache-py/tests/test_union_parity.py
index 6afbfe6b..451cd030 100644
--- a/packages/cultcache-py/tests/test_union_parity.py
+++ b/packages/cultcache-py/tests/test_union_parity.py
@@ -285,8 +285,10 @@ class UnionCodecRuleTests(unittest.TestCase):
         )
 
     def test_unit_arm_is_written_with_empty_slots_and_reads_back(self) -> None:
-        self.assertEqual(unit_outcome.encode(UnionArm("Denied")), [2, []])
-        self.assertEqual(unit_outcome.decode([2, []]), UnionArm("Denied", None))
+        # The bytes are the C# reference's (tests/vectors/union-parity/unit-arm-denied.msgpack), not spelled here.
+        reference = _vector("unit-arm-denied.msgpack")
+        self.assertEqual(msgpack.packb(unit_outcome.encode(UnionArm("Denied"))), reference)
+        self.assertEqual(unit_outcome.decode(msgpack.unpackb(reference, raw=False)), UnionArm("Denied", None))
 
     def test_unit_arm_refuses_a_value_to_carry(self) -> None:
         with self.assertRaises(ValueError) as caught:
@@ -738,8 +740,12 @@ class UnionDecodeBoundsTests(unittest.TestCase):
                 self.assertEqual(str(caught.exception), expected)
 
     def test_the_ends_of_the_int32_key_domain_are_unit_arms_that_read_and_write(self) -> None:
-        self.assertEqual(unit_outcome.decode([-(2**31), []]), UnionArm("Floor"))
-        self.assertEqual(unit_outcome.encode(UnionArm("Ceiling")), [2**31 - 1, []])
+        floor = _vector("unit-arm-floor.msgpack")
+        ceiling = _vector("unit-arm-ceiling.msgpack")
+        self.assertEqual(unit_outcome.decode(msgpack.unpackb(floor, raw=False)), UnionArm("Floor"))
+        self.assertEqual(msgpack.packb(unit_outcome.encode(UnionArm("Floor"))), floor)
+        self.assertEqual(unit_outcome.decode(msgpack.unpackb(ceiling, raw=False)), UnionArm("Ceiling"))
+        self.assertEqual(msgpack.packb(unit_outcome.encode(UnionArm("Ceiling"))), ceiling)
 
 
 if __name__ == "__main__":
diff --git a/packages/cultcache-rs/tests/union_parity.rs b/packages/cultcache-rs/tests/union_parity.rs
index 75154f79..7d245d67 100644
--- a/packages/cultcache-rs/tests/union_parity.rs
+++ b/packages/cultcache-rs/tests/union_parity.rs
@@ -100,6 +100,10 @@ enum Outcome {
 enum UnitOutcome {
     #[cultcache(key = 2)]
     Denied,
+    #[cultcache(key = -2147483648)]
+    Floor,
+    #[cultcache(key = 2147483647)]
+    Ceiling,
 }
 
 #[derive(Clone, Debug, PartialEq, CultSlots)]
@@ -250,8 +254,16 @@ fn union_surface_decodes_and_reencodes() {
 
 #[test]
 fn a_unit_arm_is_a_key_and_an_empty_slots_array() {
-    assert_eq!(rmp_serde::to_vec(&UnitOutcome::Denied).unwrap(), [0x92, 0x02, 0x90]);
-    assert_eq!(rmp_serde::from_slice::<UnitOutcome>(&[0x92, 0x02, 0x90]).unwrap(), UnitOutcome::Denied);
+    // The bytes are the C# reference's, not spelled here.
+    for (file, arm) in [
+        ("unit-arm-denied.msgpack", UnitOutcome::Denied),
+        ("unit-arm-floor.msgpack", UnitOutcome::Floor),
+        ("unit-arm-ceiling.msgpack", UnitOutcome::Ceiling),
+    ] {
+        let reference = vector(file);
+        assert_eq!(rmp_serde::to_vec(&arm).unwrap(), reference, "{file}");
+        assert_eq!(rmp_serde::from_slice::<UnitOutcome>(&reference).unwrap(), arm, "{file}");
+    }
 }
 
 #[test]
diff --git a/packages/cultcache-ts/test/union-parity.test.ts b/packages/cultcache-ts/test/union-parity.test.ts
index 15bfaeb9..46dcb42f 100644
--- a/packages/cultcache-ts/test/union-parity.test.ts
+++ b/packages/cultcache-ts/test/union-parity.test.ts
@@ -219,8 +219,13 @@ test("the union surface vector decodes to the constructed value and re-encodes b
   assert.equal(hex(writeWire(surfaceCodec.encode(golden))), hex(committed));
 });
 
-test("a unit arm is written [key, []] and its payload must be an array", () => {
-  assert.deepEqual(outcome.encode({ kind: "Denied" }), [2, []]);
+test("a unit arm is written as the C# reference writes it, and its payload must be an array", async () => {
+  // The bytes are tests/vectors/union-parity/unit-arm-*.msgpack, written by the reference, not spelled here.
+  for (const [file, kind] of [["denied", "Denied"], ["floor", "Floor"], ["ceiling", "Ceiling"]] as const) {
+    const reference = await read(`unit-arm-${file}.msgpack`);
+    assert.deepEqual(outcome.decode(readWire(reference)), { kind }, file);
+    assert.equal(hex(writeWire(outcome.encode({ kind }))), hex(reference), file);
+  }
   assert.throws(() => outcome.decode([2, null]), /Union Outcome/u);
 });