diff --git a/SVSim.BattleNode/Lifecycle/ScriptedLifecycle.cs b/SVSim.BattleNode/Lifecycle/ScriptedLifecycle.cs
index ccd84da..801d90b 100644
--- a/SVSim.BattleNode/Lifecycle/ScriptedLifecycle.cs
+++ b/SVSim.BattleNode/Lifecycle/ScriptedLifecycle.cs
@@ -1,3 +1,4 @@
+using System.Collections.Immutable;
 using SVSim.BattleNode.Bridge;
 using SVSim.BattleNode.Protocol;
 using SVSim.BattleNode.Protocol.Bodies;
@@ -59,9 +60,12 @@ public static class ScriptedLifecycle
 
     /// <summary>
     /// Initial 3-card hand idxs from <see cref="BuildDeal"/>. Each position in this array
-    /// is one card; the value is the card's deck idx.
+    /// is one card; the value is the card's deck idx. <see cref="ImmutableArray{T}"/> enforces
+    /// the "read-only constant" contract at the type level — callers cannot mutate it, even
+    /// accidentally (the prior <c>long[]</c> allowed in-place modification by anyone with the
+    /// field reference).
     /// </summary>
-    private static readonly long[] InitialHand = { 1, 2, 3 };
+    private static readonly ImmutableArray<long> InitialHand = ImmutableArray.Create<long>(1, 2, 3);
 
     /// <summary>
     /// Compute the player's hand after a mulligan. For every idx in <paramref name="swapIndices"/>
@@ -70,7 +74,7 @@ public static class ScriptedLifecycle
     /// </summary>
     public static long[] ComputeHandAfterSwap(IReadOnlyList<long> swapIndices)
     {
-        var hand = (long[])InitialHand.Clone();
+        var hand = InitialHand.ToArray();
         var nextDeckIdx = 4L;
         for (var pos = 0; pos < hand.Length; pos++)
         {