diff --git a/SVSim.BattleEngine.Tests/HeadlessFixture.cs b/SVSim.BattleEngine.Tests/HeadlessFixture.cs
index cda2131..1309baa 100644
--- a/SVSim.BattleEngine.Tests/HeadlessFixture.cs
+++ b/SVSim.BattleEngine.Tests/HeadlessFixture.cs
@@ -221,14 +221,13 @@ namespace SVSim.BattleEngine.Tests
                 .SetValue(null, "headless-udid");
         }
 
-        // Simple deterministic 40-card deck seed for multi-instance tests. The seed is unused for now
-        // (every slot is the same vanilla FollowerId), but takes a seed parameter so callers can vary
-        // it without API churn if/when we want a heterogeneous deck. Card 100011010 is loaded as part
-        // of EnsureProcessGlobals' HeadlessCardMaster.Load batch so SessionBattleEngine.Setup resolves
-        // each entry without re-loading.
-        public static long[] SampleDeck(int seed)
+        // Simple deterministic 40-card deck for multi-instance tests: every slot is the same vanilla
+        // FollowerId. Card 100011010 is loaded as part of EnsureProcessGlobals' HeadlessCardMaster.Load
+        // batch so SessionBattleEngine.Setup resolves each entry without re-loading. Kept a single
+        // shape — the multi-instance property being verified (per-session ambient isolation across
+        // parallel battles) is driven by distinct masterSeeds on the engines, not by deck variation.
+        public static long[] SampleDeck()
         {
-            var rng = new System.Random(seed);
             var deck = new long[40];
             for (int i = 0; i < 40; i++) deck[i] = FollowerId;
             return deck;
diff --git a/SVSim.BattleEngine.Tests/MultiInstanceEngineTests.cs b/SVSim.BattleEngine.Tests/MultiInstanceEngineTests.cs
index a0fbb80..eb78bec 100644
--- a/SVSim.BattleEngine.Tests/MultiInstanceEngineTests.cs
+++ b/SVSim.BattleEngine.Tests/MultiInstanceEngineTests.cs
@@ -23,34 +23,38 @@ public class MultiInstanceEngineTests
     [Test]
     public async Task TwoBattles_ResolveIndependently_OnDifferentTasks()
     {
-        var deckA1 = HeadlessEngineEnv.SampleDeck(seed: 1);
-        var deckA2 = HeadlessEngineEnv.SampleDeck(seed: 2);
-        var deckB1 = HeadlessEngineEnv.SampleDeck(seed: 3);
-        var deckB2 = HeadlessEngineEnv.SampleDeck(seed: 4);
-
         var engineA = new SessionBattleEngine();
         var engineB = new SessionBattleEngine();
-        engineA.Setup(masterSeed: 111, deckA1, deckA2, seatAClass: 1, seatBClass: 2);
-        engineB.Setup(masterSeed: 222, deckB1, deckB2, seatAClass: 5, seatBClass: 7);
+        engineA.Setup(masterSeed: 111, HeadlessEngineEnv.SampleDeck(), HeadlessEngineEnv.SampleDeck(),
+            seatAClass: 1, seatBClass: 2);
+        engineB.Setup(masterSeed: 222, HeadlessEngineEnv.SampleDeck(), HeadlessEngineEnv.SampleDeck(),
+            seatAClass: 5, seatBClass: 7);
 
         var taskA = Task.Run(() => DriveBasicTurns(engineA));
         var taskB = Task.Run(() => DriveBasicTurns(engineB));
         await Task.WhenAll(taskA, taskB);
 
+        // Pin the engines' post-Setup state to concrete starting values: LeaderLife=20 (InitLeaderLife's
+        // DefaultLeaderLife, applied by SessionBattleEngine.Setup), Pp=0 (pre-first-turn, no PP refill
+        // has run), HandCount=0 (Setup builds the deck/leader graph but doesn't deal an opening hand —
+        // mulligan/draw happens once a turn-start phase runs, which DriveBasicTurns doesn't trigger).
+        // Both engines must report the SAME starting state regardless of distinct masterSeeds, which is
+        // the cross-contamination property under test: ambient isolation means neither engine's reads
+        // can leak into the other's seat lookups.
         Assert.That(engineA.LeaderLife(true), Is.EqualTo(20));
         Assert.That(engineB.LeaderLife(true), Is.EqualTo(20));
-        Assert.That(engineA.Pp(true), Is.GreaterThanOrEqualTo(0));
-        Assert.That(engineB.Pp(true), Is.GreaterThanOrEqualTo(0));
+        Assert.That(engineA.Pp(true), Is.EqualTo(0));
+        Assert.That(engineB.Pp(true), Is.EqualTo(0));
+        Assert.That(engineA.HandCount(true), Is.EqualTo(0));
+        Assert.That(engineB.HandCount(true), Is.EqualTo(0));
     }
 
     [Test]
-    public async Task StressN_RandomDecks_BaselineMatches([Values(4, 8, 16)] int n)
+    public async Task StressN_BaselineMatches([Values(4, 8, 16)] int n)
     {
         var inputs = new (int seed, long[] deckA, long[] deckB)[n];
         for (int i = 0; i < n; i++)
-            inputs[i] = (1000 + i,
-                HeadlessEngineEnv.SampleDeck(seed: 100 + i * 2),
-                HeadlessEngineEnv.SampleDeck(seed: 101 + i * 2));
+            inputs[i] = (1000 + i, HeadlessEngineEnv.SampleDeck(), HeadlessEngineEnv.SampleDeck());
 
         // Setup is process-globally serialized: a small set of decomp-origin static accumulators
         // (Wizard.LocalLog._lastTraceLogStringBuilder, etc.) is touched during BattleManagerBase ctor.