diff --git a/SVSim.UnitTests/BattleNode/Integration/HeadlessConductorTests.cs b/SVSim.UnitTests/BattleNode/Integration/HeadlessConductorTests.cs
index 12794fb..d0342b2 100644
--- a/SVSim.UnitTests/BattleNode/Integration/HeadlessConductorTests.cs
+++ b/SVSim.UnitTests/BattleNode/Integration/HeadlessConductorTests.cs
@@ -12,6 +12,7 @@ namespace SVSim.UnitTests.BattleNode.Integration;
 /// SVSim.UnitTests process.
 /// </summary>
 [TestFixture]
+[NonParallelizable]
 public class HeadlessConductorTests
 {
     [Test]
diff --git a/SVSim.UnitTests/BattleNode/Integration/NodeNativeBattleHarness.cs b/SVSim.UnitTests/BattleNode/Integration/NodeNativeBattleHarness.cs
index d743385..3811415 100644
--- a/SVSim.UnitTests/BattleNode/Integration/NodeNativeBattleHarness.cs
+++ b/SVSim.UnitTests/BattleNode/Integration/NodeNativeBattleHarness.cs
@@ -39,14 +39,20 @@ internal sealed class NodeNativeBattleHarness : IDisposable
     public const long DefaultSeatAViewerId = 1001;
     public const long DefaultSeatBViewerId = 1002;
 
-    /// <summary>Spellboost cost-reducer card (looking ahead to M-HC-3). Present in cards.json.</summary>
+    /// <summary>Spellboost cost-reducer card (looking ahead to M-HC-3). Known id present in cards.json
+    /// (sourced from tk2 battle capture / existing engine tests); a cards.json regeneration that drops
+    /// it will produce a traceable failure here.</summary>
     public const long SpellboostCardId = 101314020;
 
-    /// <summary>A second spellboost card seen in the tk2 capture; present in cards.json.</summary>
+    /// <summary>A second spellboost card seen in the tk2 capture. Known id present in cards.json
+    /// (sourced from tk2 battle capture / existing engine tests); a cards.json regeneration that drops
+    /// it will produce a traceable failure here.</summary>
     public const long SpellboostCardIdAlt = 100314020;
 
     /// <summary>A plain vanilla follower the engine resolution path proved out
-    /// (HeadlessFixture.FollowerId). The bulk of the deterministic deck.</summary>
+    /// (HeadlessFixture.FollowerId). The bulk of the deterministic deck. Known id present in cards.json
+    /// (sourced from tk2 battle capture / existing engine tests); a cards.json regeneration that drops
+    /// it will produce a traceable failure here.</summary>
     public const long VanillaFollowerId = 100011010;
 
     public BattleSessionState State { get; }
@@ -135,8 +141,10 @@ internal sealed class NodeNativeBattleHarness : IDisposable
     public void Dispose() { /* engine holds no unmanaged resources; nothing to release. */ }
 
     /// <summary>Minimal test-only <see cref="IBattleParticipant"/> exposing only the
-    /// <see cref="ViewerId"/> + <see cref="Context"/> that the harness reads. All broker members are
-    /// no-ops — the harness drives the engine directly, never the session relay.</summary>
+    /// <see cref="ViewerId"/> + <see cref="Context"/> that the harness reads. Broker members
+    /// (<c>PushAsync</c>, <c>RunAsync</c>, <c>TerminateAsync</c>) throw <see cref="NotSupportedException"/>
+    /// — the harness drives the engine directly, so a frame must never reach the participant relay.
+    /// Silent no-ops would let a misrouted push pass undetected.</summary>
     internal sealed class StubParticipant : IBattleParticipant
     {
         public long ViewerId { get; }
@@ -152,9 +160,12 @@ internal sealed class NodeNativeBattleHarness : IDisposable
         public event Func<MsgEnvelope, CancellationToken, Task>? FrameEmitted;
 #pragma warning restore CS0067
 
-        public Task PushAsync(MsgEnvelope envelope, Stock stock, CancellationToken ct) => Task.CompletedTask;
-        public Task RunAsync(CancellationToken ct) => Task.CompletedTask;
-        public Task TerminateAsync(BattleFinishReason reason) => Task.CompletedTask;
+        public Task PushAsync(MsgEnvelope envelope, Stock stock, CancellationToken ct) =>
+            throw new NotSupportedException("StubParticipant.PushAsync — harness drives the engine directly; a frame must not reach the participant relay.");
+        public Task RunAsync(CancellationToken ct) =>
+            throw new NotSupportedException("StubParticipant.RunAsync should not be called in harness tests.");
+        public Task TerminateAsync(BattleFinishReason reason) =>
+            throw new NotSupportedException("StubParticipant.TerminateAsync should not be called in harness tests.");
         public ValueTask DisposeAsync() => ValueTask.CompletedTask;
     }
 }