diff --git a/SVSim.BattleEngine.Tests/SVSim.BattleEngine.Tests.csproj b/SVSim.BattleEngine.Tests/SVSim.BattleEngine.Tests.csproj
index 83a312d..b8073e2 100644
--- a/SVSim.BattleEngine.Tests/SVSim.BattleEngine.Tests.csproj
+++ b/SVSim.BattleEngine.Tests/SVSim.BattleEngine.Tests.csproj
@@ -18,6 +18,7 @@
+
diff --git a/SVSim.BattleEngine.Tests/SessionEngine/SessionEngineConstructionTests.cs b/SVSim.BattleEngine.Tests/SessionEngine/SessionEngineConstructionTests.cs
new file mode 100644
index 0000000..402efb7
--- /dev/null
+++ b/SVSim.BattleEngine.Tests/SessionEngine/SessionEngineConstructionTests.cs
@@ -0,0 +1,17 @@
+using System.Linq;
+using NUnit.Framework;
+using SVSim.BattleNode.Sessions.Engine;
+
+namespace SVSim.BattleEngine.Tests.SessionEngine
+{
+ [TestFixture]
+ public class SessionEngineConstructionTests
+ {
+ [Test]
+ public void SessionBattleEngine_instantiates_and_is_not_ready_before_setup()
+ {
+ var engine = new SessionBattleEngine();
+ Assert.That(engine.IsReady, Is.False);
+ }
+ }
+}
diff --git a/SVSim.BattleNode/Sessions/Engine/EngineIngestResult.cs b/SVSim.BattleNode/Sessions/Engine/EngineIngestResult.cs
new file mode 100644
index 0000000..a275713
--- /dev/null
+++ b/SVSim.BattleNode/Sessions/Engine/EngineIngestResult.cs
@@ -0,0 +1,9 @@
+namespace SVSim.BattleNode.Sessions.Engine;
+
+/// Outcome of feeding one client frame to the engine (design ND6). A divergence/reject is a
+/// DETECTED-DESYNC EVENT surfaced to the caller — never silently absorbed. Phase-2 policy: log.
+internal sealed record EngineIngestResult(bool Accepted, bool Diverged, string? RejectReason)
+{
+ public static EngineIngestResult Ok() => new(Accepted: true, Diverged: false, RejectReason: null);
+ public static EngineIngestResult Reject(string reason) => new(Accepted: false, Diverged: true, RejectReason: reason);
+}
diff --git a/SVSim.BattleNode/Sessions/Engine/SessionBattleEngine.cs b/SVSim.BattleNode/Sessions/Engine/SessionBattleEngine.cs
new file mode 100644
index 0000000..f3dbf2c
--- /dev/null
+++ b/SVSim.BattleNode/Sessions/Engine/SessionBattleEngine.cs
@@ -0,0 +1,28 @@
+extern alias engine;
+using engine::SVSim.BattleEngine.Rng;
+using SVSim.BattleNode.Protocol;
+
+namespace SVSim.BattleNode.Sessions.Engine;
+
+/// One authoritative engine per BattleSession, seated as both players (design ND2). A faithful
+/// SHADOW: it mirrors each client's resolved play, never overrides/rejects/originates (ND1). Ingest is
+/// the engine's own NetworkBattleReceiver.ReceivedMessage (ND4); isPlayer selects the seat (F-N-2).
+internal sealed class SessionBattleEngine
+{
+ private HeadlessNetworkBattleMgr? _mgr;
+
+ /// True once Setup has built the two-seat battle.
+ public bool IsReady => _mgr is not null;
+
+ /// Construct the two-seat network battle from both decks + the master seed (design F-N-5).
+ /// / are the per-side deck orders the node
+ /// already computed (BattleSessionState.GetShuffledDeck) and handed each client.
+ public void Setup(int masterSeed,
+ IReadOnlyList seatADeck, IReadOnlyList seatBDeck)
+ => throw new NotImplementedException("Filled by Task 3 (construction probe).");
+
+ /// Ingest one client frame into the engine for the given seat.
+ /// maps the sender to the engine's player(true)/opponent(false) seat (F-N-2).
+ public EngineIngestResult Receive(MsgEnvelope env, bool isPlayerSeat)
+ => throw new NotImplementedException("Filled by Task 4 (ingest probe).");
+}
diff --git a/SVSim.BattleNode/Sessions/Engine/SessionContentsCreator.cs b/SVSim.BattleNode/Sessions/Engine/SessionContentsCreator.cs
new file mode 100644
index 0000000..900f46a
--- /dev/null
+++ b/SVSim.BattleNode/Sessions/Engine/SessionContentsCreator.cs
@@ -0,0 +1,40 @@
+extern alias engine;
+using engine::Wizard.BattleMgr;
+using engine::Wizard.Battle.Phase;
+using engine::Wizard.Battle.Recovery;
+using engine::Wizard.Battle.Replay;
+using engine::Wizard.Battle.Resource;
+using engine::Wizard.Battle.View.Vfx;
+
+namespace SVSim.BattleNode.Sessions.Engine;
+
+/// The node's production IBattleMgrContentsCreator. Mirrors the test-side
+/// HeadlessContentsCreator (HeadlessFixture.cs) but carries the per-battle master seed so the
+/// engine's RNG stream is born aligned with the seed the node handed both clients (design F-N-5).
+/// The non-RandomSeed members are the no-op recovery/replay/resource/vfx/phase creators the
+/// NetworkStandardBattleMgr ctor dereferences — the engine's own Null* implementations, same set the
+/// headless test harness uses.
+internal sealed class SessionContentsCreator : IBattleMgrContentsCreator
+{
+ public SessionContentsCreator(int masterSeed) => RandomSeed = masterSeed;
+
+ public int RandomSeed { get; }
+
+ // No-op managers: the ctor's FirstRecoverySetting/FirstReplaySetting dereference these; recovery/
+ // replay recording is irrelevant to a shadow engine, so use the engine's own null implementations.
+ public IRecoveryManager RecoveryManager { get; } = new NullRecoveryManager();
+ public IRecoveryRecordManager RecoveryRecordManager { get; } = new NullRecoveryRecordManager();
+ public IReplayRecordManager ReplayRecordManager { get; } = new NullReplayRecordManager();
+
+ public IBattleResourceMgr CreateResourceMgr() => new BattleResourceMgr();
+ public VfxMgr CreateVfxMgr() => new VfxMgr();
+ public IPhaseCreator CreatePhaseCreator(engine::BattleManagerBase battleMgr) =>
+ new SessionPhaseCreator(battleMgr);
+}
+
+/// Node analogue of the test HeadlessPhaseCreator / the engine's SingleBattlePhaseCreator
+/// (cut from the M1 copy set as an entry-point ctor): inherits PhaseCreatorBase wholesale.
+internal sealed class SessionPhaseCreator : PhaseCreatorBase
+{
+ public SessionPhaseCreator(engine::BattleManagerBase battleMgr) : base(battleMgr) { }
+}