diff --git a/SVSim.BattleNode/Sessions/Dispatch/BattleSessionState.cs b/SVSim.BattleNode/Sessions/Dispatch/BattleSessionState.cs
index 3a53df5..b4873b8 100644
--- a/SVSim.BattleNode/Sessions/Dispatch/BattleSessionState.cs
+++ b/SVSim.BattleNode/Sessions/Dispatch/BattleSessionState.cs
@@ -1,11 +1,31 @@
+using SVSim.BattleNode.Sessions;
+
 namespace SVSim.BattleNode.Sessions.Dispatch;
 
-/// <summary>Mutable per-session state shared across frame handlers. Today: the session-level
-/// phase (only ever advanced to <see cref="BattleSessionPhase.Terminal"/>) and the mulligan
-/// barrier's post-swap hands. FUTURE (PvP equivalency, NOT this refactor): per-side idx->cardId
-/// maps + reveal-gating state land here.</summary>
+/// <summary>Mutable per-session state shared across frame handlers. The mulligan barrier's
+/// post-swap hands, plus (PvP-equivalency, vanilla slice) the per-side idx->cardId map used to
+/// synthesize the opponent-facing <c>knownList</c>. FUTURE: a token map (cardIds mined from
+/// orderList <c>add</c> ops, idx>30) + a reveal-gate set land alongside <see cref="IdxToCardId"/>.</summary>
 internal sealed class BattleSessionState
 {
     public BattleSessionPhase SessionPhase { get; set; } = BattleSessionPhase.AwaitingInitNetwork;
     public Dictionary<IBattleParticipant, long[]> PostSwapHands { get; } = new();
+
+    /// <summary>Per-side idx->cardId, seeded lazily from <see cref="MatchContext.SelfDeckCardIds"/>.
+    /// Deck cards only (idx 1..deckCount); tokens (idx>deckCount) are deferred.</summary>
+    public Dictionary<IBattleParticipant, Dictionary<int, long>> IdxToCardId { get; } = new();
+
+    /// <summary>The sender's idx->cardId map, seeding it from its <see cref="MatchContext"/> on first
+    /// use. <c>BuildPlayerDeck</c> assigns deck idx = position+1, so entry (i+1) -> cardIds[i].</summary>
+    public IReadOnlyDictionary<int, long> GetOrSeedDeckMap(IBattleParticipant side)
+    {
+        if (!IdxToCardId.TryGetValue(side, out var map))
+        {
+            map = new Dictionary<int, long>();
+            var deck = side.Context.SelfDeckCardIds;
+            for (var i = 0; i < deck.Count; i++) map[i + 1] = deck[i];
+            IdxToCardId[side] = map;
+        }
+        return map;
+    }
 }
diff --git a/SVSim.UnitTests/BattleNode/Sessions/BattleSessionStateTests.cs b/SVSim.UnitTests/BattleNode/Sessions/BattleSessionStateTests.cs
new file mode 100644
index 0000000..873984a
--- /dev/null
+++ b/SVSim.UnitTests/BattleNode/Sessions/BattleSessionStateTests.cs
@@ -0,0 +1,50 @@
+using NUnit.Framework;
+using SVSim.BattleNode.Bridge;
+using SVSim.BattleNode.Sessions;
+using SVSim.BattleNode.Sessions.Dispatch;
+
+namespace SVSim.UnitTests.BattleNode.Sessions;
+
+[TestFixture]
+public class BattleSessionStateTests
+{
+    private sealed class StubParticipant : IBattleParticipant
+    {
+        public long ViewerId { get; }
+        public MatchContext Context { get; }
+        public event Func<SVSim.BattleNode.Protocol.MsgEnvelope, CancellationToken, Task>? FrameEmitted;
+        public StubParticipant(long id, MatchContext ctx) { ViewerId = id; Context = ctx; }
+        public Task PushAsync(SVSim.BattleNode.Protocol.MsgEnvelope e, bool n, CancellationToken c) => Task.CompletedTask;
+        public Task RunAsync(CancellationToken c) => Task.CompletedTask;
+        public Task TerminateAsync(BattleFinishReason r) => Task.CompletedTask;
+        public ValueTask DisposeAsync() => ValueTask.CompletedTask;
+        private void Touch() => FrameEmitted?.Invoke(null!, default);
+    }
+
+    private static MatchContext Ctx(params long[] deck) => new(
+        SelfDeckCardIds: deck, ClassId: "1", CharaId: "1", CardMasterName: "cm",
+        CountryCode: "KOR", UserName: "P", SleeveId: "0", EmblemId: "0", DegreeId: "0",
+        FieldId: 0, IsOfficial: 0, BattleType: 11);
+
+    [Test]
+    public void GetOrSeedDeckMap_maps_idx_1based_to_deck_cardIds()
+    {
+        var state = new BattleSessionState();
+        var p = new StubParticipant(1, Ctx(900L, 901L, 902L));
+
+        var map = state.GetOrSeedDeckMap(p);
+
+        Assert.That(map[1], Is.EqualTo(900L));
+        Assert.That(map[2], Is.EqualTo(901L));
+        Assert.That(map[3], Is.EqualTo(902L));
+        Assert.That(map.ContainsKey(4), Is.False);
+    }
+
+    [Test]
+    public void GetOrSeedDeckMap_is_idempotent_same_instance()
+    {
+        var state = new BattleSessionState();
+        var p = new StubParticipant(1, Ctx(900L));
+        Assert.That(state.GetOrSeedDeckMap(p), Is.SameAs(state.GetOrSeedDeckMap(p)));
+    }
+}