diff --git a/SVSim.BattleNode/Sessions/Engine/SessionBattleEngine.cs b/SVSim.BattleNode/Sessions/Engine/SessionBattleEngine.cs
index 814867e..947295f 100644
--- a/SVSim.BattleNode/Sessions/Engine/SessionBattleEngine.cs
+++ b/SVSim.BattleNode/Sessions/Engine/SessionBattleEngine.cs
@@ -161,6 +161,14 @@ internal sealed class SessionBattleEngine
     /// a card dealt from the seeded deck.</summary>
     public int HandCardIndex(bool playerSeat, int handPos) => Seat(playerSeat).HandCardList[handPos].Index;
 
+    /// <summary>The real <c>CardId</c> (wire identity) of the in-play follower at <paramref name="boardPos"/>
+    /// (0-based, skipping the leader/Class card at ClassAndInPlayCardList[0] — same convention as
+    /// <see cref="BoardCount"/>). Used to assert an opponent reveal seated the substituted card with its
+    /// true identity (M-HC-2): before the reveal the slot holds a hidden dummy (cardId 0); after, the
+    /// engine-resolved actual card carries the wire cardId.</summary>
+    public int InPlayCardId(bool playerSeat, int boardPos) =>
+        Seat(playerSeat).ClassAndInPlayCardList[boardPos + 1].CardId;
+
     private engine::BattlePlayerBase Seat(bool playerSeat) =>
         (_mgr ?? throw new InvalidOperationException("read before Setup")).GetBattlePlayer(playerSeat);
 
diff --git a/SVSim.UnitTests/BattleNode/Integration/HeadlessConductorTests.cs b/SVSim.UnitTests/BattleNode/Integration/HeadlessConductorTests.cs
index 28c891a..fed1845 100644
--- a/SVSim.UnitTests/BattleNode/Integration/HeadlessConductorTests.cs
+++ b/SVSim.UnitTests/BattleNode/Integration/HeadlessConductorTests.cs
@@ -92,6 +92,28 @@ public class HeadlessConductorTests
     private static Dictionary<string, object?> TurnStartBody() => new() { ["spin"] = 0 };
     private static Dictionary<string, object?> TurnEndBody() => new() { ["turnState"] = 0 };
 
+    // An opponent play that REVEALS the played card. The wire shape is taken verbatim from
+    // battle_test_cl2.ndjson's first opponent PlayActions frame:
+    //   { playIdx, type:30, knownList:[{idx, cardId, to:30, spellboost:0, attachTarget:""}] }
+    // type 30 == PLAY_HAND; knownList[].idx == the hidden dummy's engine Index; cardId == the real
+    // identity to substitute; to 30 == NetworkCardPlaceState.Field (the card lands in play).
+    private static Dictionary<string, object?> RevealPlayBody(int idx, long cardId) => new()
+    {
+        ["playIdx"] = idx,
+        ["type"] = 30,
+        ["knownList"] = new List<object?>
+        {
+            new Dictionary<string, object?>
+            {
+                ["idx"] = idx,
+                ["cardId"] = cardId,
+                ["to"] = 30,
+                ["spellboost"] = 0,
+                ["attachTarget"] = "",
+            },
+        },
+    };
+
     [Test]
     public void Swap_seats_post_mulligan_hand_headless()
     {
@@ -176,6 +198,55 @@ public class HeadlessConductorTests
         Assert.That(harness.LeaderLife(playerSeat: false), Is.EqualTo(20), "seat B leader life");
     }
 
+    [Test]
+    public void Opponent_reveal_seats_card_on_seat_B_headless()
+    {
+        // Seat B's deck idx 1 is a known vanilla follower, so the reveal's wire cardId maps to a real
+        // card the opponent can play to the board. (Seat A's deck is left at default — irrelevant here.)
+        var seatBDeck = new List<long> { NodeNativeBattleHarness.VanillaFollowerId };
+        seatBDeck.AddRange(NodeNativeBattleHarness.DefaultDeck());
+        seatBDeck = seatBDeck.GetRange(0, 30);
+
+        using var harness = NodeNativeBattleHarness.Create(seatBDeck: seatBDeck);
+
+        // --- drive to seat B's turn (reuse Task 3's two-turn sequence) ---------------------------
+        Assert.That(harness.Push(NetworkBattleUri.Deal, DealBody(), isPlayerSeat: true).Accepted,
+            Is.True, "Deal");
+        Assert.That(harness.Push(NetworkBattleUri.Swap, SwapBody(), isPlayerSeat: true).Accepted,
+            Is.True, "Swap");
+        Assert.That(harness.Push(NetworkBattleUri.Ready, ReadyBody(), isPlayerSeat: true).Accepted,
+            Is.True, "Ready");
+        Assert.That(harness.Push(NetworkBattleUri.TurnStart, TurnStartBody(), isPlayerSeat: true).Accepted,
+            Is.True, "turn1 TurnStart");
+        Assert.That(harness.Push(NetworkBattleUri.TurnEnd, TurnEndBody(), isPlayerSeat: true).Accepted,
+            Is.True, "turn1 TurnEnd");
+        Assert.That(harness.Push(NetworkBattleUri.TurnStart, TurnStartBody(), isPlayerSeat: false).Accepted,
+            Is.True, "turn2 TurnStart (seat B active)");
+
+        // Seat B's opening hand is hidden (deck reads full minus its single turn-1 draw); its cards
+        // have NOT been disclosed to the relay yet. The dummy at engine Index 1 is the deck-idx-1 card
+        // (the vanilla follower), seated in a hidden zone — NOT on the board. Confirm seat B's board is
+        // empty BEFORE the reveal, so the post-reveal +1 is decisively the reveal seating the card.
+        // (Node-native, the harness seeds each side's cards with their real id rather than cardId-0
+        // dummies — it knows both decks — so the reveal substitution is identity-preserving here; the
+        // board delta is what proves ReplaceReceivedCard.ReplaceCard -> CreateActualCard resolved the
+        // card onto the board headless. M-HC-5 exercises a true cardId-0 -> cardId substitution.)
+        var boardBefore = harness.BoardCount(playerSeat: false);
+        Assert.That(boardBefore, Is.EqualTo(0), "seat B has no board followers before the reveal");
+
+        // --- the reveal: an opponent PlayActions frame carrying a knownList that discloses idx 1 ---
+        const long revealedCardId = NodeNativeBattleHarness.VanillaFollowerId;
+        var reveal = harness.Push(
+            NetworkBattleUri.PlayActions, RevealPlayBody(idx: 1, cardId: revealedCardId),
+            isPlayerSeat: false);
+
+        Assert.That(reveal.Accepted, Is.True, $"opponent reveal rejected: {reveal.RejectReason}");
+        Assert.That(harness.BoardCount(playerSeat: false), Is.EqualTo(boardBefore + 1),
+            "the revealed follower must seat on seat B's board");
+        Assert.That(harness.InPlayCardId(playerSeat: false, boardPos: 0), Is.EqualTo((int)revealedCardId),
+            "the seated card's identity must equal the wire cardId from the reveal");
+    }
+
     [Test]
     public void Deal_seats_three_card_hand_headless()
     {
diff --git a/SVSim.UnitTests/BattleNode/Integration/NodeNativeBattleHarness.cs b/SVSim.UnitTests/BattleNode/Integration/NodeNativeBattleHarness.cs
index f9bd6d9..1e261e9 100644
--- a/SVSim.UnitTests/BattleNode/Integration/NodeNativeBattleHarness.cs
+++ b/SVSim.UnitTests/BattleNode/Integration/NodeNativeBattleHarness.cs
@@ -131,6 +131,11 @@ internal sealed class NodeNativeBattleHarness : IDisposable
     /// Play frame would carry to play it).</summary>
     public int PlayerHandCardIndex(int handPos) => Engine.HandCardIndex(playerSeat: true, handPos);
 
+    /// <summary>The real wire <c>CardId</c> of the in-play follower at <paramref name="boardPos"/> on the
+    /// given seat (0-based, leader excluded). Asserts an opponent reveal seated the substituted identity
+    /// (M-HC-2).</summary>
+    public int InPlayCardId(bool playerSeat, int boardPos) => Engine.InPlayCardId(playerSeat, boardPos);
+
     /// <summary>Build an envelope for <paramref name="body"/> and ingest it into the engine for the
     /// given seat (player == seat A). Mirrors <c>BattleNodeFlowTests.MakeEnvelopeWith</c> +
     /// <c>SessionBattleEngine.Receive</c>.</summary>