using System.Collections.Generic;
using System.Linq;
using NUnit.Framework;
using SVSim.BattleNode.Protocol;
using SVSim.BattleNode.Protocol.Bodies;
using SVSim.BattleNode.Sessions;
using SVSim.BattleNode.Sessions.Dispatch;
using SVSim.BattleNode.Sessions.Dispatch.Handlers;

namespace SVSim.UnitTests.BattleNode.Integration;

/// <summary>
/// Headless-Conductor milestone tests (M-HC-*). The oracle is a node-native battle:
/// a FIXED master seed + FIXED decks drive the engine's receive path headless, and we
/// assert on engine board-state. By construction the node assigns idx = position in the
/// shuffled order, so the engine's headless draw reproduces the node's draw order.
///
/// Task 1 (M-HC-0a) exit criterion: the engine seats headless (IsReady) in the
/// SVSim.UnitTests process.
///
/// Task 2 (M-HC-0b) exit criterion: a node-generated <c>Deal</c> seats the 3-card hand and a
/// vanilla hand-card <c>Play</c> resolves on ENGINE board state (card left hand, PP dropped
/// by cost, board reflects the play) — driven through the receive CONDUCTOR, not the
/// direct ActionProcessor path the M2-M12 oracles use.
///
/// Task 3 (M-HC-1) exit criterion: the mulligan ops (<c>Swap</c> seats the post-mulligan hand —
/// idx-3 swapped for the next unused deck idx-4) and turn ops (<c>Ready</c>/<c>TurnStart</c>/
/// <c>TurnEnd</c>) resolve headless, so two full turns of a node-native battle track on engine
/// state (hand/board/PP/deck/turn/leader-life on both seats match the deterministic progression
/// at each boundary). All driven through the same receive conductor.
///
/// Task 5 (M-HC-3a) exit criterion: the opponent-facing <c>knownList[].cost</c> carries the
/// engine-RESOLVED play-time cost (the discounted cost the engine actually charged), closing the
/// spellboost cost-desync by construction. Proven both at the engine read (PlayedCardCost off a
/// charge-seeded reducer) and the handler emit (PlayActionsHandler -> PlayActionsBroadcastBody).
/// NOTE: a BOARD-DEPENDENT cost reducer (e.g. <c>when_evolve_other</c>) is DEFERRED to M-HC-4 —
/// evolve does not yet resolve headless. Because cost is read straight off the resolved engine,
/// board modifiers are captured by construction once their ops resolve, so no separate emit-site
/// change is needed when M-HC-4 lands; only a board-dependent validation case is owed there.
/// </summary>
[TestFixture]
[NonParallelizable]
public class HeadlessConductorTests
{
    [Test]
    public void Harness_seats_engine_headless_and_is_ready()
    {
        using var harness = NodeNativeBattleHarness.Create();

        Assert.That(harness.IsReady, Is.True,
            "Engine must seat headless: EngineGlobalInit ran + both decks seeded. " +
            "If false, the most likely cause is a missing cards.json content link in " +
            "SVSim.UnitTests.csproj (EngineGlobalInit reads AppContext.BaseDirectory/Data/cards.json).");

        // Non-vacuous: a seated engine has live board state for BOTH seats. Reading these off a
        // not-really-set-up engine would throw (Seat() guards on _mgr). Leader life is the headless
        // default (20) before any frame is ingested.
        Assert.That(harness.LeaderLife(playerSeat: true), Is.EqualTo(20), "seat A leader life");
        Assert.That(harness.LeaderLife(playerSeat: false), Is.EqualTo(20), "seat B leader life");
    }

    // The node's BuildDeal opening hand: pos->idx (0,1),(1,2),(2,3). hand == deck idx 1,2,3, i.e.
    // the top 3 of the node-native shuffled deck. Both seats deal the same idx triple.
    private static Dictionary<string, object?> DealBody() => new()
    {
        ["self"] = PosIdxList((0, 1), (1, 2), (2, 3)),
        ["oppo"] = PosIdxList((0, 1), (1, 2), (2, 3)),
    };

    // A minimal vanilla hand-card play: type 30 == PLAY_HAND; playIdx is the played card's index.
    // No targetList/orderList — a vanilla follower auto-resolves with no selection.
    private static Dictionary<string, object?> PlayBody(int playIdx) => new()
    {
        ["playIdx"] = playIdx,
        ["type"] = 30,
    };

    // A pos->idx list (the wire shape NetworkParameter.self/oppo carry: an ordered list of
    // {pos, idx} dicts). The receiver re-sorts by pos into the seat's idx list.
    private static List<object?> PosIdxList(params (int pos, int idx)[] entries)
    {
        var list = new List<object?>(entries.Length);
        foreach (var (pos, idx) in entries)
            list.Add(new Dictionary<string, object?> { ["pos"] = pos, ["idx"] = idx });
        return list;
    }

    // Server-authored Swap RESPONSE frame (the shadow ingests this, NOT the client's {idxList}
    // Submit). It carries the POST-mulligan self hand as pos->idx. Swapping the pos-2 card (deck
    // idx 3) pulls the next unused deck idx (4) — exactly battle_test_cl1's Swap receive frame.
    private static Dictionary<string, object?> SwapBody() => new()
    {
        ["self"] = PosIdxList((0, 1), (1, 2), (2, 4)),
    };

    // Server-authored Ready frame: both hands known + the idxChangeSeed/spin the receiver
    // consumes to seal the mulligan and start turn 1. Mirrors battle_test_cl1's Ready receive.
    private static Dictionary<string, object?> ReadyBody() => new()
    {
        ["self"] = PosIdxList((0, 1), (1, 2), (2, 4)), // same post-mulligan self hand as SwapBody — Ready re-echoes it
        ["oppo"] = PosIdxList((0, 1), (1, 2), (2, 3)),
        ["idxChangeSeed"] = 857671914,
        ["spin"] = 0,
    };

    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()
    {
        using var harness = NodeNativeBattleHarness.Create();

        var deal = harness.Push(NetworkBattleUri.Deal, DealBody(), isPlayerSeat: true);
        Assert.That(deal.Accepted, Is.True, $"Deal rejected: {deal.RejectReason}");
        Assert.That(harness.HandCount(playerSeat: true), Is.EqualTo(3), "post-Deal hand");

        var swap = harness.Push(NetworkBattleUri.Swap, SwapBody(), isPlayerSeat: true);
        Assert.That(swap.Accepted, Is.True, $"Swap rejected: {swap.RejectReason}");
        Assert.That(harness.HandCount(playerSeat: true), Is.EqualTo(3),
            "the swapped slot is replaced, not removed — hand stays at 3");

        // The pos-2 card was the deck-idx-3 card; the swap replaces it with the deck-idx-4 card.
        // The kept cards (idx 1, 2) stay put. Assert the engine hand holds idx {1,2,4}.
        var handIdxs = new[]
        {
            harness.PlayerHandCardIndex(0),
            harness.PlayerHandCardIndex(1),
            harness.PlayerHandCardIndex(2),
        };
        Assert.That(handIdxs, Is.EquivalentTo(new[] { 1, 2, 4 }),
            "post-mulligan hand must hold deck idx 1,2,4 (idx-3 swapped for the next unused idx-4)");
    }

    [Test]
    public void Two_turns_track_on_engine_state_headless()
    {
        // The oracle is the engine's OWN deterministic node-native progression off the fixed seed:
        // every value below is the engine-resolved state, reproducible by construction. The shadow
        // ingests the same server-authored frame stream the live node emits (Deal/Swap/Ready then
        // per-turn TurnStart/TurnEnd — the exact receive frames captured in battle_test_cl1.ndjson).
        using var harness = NodeNativeBattleHarness.Create();

        // --- mulligan barrier: Deal, Swap, Ready -------------------------------------------------
        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");
        var ready = harness.Push(NetworkBattleUri.Ready, ReadyBody(), isPlayerSeat: true);
        Assert.That(ready.Accepted, Is.True, $"Ready rejected: {ready.RejectReason}");

        // After Ready the mulligan is sealed and the main phase is entered, but no turn has been
        // opened yet (TurnStart does the ramp + draw). Seat A holds its post-mulligan 3-card hand;
        // the opponent's hand stays hidden until its reveal frames land (Task 4) — node-native, the
        // opponent's opening hand is never disclosed to the relay before its own turn.
        Assert.That(harness.HandCount(playerSeat: true), Is.EqualTo(3), "seat A hand after Ready");
        Assert.That(harness.Turn(playerSeat: true), Is.EqualTo(0), "no turn opened yet after Ready");

        // --- turn 1 (seat A active) -------------------------------------------------------------
        // Seat A is the engine's player seat and is NOT game-first here, so turn-1 draws TWO cards
        // (the standard second-player turn-1 draw). PP ramps to 1.
        var t1 = harness.Push(NetworkBattleUri.TurnStart, TurnStartBody(), isPlayerSeat: true);
        Assert.That(t1.Accepted, Is.True, $"turn1 TurnStart rejected: {t1.RejectReason}");
        Assert.That(harness.Turn(playerSeat: true), Is.EqualTo(1), "seat A turn counter");
        Assert.That(harness.Pp(playerSeat: true), Is.EqualTo(1), "turn 1 ramps seat A max PP to 1");
        Assert.That(harness.HandCount(playerSeat: true), Is.EqualTo(5),
            "turn-1 second-player draw is 2 cards (3 -> 5)");
        Assert.That(harness.DeckCount(playerSeat: true), Is.EqualTo(25), "seat A deck after draw");

        // End seat A's turn.
        var t1End = harness.Push(NetworkBattleUri.TurnEnd, TurnEndBody(), isPlayerSeat: true);
        Assert.That(t1End.Accepted, Is.True, $"turn1 TurnEnd rejected: {t1End.RejectReason}");

        // --- turn 2 (seat B active) -------------------------------------------------------------
        // Seat B opens its first turn: PP ramps to 1 and it draws its turn-1 card. (Seat B's deck
        // started full at 30 because its opening hand is dealt into hidden zones, not its
        // HandCardList, until reveal — so its first visible draw moves deck 30 -> 29, hand 0 -> 1.)
        var t2 = harness.Push(NetworkBattleUri.TurnStart, TurnStartBody(), isPlayerSeat: false);
        Assert.That(t2.Accepted, Is.True, $"turn2 TurnStart rejected: {t2.RejectReason}");
        Assert.That(harness.Turn(playerSeat: false), Is.EqualTo(1), "seat B turn counter");
        Assert.That(harness.Pp(playerSeat: false), Is.EqualTo(1), "turn 2 ramps seat B max PP to 1");
        Assert.That(harness.HandCount(playerSeat: false), Is.EqualTo(1), "seat B turn-1 draw");
        // Seat B's opening hand was dealt into hidden zones (not HandCardList), so its deck started at 30;
        // the single turn-1 draw brings it to 29.
        Assert.That(harness.DeckCount(playerSeat: false), Is.EqualTo(29), "seat B deck after turn-1 draw");

        // Both leaders untouched (no damage dealt across the two opening turns) — state tracks
        // cleanly on BOTH seats at the turn boundary.
        Assert.That(harness.LeaderLife(playerSeat: true), Is.EqualTo(20), "seat A leader life");
        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 whatever card the
        // shuffle seated at that index (shuffledDeck[0]), parked 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
        // — it knows both decks — so this test's reveal substitution is identity-preserving by choice;
        // CreateActualCard builds the card purely from the wire cardId regardless of which card the
        // shuffle parked at Index 1. The board delta is what proves ReplaceReceivedCard.ReplaceCard ->
        // CreateActualCard resolved the card onto the board headless. The companion test
        // Opponent_reveal_overrides_seeded_identity_headless stresses a MISMATCHED cardId to prove the
        // wire id — not the seeded identity — is what gets seated.)
        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 Opponent_reveal_overrides_seeded_identity_headless()
    {
        // This is the substitution half of M-HC-2: prove the seated card's POST-reveal identity is the
        // WIRE cardId even when it DIFFERS from whatever the shuffle parked at that engine Index.
        // ReplaceReceivedCard.CreateActualCard builds the card purely from cardData.CardId, independent
        // of the seated dummy's id — so a reveal whose cardId mismatches the seed must OVERRIDE it.
        //
        // Z (seeded) vs W (revealed) are DIFFERENT cost-1 vanilla followers, both present + creatable in
        // cards.json:
        //   Z = 100011010 — the proven vanilla follower (char_type 1, cost 1). Seat B's deck is made
        //       UNIFORMLY of Z, so whichever idx the shuffle parked at Index 1 is unambiguously Z.
        //   W = 101211120 — a different cost-1 vanilla follower (char_type 1, cost 1, no skill). Cost 1
        //       seats at seat B's first-turn PP (1). The id is NOT in seat B's deck, so the only way it
        //       can appear on the board is the reveal substituting it in.
        const long Z = NodeNativeBattleHarness.VanillaFollowerId; // 100011010
        const long W = NodeNativeBattleHarness.AltVanillaFollowerId; // 101211120
        Assert.That(W, Is.Not.EqualTo(Z), "Z and W must differ for the substitution to be observable");

        // Uniform Z deck for seat B (every dummy is Z regardless of shuffle). Seat A left at default.
        var seatBDeck = Enumerable.Repeat(Z, 30).ToList();

        using var harness = NodeNativeBattleHarness.Create(seatBDeck: seatBDeck);

        // --- drive to seat B's turn (same two-turn sequence as the sibling reveal test) -------------
        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)");

        var boardBefore = harness.BoardCount(playerSeat: false);
        Assert.That(boardBefore, Is.EqualTo(0), "seat B has no board followers before the reveal");

        // The reveal discloses idx 1 (seeded as Z) with the MISMATCHED wire cardId W.
        var reveal = harness.Push(
            NetworkBattleUri.PlayActions, RevealPlayBody(idx: 1, cardId: W), 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");
        // The decisive assertion: the seated identity is W (the wire cardId), NOT Z (the seeded id).
        // Because the deck is uniformly Z, this can only pass if the reveal OVERRODE the seeded identity.
        Assert.That(harness.InPlayCardId(playerSeat: false, boardPos: 0), Is.EqualTo((int)W),
            "the seated card must be the wire cardId W, overriding the seeded Z identity at that idx");
    }

    // === M-HC-4a: attack resolves headless =======================================================

    [Test]
    public void Attack_on_enemy_leader_resolves_on_engine_state_headless()
    {
        // Seat A plays a vanilla follower on turn 1, then on its NEXT turn (past summoning sickness)
        // attacks seat B's leader. Assert seat B's leader life drops by the follower's attack (1) and the
        // attacker is spent. Driven entirely through the receive conductor (Push -> engine.Receive).
        //
        // Uniform vanilla deck so the card dealt at engine Index 1 is unambiguously the 1/2 vanilla.
        var deck = Enumerable.Repeat(NodeNativeBattleHarness.VanillaFollowerId, 30).ToList();
        using var harness = NodeNativeBattleHarness.Create(seatADeck: deck);

        // --- mulligan + open seat A turn 1 ------------------------------------------------------------
        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");

        // Play the vanilla (engine Index 1, cost 1) onto seat A's board.
        Assert.That(harness.Push(NetworkBattleUri.PlayActions, PlayBody(1), isPlayerSeat: true).Accepted,
            Is.True, "turn1 vanilla play");
        Assert.That(harness.BoardCount(playerSeat: true), Is.EqualTo(1), "seat A follower on board after play");

        // The just-played follower has summoning sickness this turn (can't attack yet).
        Assert.That(harness.InPlayCardAttackable(playerSeat: true, boardPos: 0), Is.False,
            "a follower has summoning sickness the turn it is played");

        int attackerIdx = harness.InPlayCardIndex(playerSeat: true, boardPos: 0);
        int attackerAtk = harness.InPlayCardAtk(playerSeat: true, boardPos: 0);
        Assert.That(attackerAtk, Is.EqualTo(1), "the vanilla follower's attack stat is 1");

        // --- advance to seat A's NEXT turn (turn 3) so the follower is past summoning sickness ---------
        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 (B)");
        Assert.That(harness.Push(NetworkBattleUri.TurnEnd, TurnEndBody(), isPlayerSeat: false).Accepted, Is.True, "turn2 TurnEnd (B)");
        Assert.That(harness.Push(NetworkBattleUri.TurnStart, TurnStartBody(), isPlayerSeat: true).Accepted, Is.True, "turn3 TurnStart (A)");

        Assert.That(harness.InPlayCardAttackable(playerSeat: true, boardPos: 0), Is.True,
            "the follower can attack on seat A's next turn (summoning sickness cleared)");

        int leaderLifeBefore = harness.LeaderLife(playerSeat: false);
        Assert.That(leaderLifeBefore, Is.EqualTo(20), "seat B leader untouched before the attack");

        // --- the attack: seat A follower -> seat B leader (Index 0, on the enemy seat) ----------------
        var attack = harness.Push(
            NetworkBattleUri.PlayActions,
            NodeNativeBattleHarness.AttackBody(attackerIdx, targetIdx: 0, targetOnEnemySeat: true),
            isPlayerSeat: true);

        Assert.That(attack.Accepted, Is.True, $"attack rejected: {attack.RejectReason}");
        Assert.That(harness.LeaderLife(playerSeat: false), Is.EqualTo(leaderLifeBefore - attackerAtk),
            "seat B leader life must drop by the attacker's attack stat");
        Assert.That(harness.InPlayCardAttackable(playerSeat: true, boardPos: 0), Is.False,
            "the attacker is spent after attacking (can't attack again this turn)");
    }

    [Test]
    public void Follower_vs_follower_attack_is_a_lethal_trade_headless()
    {
        // Seat A plays a 1/1 vanilla; seat B reveals a 1/1 vanilla (M-HC-2 reveal pattern). On seat A's
        // next turn the follower attacks seat B's follower. Each deals 1 to a 1-life body -> a lethal
        // trade: both followers' life drops and both leave the board.
        var oneOne = NodeNativeBattleHarness.VanillaOneOneFollowerId;
        var seatADeck = Enumerable.Repeat(oneOne, 30).ToList();
        var seatBDeck = Enumerable.Repeat(oneOne, 30).ToList();
        using var harness = NodeNativeBattleHarness.Create(seatADeck: seatADeck, seatBDeck: seatBDeck);

        // --- mulligan + seat A turn 1: play the 1/1 -------------------------------------------------
        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.PlayActions, PlayBody(1), isPlayerSeat: true).Accepted, Is.True, "turn1 play 1/1");
        Assert.That(harness.BoardCount(playerSeat: true), Is.EqualTo(1), "seat A 1/1 on board");
        int attackerIdx = harness.InPlayCardIndex(playerSeat: true, boardPos: 0);

        // --- seat B turn 2: reveal a 1/1 onto seat B's board ------------------------------------------
        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 (B)");
        Assert.That(harness.BoardCount(playerSeat: false), Is.EqualTo(0), "seat B board empty before reveal");
        Assert.That(harness.Push(NetworkBattleUri.PlayActions, RevealPlayBody(idx: 1, cardId: oneOne), isPlayerSeat: false).Accepted,
            Is.True, "seat B reveal-play 1/1");
        Assert.That(harness.BoardCount(playerSeat: false), Is.EqualTo(1), "seat B 1/1 on board after reveal");
        int targetIdx = harness.InPlayCardIndex(playerSeat: false, boardPos: 0);

        // --- back to seat A (turn 3): the 1/1 is past summoning sickness ------------------------------
        Assert.That(harness.Push(NetworkBattleUri.TurnEnd, TurnEndBody(), isPlayerSeat: false).Accepted, Is.True, "turn2 TurnEnd (B)");
        Assert.That(harness.Push(NetworkBattleUri.TurnStart, TurnStartBody(), isPlayerSeat: true).Accepted, Is.True, "turn3 TurnStart (A)");
        Assert.That(harness.InPlayCardAttackable(playerSeat: true, boardPos: 0), Is.True, "attacker past summoning sickness");

        Assert.That(harness.InPlayCardLife(playerSeat: true, boardPos: 0), Is.EqualTo(1), "attacker 1/1 full life before trade");
        Assert.That(harness.InPlayCardLife(playerSeat: false, boardPos: 0), Is.EqualTo(1), "target 1/1 full life before trade");

        // --- attack follower -> follower (target on enemy seat B) ------------------------------------
        var attack = harness.Push(
            NetworkBattleUri.PlayActions,
            NodeNativeBattleHarness.AttackBody(attackerIdx, targetIdx, targetOnEnemySeat: true),
            isPlayerSeat: true);

        Assert.That(attack.Accepted, Is.True, $"follower trade rejected: {attack.RejectReason}");
        // 1/1 vs 1/1: each takes 1 -> both at 0 life -> both die and leave the board (lethal trade).
        Assert.That(harness.BoardCount(playerSeat: true), Is.EqualTo(0), "attacker 1/1 died in the trade");
        Assert.That(harness.BoardCount(playerSeat: false), Is.EqualTo(0), "target 1/1 died in the trade");
        Assert.That(harness.LeaderLife(playerSeat: false), Is.EqualTo(20),
            "neither leader takes damage in a follower-vs-follower trade");
    }

    [Test]
    public void Deal_seats_three_card_hand_headless()
    {
        using var harness = NodeNativeBattleHarness.Create();

        var result = harness.Push(NetworkBattleUri.Deal, DealBody(), isPlayerSeat: true);

        Assert.That(result.Accepted, Is.True, $"Deal rejected: {result.RejectReason}");
        Assert.That(harness.HandCount(playerSeat: true), Is.EqualTo(3),
            "Deal must seat the 3-card opening hand on the player seat.");
    }

    [Test]
    public void Vanilla_play_resolves_on_engine_state_headless()
    {
        // Deck idx 1/2/3 are the top three of the shuffled deck; arrange idx-1 to be a known vanilla
        // follower so the Play assertion is decisive. Put the vanilla follower first; the rest of the
        // default deck (spellboost + vanillas) follows.
        var deck = new List<long> { NodeNativeBattleHarness.VanillaFollowerId };
        deck.AddRange(NodeNativeBattleHarness.DefaultDeck());
        deck = deck.GetRange(0, 30);

        using var harness = NodeNativeBattleHarness.Create(seatADeck: deck);

        var deal = harness.Push(NetworkBattleUri.Deal, DealBody(), isPlayerSeat: true);
        Assert.That(deal.Accepted, Is.True, $"Deal rejected: {deal.RejectReason}");
        Assert.That(harness.HandCount(playerSeat: true), Is.EqualTo(3), "post-Deal hand");

        var ppBefore = harness.Pp(playerSeat: true);
        var handBefore = harness.HandCount(playerSeat: true);
        var boardBefore = harness.BoardCount(playerSeat: true);

        // The played card is at hand index 1 (deck idx 1 -> the first dealt card; engine card Index
        // mirrors deck position+1). The shuffle determines which deck idx-1 maps to; we only need a
        // vanilla follower in the opening hand. Use the first dealt idx.
        var playIdx = harness.PlayerHandCardIndex(0);
        var play = harness.Push(NetworkBattleUri.PlayActions, PlayBody(playIdx), isPlayerSeat: true);

        Assert.That(play.Accepted, Is.True, $"Play rejected: {play.RejectReason}");
        Assert.That(harness.HandCount(playerSeat: true), Is.EqualTo(handBefore - 1),
            "the played card must leave the hand");
        Assert.That(harness.BoardCount(playerSeat: true), Is.EqualTo(boardBefore + 1),
            "a follower play must add one to the board");
        Assert.That(harness.Pp(playerSeat: true), Is.LessThan(ppBefore),
            "PP must drop by the played card's cost");
    }

    // === M-HC-3a: engine-resolved cost on the knownList ==========================================

    // The spellboost cost-reducer 101314020 (base cost 5). Its when_spell_charge cost_change skill
    // (skill_option add=ADD_CHARGE_COUNT*-1) reduces its OWN cost by 1 per accumulated spellboost
    // charge — so resolved cost == max(0, 5 - charge). The harness seeds the charge directly
    // (SeedHandCardSpellboostCost registers the same CostAddModifier(-1)/charge the engine's own
    // Skill_cost_change builds) because pumping real charge needs the VFX-coupled spell-charge chain.
    private const long SpellboostReducerId = NodeNativeBattleHarness.SpellboostCardId; // 101314020
    private const int SpellboostReducerBaseCost = 5;

    // A deck made UNIFORMLY of the spellboost reducer, so whatever idx the shuffle parks at engine
    // Index 1 (the first dealt card) is unambiguously the reducer — no need to chase the shuffled
    // position. (A non-uniform deck would shuffle the reducer off idx 1; the cost read would then be a
    // vanilla's base 1, masking the discount — that is exactly the first RED this surfaced.)
    private static IReadOnlyList<long> ReducerDeck() => Enumerable.Repeat(SpellboostReducerId, 30).ToList();

    [TestCase(0, SpellboostReducerBaseCost)] // no charge -> base cost (5)
    [TestCase(4, 1)]                         // 4 charges -> 5 - 4 = 1
    [TestCase(5, 0)]                         // 5 charges -> max(0, 5 - 5) = 0
    public void PlayedCardCost_reads_engine_resolved_discounted_cost(int charge, int expectedCost)
    {
        // ENGINE-READ proof (the count->cost resolution off the real Cost getter). Drive a node-native
        // battle to seat A's turn 1, seed the reducer's spellboost charge, play it, and read the cost the
        // engine actually charged. expectedCost is base(5) - charge, the engine's authentic resolution —
        // and the differing values across charge levels are the non-vacuity (a wrong charge -> wrong cost).
        using var harness = NodeNativeBattleHarness.Create(seatADeck: ReducerDeck());

        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");

        // The reducer dealt at engine Index 1 (deck position 0). Seed the charge on it WHILE it is in hand,
        // then confirm the engine's Cost getter resolved the discount BEFORE the play (pre-play pin).
        int seededHandCost = harness.Engine.SeedHandCardSpellboostCost(playerSeat: true, idx: 1, charge);
        Assert.That(seededHandCost, Is.EqualTo(expectedCost),
            $"engine hand-card Cost must resolve base({SpellboostReducerBaseCost}) - charge({charge})");

        // Play it. With max(0,5-charge) <= 1 for charge 4/5, and charge 0 keeping cost 5 (PP 1 can't pay
        // 5), we only need the cost READ to be correct — but assert acceptance where affordable.
        var play = harness.Push(NetworkBattleUri.PlayActions, PlayBody(1), isPlayerSeat: true);
        if (expectedCost <= 1)
            Assert.That(play.Accepted, Is.True, $"affordable reducer play rejected: {play.RejectReason}");

        // The PAYOFF read: PlayedCardCost returns the engine-resolved play-time cost. For an affordable
        // play this is the captured PlayedCost (post-resolution, card now in cemetery — it is a spell);
        // for the unaffordable charge-0 case the card stays in hand and the live Cost (5) is read. Either
        // way the value equals the engine's resolved discounted cost.
        Assert.That(harness.Engine.PlayedCardCost(playerSeat: true, idx: 1),
            Is.EqualTo(expectedCost),
            $"PlayedCardCost must equal the engine-resolved cost {expectedCost} at charge {charge}");
    }

    [Test]
    public void Vanilla_play_PlayedCardCost_is_base_cost()
    {
        // A vanilla follower has no cost modifier, so the engine resolves its base cost (1) by
        // construction — the cost the knownList will carry for a non-boosted play.
        var deck = new List<long> { NodeNativeBattleHarness.VanillaFollowerId };
        deck.AddRange(NodeNativeBattleHarness.DefaultDeck());
        deck = deck.GetRange(0, 30);
        using var harness = NodeNativeBattleHarness.Create(seatADeck: deck);

        Assert.That(harness.Push(NetworkBattleUri.Deal, DealBody(), isPlayerSeat: true).Accepted, Is.True);
        var playIdx = harness.PlayerHandCardIndex(0);
        Assert.That(harness.Push(NetworkBattleUri.PlayActions, PlayBody(playIdx), isPlayerSeat: true).Accepted,
            Is.True, "vanilla play");

        Assert.That(harness.Engine.PlayedCardCost(playerSeat: true, idx: playIdx), Is.EqualTo(1),
            "a cost-1 vanilla follower resolves to base cost 1");
    }

    [Test]
    public void PlayedCardCost_degrades_to_fallback_for_unknown_idx()
    {
        // Graceful degradation: an idx with no resolved card returns the fallback (non-engine sessions
        // and unmapped idxs never crash the handler).
        using var harness = NodeNativeBattleHarness.Create();
        Assert.That(harness.Engine.PlayedCardCost(playerSeat: true, idx: 9999, fallback: 7), Is.EqualTo(7));
    }

    // --- HANDLER-EMIT proof: the cost reaches the opponent-facing knownList[].cost ----------------

    // A PlayActions wire frame the HANDLER consumes: it needs an orderList move op for the played idx so
    // BuildPlayedCard can synthesize the entry (the engine resolves the play from playIdx/type alone, but
    // the opponent-facing synthesis is driven by the wire orderList). to:30 == Field.
    private static Dictionary<string, object?> HandlerPlayBody(int playIdx) => new()
    {
        ["playIdx"] = playIdx,
        ["type"] = 30,
        ["orderList"] = new List<object?>
        {
            new Dictionary<string, object?>
            {
                ["move"] = new Dictionary<string, object?>
                {
                    ["idx"] = new List<object?> { (long)playIdx },
                    ["isSelf"] = 1L, ["from"] = 10L, ["to"] = 30L,
                },
            },
        },
    };

    [Test]
    public void Handler_emits_engine_resolved_cost_on_knownList()
    {
        // The end-to-end payoff: build a FrameDispatchContext over the harness (engine + state +
        // participants), drive to seat A's turn, seed the reducer's charge, INGEST the play (so the engine
        // resolves + captures PlayedCost), then run PlayActionsHandler.Handle and inspect the emitted
        // knownList[0].cost. It must equal the engine-resolved discounted cost (NOT the base cost) —
        // proving the cost-desync is closed by construction at the emit site.
        const int charge = 4;
        const int expectedCost = SpellboostReducerBaseCost - charge; // 1
        using var harness = NodeNativeBattleHarness.Create(seatADeck: ReducerDeck());

        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");

        // Seed the charge so the engine resolves the reducer at cost 1 (affordable on PP 1).
        Assert.That(harness.Engine.SeedHandCardSpellboostCost(playerSeat: true, idx: 1, charge),
            Is.EqualTo(expectedCost), "pre-play resolved hand cost");

        // Ingest the play into the engine (seat A == player) so PlayedCost is captured at resolution.
        var playBody = HandlerPlayBody(1);
        Assert.That(harness.Push(NetworkBattleUri.PlayActions, playBody, isPlayerSeat: true).Accepted,
            Is.True, "reducer play ingest");

        // Build the dispatch context the way BattleSession.BuildContext does, with both stubs advanced to
        // AfterReady so the PvP relay gate (BothSidesAfterReady) passes. From == seat A (the sender).
        harness.SeatA.Phase = HandshakePhase.AfterReady;
        harness.SeatB.Phase = HandshakePhase.AfterReady;
        var env = new MsgEnvelope(
            NetworkBattleUri.PlayActions, ViewerId: harness.SeatA.ViewerId, Uuid: "udid-test", Bid: null,
            RetryAttempt: 0, Cat: EmitCategory.Battle, PubSeq: null, PlaySeq: null,
            Body: new RawBody(playBody));
        var ctx = new FrameDispatchContext
        {
            A = harness.SeatA, B = harness.SeatB, From = harness.SeatA, Other = harness.SeatB,
            Env = env, BattleId = "test-battle", State = harness.State, Engine = harness.Engine,
        };

        var routes = new PlayActionsHandler().Handle(ctx);

        Assert.That(routes, Has.Count.EqualTo(1), "one route to the opponent");
        var body = routes[0].Frame.Body as PlayActionsBroadcastBody;
        Assert.That(body, Is.Not.Null, "frame body is a PlayActionsBroadcastBody");
        Assert.That(body!.KnownList, Is.Not.Null.And.Count.EqualTo(1), "one knownList entry (the played card)");
        Assert.That(body.KnownList![0].CardId, Is.EqualTo(SpellboostReducerId), "the reducer's identity");
        // THE assertion: the emitted cost is the engine-resolved DISCOUNTED cost (1), not the base (5).
        Assert.That(body.KnownList[0].Cost, Is.EqualTo(expectedCost),
            "knownList[].cost must be the engine-resolved discounted cost, not the base cost");
        Assert.That(body.KnownList[0].Cost, Is.Not.EqualTo(SpellboostReducerBaseCost),
            "non-vacuity: the emitted cost must NOT be the un-discounted base cost");
    }

    // === M-HC-3b: REAL spell-charge accumulation (no seam) =======================================

    // The spellboost GRANTOR 118311030: a cost-3 follower whose when_play spell_charge skill
    // (add_charge=1, target character=me&target=hand&card_type=all) adds +1 spell-charge to EVERY card in
    // the caster's hand on each play. Drives the reducer's charge for real headless — no SeedHandCardSpellboostCost
    // seam. (Its authored SECOND charge skill, add_charge=5, does NOT fire headless — only +1 lands per play;
    // recorded as a known fidelity follow-up, irrelevant to this regression which needs only the +1.)
    private const long SpellboostGrantorId = 118311030;

    // A deck of alternating reducers + grantors so both reliably populate the opening hand and early draws
    // (a single front-loaded reducer would shuffle out of reach). 15 of each = 30.
    private static IReadOnlyList<long> ReducerAndGrantorDeck()
    {
        var deck = new List<long>(30);
        for (int i = 0; i < 15; i++) { deck.Add(SpellboostReducerId); deck.Add(SpellboostGrantorId); }
        return deck;
    }

    // Find the engine Index of the first hand card on seat A with the given wire cardId (the hand is
    // shuffled, so we locate by identity, not position). -1 if not present.
    private static int FindHandIdxByCardId(NodeNativeBattleHarness harness, long cardId)
    {
        for (int i = 0; i < harness.HandCount(playerSeat: true); i++)
            if (harness.HandCardId(playerSeat: true, i) == (int)cardId)
                return harness.HandCardIndex(playerSeat: true, i);
        return -1;
    }

    // Ramp seat A to its turn `targetTurn` by alternating TurnStart/TurnEnd A/B; leaves seat A's turn OPEN.
    private void RampToSeatATurn(NodeNativeBattleHarness harness, int targetTurn)
    {
        bool seatA = true;
        while (true)
        {
            Assert.That(harness.Push(NetworkBattleUri.TurnStart, TurnStartBody(), isPlayerSeat: seatA).Accepted,
                Is.True, "TurnStart");
            if (seatA && harness.Turn(playerSeat: true) == targetTurn) return;
            Assert.That(harness.Push(NetworkBattleUri.TurnEnd, TurnEndBody(), isPlayerSeat: seatA).Accepted,
                Is.True, "TurnEnd");
            seatA = !seatA;
        }
    }

    [Test]
    public void Real_spell_charge_drops_engine_cost_and_count_no_seam()
    {
        // The committed M-HC-3b closure guard: drive a REAL spell-charge sequence headless (NO
        // SeedHandCardSpellboostCost seam) and assert the engine-sourced COST and SPELLBOOST COUNT the node
        // now emits are both correct by construction. Proves the retired wire-derived bookkeeping is
        // redundant: the engine accumulates the charge itself (each grantor play runs the reducer's own
        // AddSpellChargeCount) and resolves the discount.
        using var harness = NodeNativeBattleHarness.Create(seatADeck: ReducerAndGrantorDeck());

        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");

        // Ramp to seat A turn 3 (PP 3) so the cost-3 grantor is affordable.
        RampToSeatATurn(harness, targetTurn: 3);
        Assert.That(harness.Pp(playerSeat: true), Is.EqualTo(3), "seat A PP at turn 3");

        // Locate a reducer + a grantor in the (shuffled) hand by identity.
        int reducerIdx = FindHandIdxByCardId(harness, SpellboostReducerId);
        int grantorIdx = FindHandIdxByCardId(harness, SpellboostGrantorId);
        Assert.That(reducerIdx, Is.GreaterThan(0), "a reducer must be in seat A's opening hand");
        Assert.That(grantorIdx, Is.GreaterThan(0), "a grantor must be in seat A's opening hand");

        // PRE-CHARGE non-vacuity: the reducer resolves to its BASE cost (5) and 0 charge BEFORE any grant.
        Assert.That(harness.Engine.PlayedCardCost(playerSeat: true, reducerIdx, fallback: -1),
            Is.EqualTo(SpellboostReducerBaseCost), "reducer cost is base (5) before any charge");
        Assert.That(harness.Engine.PlayedCardSpellboost(playerSeat: true, reducerIdx, fallback: -1),
            Is.EqualTo(0), "reducer spell-charge is 0 before any grant");

        // Play the grantor (cost 3). Its when_play spell_charge adds +1 to every hand card — REAL engine
        // resolution, no seam. This runs through the receive conductor (Push -> engine.Receive).
        Assert.That(harness.Push(NetworkBattleUri.PlayActions, PlayBody(grantorIdx), isPlayerSeat: true).Accepted,
            Is.True, "grantor play");

        // THE engine-read assertions: the reducer (still in hand) now reads charge 1 and cost 4 (5 - 1) —
        // accumulated for real by the engine, not seeded.
        Assert.That(harness.Engine.PlayedCardSpellboost(playerSeat: true, reducerIdx, fallback: -1),
            Is.EqualTo(1), "one grantor play accumulates +1 real spell-charge on the reducer");
        Assert.That(harness.Engine.PlayedCardCost(playerSeat: true, reducerIdx, fallback: -1),
            Is.EqualTo(SpellboostReducerBaseCost - 1),
            "the engine resolves the reducer's cost down to 4 (base 5 - 1 charge), no seam");

        // PERSIST-POST-PLAY proof (the read-moment this milestone chose): advance to seat A's next turn
        // (fresh PP 4, affording the cost-4 reducer), play the reducer (a spell -> cemetery), and confirm
        // PlayedCardSpellboost/PlayedCardCost STILL read 1/4 AFTER the card left the hand — i.e. the zone
        // search reads the persisted count off the resolved card, no receive-capture needed.
        Assert.That(harness.Push(NetworkBattleUri.TurnEnd, TurnEndBody(), isPlayerSeat: true).Accepted, Is.True);
        Assert.That(harness.Push(NetworkBattleUri.TurnStart, TurnStartBody(), isPlayerSeat: false).Accepted, Is.True);
        Assert.That(harness.Push(NetworkBattleUri.TurnEnd, TurnEndBody(), isPlayerSeat: false).Accepted, Is.True);
        Assert.That(harness.Push(NetworkBattleUri.TurnStart, TurnStartBody(), isPlayerSeat: true).Accepted, Is.True);
        Assert.That(harness.Pp(playerSeat: true), Is.GreaterThanOrEqualTo(4), "seat A fresh PP affords cost-4 reducer");

        // The reducer's engine Index is stable across turns; play it now.
        Assert.That(harness.Push(NetworkBattleUri.PlayActions, PlayBody(reducerIdx), isPlayerSeat: true).Accepted,
            Is.True, "charged reducer play");
        Assert.That(harness.Engine.PlayedCardSpellboost(playerSeat: true, reducerIdx, fallback: -1),
            Is.EqualTo(1), "spell-charge persists on the played reducer (now in cemetery)");
        Assert.That(harness.Engine.PlayedCardCost(playerSeat: true, reducerIdx, fallback: -1),
            Is.EqualTo(SpellboostReducerBaseCost - 1),
            "PlayedCost captured the discounted cost (4) at play time and persists post-play");
    }

    [Test]
    public void Handler_emits_real_engine_spellboost_and_cost_on_knownList()
    {
        // The end-to-end emit payoff for M-HC-3b: a REAL-charged reducer played through the conductor, then
        // PlayActionsHandler.Handle, with BOTH knownList[].cost AND knownList[].spellboost read straight off
        // the engine (no wire-derived bookkeeping). Cost 4 (discounted) + count 1 (real charge).
        using var harness = NodeNativeBattleHarness.Create(seatADeck: ReducerAndGrantorDeck());

        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");
        RampToSeatATurn(harness, targetTurn: 3);

        int reducerIdx = FindHandIdxByCardId(harness, SpellboostReducerId);
        int grantorIdx = FindHandIdxByCardId(harness, SpellboostGrantorId);
        Assert.That(reducerIdx, Is.GreaterThan(0), "reducer in hand");
        Assert.That(grantorIdx, Is.GreaterThan(0), "grantor in hand");

        // Charge the reducer for real (one grantor play -> +1), then advance to a fresh seat A turn that
        // affords the discounted reducer.
        Assert.That(harness.Push(NetworkBattleUri.PlayActions, PlayBody(grantorIdx), isPlayerSeat: true).Accepted,
            Is.True, "grantor play");
        Assert.That(harness.Push(NetworkBattleUri.TurnEnd, TurnEndBody(), isPlayerSeat: true).Accepted, Is.True);
        Assert.That(harness.Push(NetworkBattleUri.TurnStart, TurnStartBody(), isPlayerSeat: false).Accepted, Is.True);
        Assert.That(harness.Push(NetworkBattleUri.TurnEnd, TurnEndBody(), isPlayerSeat: false).Accepted, Is.True);
        Assert.That(harness.Push(NetworkBattleUri.TurnStart, TurnStartBody(), isPlayerSeat: true).Accepted, Is.True);

        // Ingest the reducer play into the engine (so PlayedCost/SpellChargeCount are captured at resolution).
        var playBody = HandlerPlayBody(reducerIdx);
        Assert.That(harness.Push(NetworkBattleUri.PlayActions, playBody, isPlayerSeat: true).Accepted,
            Is.True, "charged reducer play ingest");

        // Build the dispatch context the way BattleSession.BuildContext does; From == seat A (the sender).
        harness.SeatA.Phase = HandshakePhase.AfterReady;
        harness.SeatB.Phase = HandshakePhase.AfterReady;
        var env = new MsgEnvelope(
            NetworkBattleUri.PlayActions, ViewerId: harness.SeatA.ViewerId, Uuid: "udid-test", Bid: null,
            RetryAttempt: 0, Cat: EmitCategory.Battle, PubSeq: null, PlaySeq: null,
            Body: new RawBody(playBody));
        var ctx = new FrameDispatchContext
        {
            A = harness.SeatA, B = harness.SeatB, From = harness.SeatA, Other = harness.SeatB,
            Env = env, BattleId = "test-battle", State = harness.State, Engine = harness.Engine,
        };

        var routes = new PlayActionsHandler().Handle(ctx);

        Assert.That(routes, Has.Count.EqualTo(1), "one route to the opponent");
        var body = routes[0].Frame.Body as PlayActionsBroadcastBody;
        Assert.That(body, Is.Not.Null, "frame body is a PlayActionsBroadcastBody");
        Assert.That(body!.KnownList, Is.Not.Null.And.Count.EqualTo(1), "one knownList entry (the played reducer)");
        Assert.That(body.KnownList![0].CardId, Is.EqualTo(SpellboostReducerId), "the reducer's identity");
        // THE assertions: cost is the engine-resolved DISCOUNTED cost (4), spellboost is the REAL count (1).
        Assert.That(body.KnownList[0].Cost, Is.EqualTo(SpellboostReducerBaseCost - 1),
            "knownList[].cost must be the engine-resolved discounted cost (4), not base (5)");
        Assert.That(body.KnownList[0].Spellboost, Is.EqualTo(1),
            "knownList[].spellboost must be the REAL engine-accumulated charge count (1), engine-sourced");
        // Non-vacuity: neither field is the un-charged default.
        Assert.That(body.KnownList[0].Cost, Is.Not.EqualTo(SpellboostReducerBaseCost),
            "non-vacuity: emitted cost is NOT the un-discounted base cost");
        Assert.That(body.KnownList[0].Spellboost, Is.Not.EqualTo(0),
            "non-vacuity: emitted spellboost is NOT 0");
    }
}