feat(games): implement blackjack game plugin with manual start and custom payouts

Adds a full blackjack game with dealer AI, hit/stand/double-down actions,
and per-player payout multipliers (house-edge model). Extends the game
framework with manualStart support and a START_GAME WebSocket message so
hosts can begin when ready. Generalizes bet settlement transaction
descriptions from chess-specific to game-agnostic.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

shared/games/blackjack/blackjack.plugin.test.ts

@@ -0,0 +1,471 @@
+import { describe, it, expect } from "bun:test";
+import { blackjackPlugin } from "./blackjack.plugin";
+import { handValue } from "./blackjack.plugin";
+import type { BlackjackState, BlackjackPlayerView, BlackjackSpectatorView, Card, PlayerHand } from "./blackjack.types";
+
+// ── Helpers ──
+
+function makeCard(rank: string, suit: string = "spades"): Card {
+    return { rank, suit } as Card;
+}
+
+/** Create a rigged multiplayer state for deterministic testing. */
+function riggedState(overrides: Partial<BlackjackState> & {
+    hands: Record<string, PlayerHand>;
+    dealerHand: Card[];
+    turnOrder: string[];
+}): BlackjackState {
+    return {
+        deck: overrides.deck ?? [makeCard("5"), makeCard("6"), makeCard("7"), makeCard("8"), makeCard("9"), makeCard("10")],
+        dealerHand: overrides.dealerHand,
+        hands: overrides.hands,
+        turnOrder: overrides.turnOrder,
+        activePlayerIndex: overrides.activePlayerIndex ?? 0,
+        phase: overrides.phase ?? "player_turns",
+    };
+}
+
+function playingHand(cards: Card[]): PlayerHand {
+    return { cards, status: "playing", result: null, resultReason: null };
+}
+
+function blackjackHand(cards: Card[]): PlayerHand {
+    return { cards, status: "blackjack", result: null, resultReason: null };
+}
+
+// ── handValue tests ──
+
+describe("handValue", () => {
+    it("calculates simple hand values", () => {
+        expect(handValue([makeCard("5"), makeCard("10")])).toBe(15);
+        expect(handValue([makeCard("K"), makeCard("Q")])).toBe(20);
+    });
+
+    it("treats ace as 11 when possible", () => {
+        expect(handValue([makeCard("A"), makeCard("10")])).toBe(21);
+    });
+
+    it("treats ace as 1 when 11 would bust", () => {
+        expect(handValue([makeCard("A"), makeCard("10"), makeCard("5")])).toBe(16);
+    });
+
+    it("handles multiple aces", () => {
+        expect(handValue([makeCard("A"), makeCard("A")])).toBe(12);
+        expect(handValue([makeCard("A"), makeCard("A"), makeCard("9")])).toBe(21);
+    });
+});
+
+// ── Plugin metadata ──
+
+describe("blackjackPlugin metadata", () => {
+    it("has correct slug and name", () => {
+        expect(blackjackPlugin.slug).toBe("blackjack");
+        expect(blackjackPlugin.name).toBe("Blackjack");
+    });
+
+    it("supports 1-6 players with manual start", () => {
+        expect(blackjackPlugin.minPlayers).toBe(1);
+        expect(blackjackPlugin.maxPlayers).toBe(6);
+        expect(blackjackPlugin.manualStart).toBe(true);
+    });
+});
+
+// ── createInitialState ──
+
+describe("createInitialState", () => {
+    it("deals 2 cards to each player and dealer", () => {
+        const state = blackjackPlugin.createInitialState(["p1", "p2", "p3"]);
+        expect(state.hands["p1"]!.cards.length).toBe(2);
+        expect(state.hands["p2"]!.cards.length).toBe(2);
+        expect(state.hands["p3"]!.cards.length).toBe(2);
+        expect(state.dealerHand.length).toBe(2);
+        expect(state.turnOrder).toEqual(["p1", "p2", "p3"]);
+    });
+
+    it("removes dealt cards from deck", () => {
+        const state = blackjackPlugin.createInitialState(["p1", "p2"]);
+        // 52 - (2*2 players + 2 dealer) = 46
+        expect(state.deck.length).toBe(46);
+    });
+
+    it("works with a single player", () => {
+        const state = blackjackPlugin.createInitialState(["solo"]);
+        expect(state.hands["solo"]!.cards.length).toBe(2);
+        expect(state.turnOrder).toEqual(["solo"]);
+    });
+});
+
+// ── Turn order ──
+
+describe("turn order", () => {
+    it("active player is the first non-blackjack player", () => {
+        const state = riggedState({
+            hands: {
+                "p1": blackjackHand([makeCard("A"), makeCard("K")]),
+                "p2": playingHand([makeCard("5"), makeCard("6")]),
+                "p3": playingHand([makeCard("7"), makeCard("8")]),
+            },
+            dealerHand: [makeCard("9"), makeCard("10")],
+            turnOrder: ["p1", "p2", "p3"],
+            activePlayerIndex: 1, // p1 skipped (blackjack)
+        });
+
+        // p1 is blackjack so active should be p2 (index 1)
+        const view = blackjackPlugin.getPlayerView(state, "p2") as BlackjackPlayerView;
+        expect(view.activePlayerId).toBe("p2");
+        expect(view.canAct).toBe(true);
+    });
+
+    it("advances to next player on stand", () => {
+        const state = riggedState({
+            hands: {
+                "p1": playingHand([makeCard("K"), makeCard("9")]),
+                "p2": playingHand([makeCard("5"), makeCard("6")]),
+            },
+            dealerHand: [makeCard("7"), makeCard("8")],
+            turnOrder: ["p1", "p2"],
+            activePlayerIndex: 0,
+        });
+
+        const result = blackjackPlugin.handleAction(state, { type: "stand" }, "p1");
+        expect(result.ok).toBe(true);
+        if (!result.ok) return;
+        expect(result.state.activePlayerIndex).toBe(1);
+        expect(result.state.phase).toBe("player_turns");
+    });
+
+    it("rejects action from non-active player", () => {
+        const state = riggedState({
+            hands: {
+                "p1": playingHand([makeCard("K"), makeCard("9")]),
+                "p2": playingHand([makeCard("5"), makeCard("6")]),
+            },
+            dealerHand: [makeCard("7"), makeCard("8")],
+            turnOrder: ["p1", "p2"],
+            activePlayerIndex: 0,
+        });
+
+        const result = blackjackPlugin.handleAction(state, { type: "hit" }, "p2");
+        expect(result.ok).toBe(false);
+    });
+});
+
+// ── handleAction: hit ──
+
+describe("handleAction — hit", () => {
+    it("adds a card to active player's hand", () => {
+        const state = riggedState({
+            hands: {
+                "p1": playingHand([makeCard("5"), makeCard("6")]),
+            },
+            dealerHand: [makeCard("K"), makeCard("7")],
+            turnOrder: ["p1"],
+            activePlayerIndex: 0,
+        });
+
+        const result = blackjackPlugin.handleAction(state, { type: "hit" }, "p1");
+        expect(result.ok).toBe(true);
+        if (!result.ok) return;
+        expect(result.state.hands["p1"]!.cards.length).toBe(3);
+    });
+
+    it("busts player and advances turn", () => {
+        const state = riggedState({
+            hands: {
+                "p1": playingHand([makeCard("K"), makeCard("Q")]),
+                "p2": playingHand([makeCard("5"), makeCard("6")]),
+            },
+            dealerHand: [makeCard("7"), makeCard("8")],
+            turnOrder: ["p1", "p2"],
+            activePlayerIndex: 0,
+            deck: [makeCard("5")], // K+Q+5 = 25 → bust
+        });
+
+        const result = blackjackPlugin.handleAction(state, { type: "hit" }, "p1");
+        expect(result.ok).toBe(true);
+        if (!result.ok) return;
+        expect(result.state.hands["p1"]!.status).toBe("bust");
+        expect(result.state.hands["p1"]!.result).toBe("lose");
+        expect(result.state.activePlayerIndex).toBe(1); // advanced to p2
+    });
+
+    it("auto-stands on 21 and advances turn", () => {
+        const state = riggedState({
+            hands: {
+                "p1": playingHand([makeCard("6"), makeCard("5")]),
+                "p2": playingHand([makeCard("7"), makeCard("8")]),
+            },
+            dealerHand: [makeCard("9"), makeCard("10")],
+            turnOrder: ["p1", "p2"],
+            activePlayerIndex: 0,
+            deck: [makeCard("10")], // 6+5+10 = 21
+        });
+
+        const result = blackjackPlugin.handleAction(state, { type: "hit" }, "p1");
+        expect(result.ok).toBe(true);
+        if (!result.ok) return;
+        expect(result.state.hands["p1"]!.status).toBe("stood");
+        expect(result.state.activePlayerIndex).toBe(1);
+    });
+});
+
+// ── handleAction: stand ──
+
+describe("handleAction — stand", () => {
+    it("resolves game when last player stands", () => {
+        const state = riggedState({
+            hands: {
+                "p1": playingHand([makeCard("K"), makeCard("9")]),
+            },
+            dealerHand: [makeCard("7"), makeCard("8")],
+            turnOrder: ["p1"],
+            activePlayerIndex: 0,
+            deck: [makeCard("2")], // dealer: 15 + 2 = 17 → stands
+        });
+
+        const result = blackjackPlugin.handleAction(state, { type: "stand" }, "p1");
+        expect(result.ok).toBe(true);
+        if (!result.ok) return;
+        expect(result.state.phase).toBe("resolved");
+        expect(result.state.hands["p1"]!.result).toBe("win");
+    });
+
+    it("dealer busts — all standing players win", () => {
+        const state = riggedState({
+            hands: {
+                "p1": playingHand([makeCard("K"), makeCard("8")]),
+                "p2": { ...playingHand([makeCard("7"), makeCard("9")]), status: "stood" as const },
+            },
+            dealerHand: [makeCard("6"), makeCard("9")],
+            turnOrder: ["p1", "p2"],
+            activePlayerIndex: 0,
+            deck: [makeCard("K")], // dealer: 6+9+K = 25 → bust
+        });
+
+        const result = blackjackPlugin.handleAction(state, { type: "stand" }, "p1");
+        expect(result.ok).toBe(true);
+        if (!result.ok) return;
+        expect(result.state.phase).toBe("resolved");
+        expect(result.state.hands["p1"]!.result).toBe("win");
+        expect(result.state.hands["p2"]!.result).toBe("win");
+    });
+
+    it("mixed results — one wins, one loses, one pushes", () => {
+        const state = riggedState({
+            hands: {
+                "p1": { ...playingHand([makeCard("K"), makeCard("9")]), status: "stood" as const },  // 19
+                "p2": { ...playingHand([makeCard("7"), makeCard("8")]), status: "stood" as const },  // 15
+                "p3": playingHand([makeCard("Q"), makeCard("8")]),  // 18 — active
+            },
+            dealerHand: [makeCard("K"), makeCard("8")],  // 18 → stands
+            turnOrder: ["p1", "p2", "p3"],
+            activePlayerIndex: 2,
+        });
+
+        const result = blackjackPlugin.handleAction(state, { type: "stand" }, "p3");
+        expect(result.ok).toBe(true);
+        if (!result.ok) return;
+        expect(result.state.hands["p1"]!.result).toBe("win");   // 19 > 18
+        expect(result.state.hands["p2"]!.result).toBe("lose");  // 15 < 18
+        expect(result.state.hands["p3"]!.result).toBe("push");  // 18 = 18
+    });
+});
+
+// ── getPlayerView ──
+
+describe("getPlayerView", () => {
+    it("hides dealer hole card during player turns", () => {
+        const state = riggedState({
+            hands: { "p1": playingHand([makeCard("K"), makeCard("5")]) },
+            dealerHand: [makeCard("7"), makeCard("Q")],
+            turnOrder: ["p1"],
+        });
+
+        const view = blackjackPlugin.getPlayerView(state, "p1") as BlackjackPlayerView;
+        expect(view.dealerHand[0]!.rank).toBe("7");
+        expect(view.dealerHand[1]!.rank as string).toBe("?");
+        expect(view.dealerFullValue).toBeNull();
+    });
+
+    it("reveals dealer hand when resolved", () => {
+        const state = riggedState({
+            hands: { "p1": { ...playingHand([makeCard("K"), makeCard("5")]), status: "stood" as const, result: "lose" as const, resultReason: "Dealer wins" } },
+            dealerHand: [makeCard("7"), makeCard("Q")],
+            turnOrder: ["p1"],
+            phase: "resolved",
+            activePlayerIndex: -1,
+        });
+
+        const view = blackjackPlugin.getPlayerView(state, "p1") as BlackjackPlayerView;
+        expect(view.dealerHand[1]!.rank).toBe("Q");
+        expect(view.dealerFullValue).toBe(17);
+    });
+
+    it("canAct is true only for active player", () => {
+        const state = riggedState({
+            hands: {
+                "p1": playingHand([makeCard("5"), makeCard("6")]),
+                "p2": playingHand([makeCard("7"), makeCard("8")]),
+            },
+            dealerHand: [makeCard("9"), makeCard("10")],
+            turnOrder: ["p1", "p2"],
+            activePlayerIndex: 0,
+        });
+
+        const view1 = blackjackPlugin.getPlayerView(state, "p1") as BlackjackPlayerView;
+        expect(view1.canAct).toBe(true);
+        expect(view1.myPlayerId).toBe("p1");
+
+        const view2 = blackjackPlugin.getPlayerView(state, "p2") as BlackjackPlayerView;
+        expect(view2.canAct).toBe(false);
+    });
+
+    it("includes all player hands in view", () => {
+        const state = riggedState({
+            hands: {
+                "p1": playingHand([makeCard("5"), makeCard("6")]),
+                "p2": playingHand([makeCard("7"), makeCard("8")]),
+            },
+            dealerHand: [makeCard("9"), makeCard("10")],
+            turnOrder: ["p1", "p2"],
+        });
+
+        const view = blackjackPlugin.getPlayerView(state, "p1") as BlackjackPlayerView;
+        expect(Object.keys(view.hands).length).toBe(2);
+        expect(view.hands["p1"]!.value).toBe(11);
+        expect(view.hands["p2"]!.value).toBe(15);
+    });
+});
+
+// ── getSpectatorView ──
+
+describe("getSpectatorView", () => {
+    it("hides dealer hole card during player turns", () => {
+        const state = riggedState({
+            hands: { "p1": playingHand([makeCard("K"), makeCard("5")]) },
+            dealerHand: [makeCard("7"), makeCard("Q")],
+            turnOrder: ["p1"],
+        });
+
+        const view = blackjackPlugin.getSpectatorView(state) as BlackjackSpectatorView;
+        expect(view.dealerHand[1]!.rank as string).toBe("?");
+    });
+
+    it("shows all player hands", () => {
+        const state = riggedState({
+            hands: {
+                "p1": playingHand([makeCard("K"), makeCard("5")]),
+                "p2": playingHand([makeCard("7"), makeCard("8")]),
+            },
+            dealerHand: [makeCard("9"), makeCard("10")],
+            turnOrder: ["p1", "p2"],
+        });
+
+        const view = blackjackPlugin.getSpectatorView(state) as BlackjackSpectatorView;
+        expect(Object.keys(view.hands).length).toBe(2);
+        expect(view.turnOrder).toEqual(["p1", "p2"]);
+    });
+});
+
+// ── isGameOver ──
+
+describe("isGameOver", () => {
+    it("returns null when game is in progress", () => {
+        const state = riggedState({
+            hands: { "p1": playingHand([makeCard("5"), makeCard("6")]) },
+            dealerHand: [makeCard("7"), makeCard("8")],
+            turnOrder: ["p1"],
+        });
+        expect(blackjackPlugin.isGameOver!(state)).toBeNull();
+    });
+
+    it("returns per-player payouts for mixed results", () => {
+        const state = riggedState({
+            hands: {
+                "p1": { cards: [makeCard("A"), makeCard("K")], status: "blackjack" as const, result: "blackjack" as const, resultReason: "Blackjack!" },
+                "p2": { cards: [makeCard("K"), makeCard("9")], status: "stood" as const, result: "win" as const, resultReason: "Higher hand" },
+                "p3": { cards: [makeCard("7"), makeCard("8")], status: "stood" as const, result: "lose" as const, resultReason: "Lower hand" },
+                "p4": { cards: [makeCard("K"), makeCard("8")], status: "stood" as const, result: "push" as const, resultReason: "Push" },
+            },
+            dealerHand: [makeCard("K"), makeCard("8")],
+            turnOrder: ["p1", "p2", "p3", "p4"],
+            phase: "resolved",
+            activePlayerIndex: -1,
+        });
+
+        const result = blackjackPlugin.isGameOver!(state)!;
+        expect(result).not.toBeNull();
+        expect(result.payouts?.["p1"]).toBe(2.5);  // blackjack
+        expect(result.payouts?.["p2"]).toBe(2);     // win
+        expect(result.payouts?.["p3"]).toBeUndefined(); // loss
+        expect(result.payouts?.["p4"]).toBe(1);     // push
+    });
+
+    it("generates a summary reason", () => {
+        const state = riggedState({
+            hands: {
+                "p1": { cards: [makeCard("K"), makeCard("9")], status: "stood" as const, result: "win" as const, resultReason: "Win" },
+                "p2": { cards: [makeCard("7"), makeCard("8")], status: "bust" as const, result: "lose" as const, resultReason: "Bust" },
+            },
+            dealerHand: [makeCard("K"), makeCard("8")],
+            turnOrder: ["p1", "p2"],
+            phase: "resolved",
+            activePlayerIndex: -1,
+        });
+
+        const result = blackjackPlugin.isGameOver!(state)!;
+        expect(result.reason).toContain("1 win");
+        expect(result.reason).toContain("1 loss");
+    });
+});
+
+// ── onPlayerDisconnect ──
+
+describe("onPlayerDisconnect", () => {
+    it("marks disconnected player as bust and advances turn", () => {
+        const state = riggedState({
+            hands: {
+                "p1": playingHand([makeCard("5"), makeCard("6")]),
+                "p2": playingHand([makeCard("7"), makeCard("8")]),
+            },
+            dealerHand: [makeCard("9"), makeCard("10")],
+            turnOrder: ["p1", "p2"],
+            activePlayerIndex: 0,
+        });
+
+        const newState = blackjackPlugin.onPlayerDisconnect!(state, "p1");
+        expect(newState.hands["p1"]!.status).toBe("bust");
+        expect(newState.hands["p1"]!.result).toBe("lose");
+        expect(newState.activePlayerIndex).toBe(1); // advanced to p2
+    });
+
+    it("resolves game if disconnected player was last active", () => {
+        const state = riggedState({
+            hands: {
+                "p1": playingHand([makeCard("5"), makeCard("6")]),
+            },
+            dealerHand: [makeCard("9"), makeCard("10")],
+            turnOrder: ["p1"],
+            activePlayerIndex: 0,
+        });
+
+        const newState = blackjackPlugin.onPlayerDisconnect!(state, "p1");
+        expect(newState.phase).toBe("resolved");
+        expect(newState.hands["p1"]!.result).toBe("lose");
+    });
+
+    it("does nothing for already-resolved game", () => {
+        const state = riggedState({
+            hands: {
+                "p1": { cards: [makeCard("K"), makeCard("9")], status: "stood" as const, result: "win" as const, resultReason: "Win" },
+            },
+            dealerHand: [makeCard("7"), makeCard("8")],
+            turnOrder: ["p1"],
+            phase: "resolved",
+            activePlayerIndex: -1,
+        });
+
+        const newState = blackjackPlugin.onPlayerDisconnect!(state, "p1");
+        expect(newState.hands["p1"]!.result).toBe("win"); // unchanged
+    });
+});

shared/games/blackjack/blackjack.plugin.ts

@@ -0,0 +1,395 @@
+import type { GamePlugin, GameResult, GameOverResult } from "../types";
+import type {
+    BlackjackState, BlackjackAction, BlackjackPlayerView,
+    BlackjackSpectatorView, PlayerHandView, Card, Suit, Rank, PlayerHand,
+} from "./blackjack.types";
+
+// ── Card helpers ──
+
+const SUITS: Suit[] = ["hearts", "diamonds", "clubs", "spades"];
+const RANKS: Rank[] = ["A", "2", "3", "4", "5", "6", "7", "8", "9", "10", "J", "Q", "K"];
+
+function createDeck(): Card[] {
+    const deck: Card[] = [];
+    for (const suit of SUITS) {
+        for (const rank of RANKS) {
+            deck.push({ suit, rank });
+        }
+    }
+    return deck;
+}
+
+function shuffleDeck(deck: Card[]): Card[] {
+    const shuffled = [...deck];
+    for (let i = shuffled.length - 1; i > 0; i--) {
+        const j = Math.floor(Math.random() * (i + 1));
+        [shuffled[i]!, shuffled[j]!] = [shuffled[j]!, shuffled[i]!];
+    }
+    return shuffled;
+}
+
+function cardValue(rank: Rank): number {
+    if (rank === "A") return 11;
+    if (["K", "Q", "J"].includes(rank)) return 10;
+    return parseInt(rank);
+}
+
+/** Calculate the best hand value (accounting for soft aces). */
+export function handValue(hand: Card[]): number {
+    let total = 0;
+    let aces = 0;
+    for (const card of hand) {
+        total += cardValue(card.rank);
+        if (card.rank === "A") aces++;
+    }
+    while (total > 21 && aces > 0) {
+        total -= 10;
+        aces--;
+    }
+    return total;
+}
+
+function isBust(hand: Card[]): boolean {
+    return handValue(hand) > 21;
+}
+
+function isNaturalBlackjack(hand: Card[]): boolean {
+    return hand.length === 2 && handValue(hand) === 21;
+}
+
+function drawCard(deck: Card[]): [Card, Card[]] {
+    return [deck[0]!, deck.slice(1)];
+}
+
+/** Play dealer hand: hit until 17+. */
+function playDealerHand(dealerHand: Card[], deck: Card[]): { dealerHand: Card[]; deck: Card[] } {
+    let hand = [...dealerHand];
+    let remaining = [...deck];
+    while (handValue(hand) < 17) {
+        const [card, rest] = drawCard(remaining);
+        hand = [...hand, card];
+        remaining = rest;
+    }
+    return { dealerHand: hand, deck: remaining };
+}
+
+/** Find the next player who still needs to act (skip blackjack/bust/stood). */
+function findNextActiveIndex(state: BlackjackState, afterIndex: number): number {
+    for (let i = afterIndex + 1; i < state.turnOrder.length; i++) {
+        const hand = state.hands[state.turnOrder[i]!];
+        if (hand && hand.status === "playing") return i;
+    }
+    return -1; // no more active players
+}
+
+/** Resolve all hands against the dealer, returning updated hands. */
+function resolveAllHands(
+    hands: Record<string, PlayerHand>,
+    dealerHand: Card[],
+): Record<string, PlayerHand> {
+    const dealerVal = handValue(dealerHand);
+    const dealerBust = isBust(dealerHand);
+    const resolved: Record<string, PlayerHand> = {};
+
+    for (const [id, hand] of Object.entries(hands)) {
+        // Already resolved (natural blackjack checked at deal time, bust on hit)
+        if (hand.result) {
+            resolved[id] = hand;
+            continue;
+        }
+
+        const playerVal = handValue(hand.cards);
+
+        if (hand.status === "bust") {
+            resolved[id] = { ...hand, result: "lose", resultReason: "Player busts" };
+        } else if (hand.status === "blackjack") {
+            // Natural blackjack vs dealer blackjack
+            if (isNaturalBlackjack(dealerHand)) {
+                resolved[id] = { ...hand, result: "push", resultReason: "Both have Blackjack" };
+            } else {
+                resolved[id] = { ...hand, result: "blackjack", resultReason: "Blackjack!" };
+            }
+        } else if (dealerBust) {
+            resolved[id] = { ...hand, result: "win", resultReason: "Dealer busts" };
+        } else if (playerVal > dealerVal) {
+            resolved[id] = { ...hand, result: "win", resultReason: "Higher hand" };
+        } else if (playerVal < dealerVal) {
+            resolved[id] = { ...hand, result: "lose", resultReason: "Dealer has higher hand" };
+        } else {
+            resolved[id] = { ...hand, result: "push", resultReason: "Push" };
+        }
+    }
+
+    return resolved;
+}
+
+/** Build the dealer hand for views — hole card hidden during player_turns. */
+function dealerVisibleHand(state: BlackjackState): Card[] {
+    if (state.phase === "resolved") return state.dealerHand;
+    return [state.dealerHand[0]!, { suit: "spades", rank: "?" as Rank }];
+}
+
+/** Convert internal PlayerHand to a view. */
+function toHandView(hand: PlayerHand): PlayerHandView {
+    return {
+        cards: hand.cards,
+        value: handValue(hand.cards),
+        status: hand.status,
+        result: hand.result,
+        resultReason: hand.resultReason,
+    };
+}
+
+/** Transition to dealer turn + resolve, or just resolve if all busted/blackjacked. */
+function finishPlayerTurns(state: BlackjackState): BlackjackState {
+    // Check if any player stood (dealer needs to play)
+    const anyStood = Object.values(state.hands).some(h => h.status === "stood");
+
+    let dealerHand = state.dealerHand;
+    let deck = state.deck;
+
+    if (anyStood) {
+        const dealer = playDealerHand(dealerHand, deck);
+        dealerHand = dealer.dealerHand;
+        deck = dealer.deck;
+    }
+
+    const resolvedHands = resolveAllHands(state.hands, dealerHand);
+
+    return {
+        ...state,
+        deck,
+        dealerHand,
+        hands: resolvedHands,
+        activePlayerIndex: -1,
+        phase: "resolved",
+    };
+}
+
+// ── Plugin ──
+
+export const blackjackPlugin: GamePlugin<BlackjackState, BlackjackAction> = {
+    slug: "blackjack",
+    name: "Blackjack",
+    minPlayers: 1,
+    maxPlayers: 6,
+    manualStart: true,
+
+    createInitialState(players: string[], _options?: Record<string, unknown>): BlackjackState {
+        let deck = shuffleDeck(createDeck());
+        const turnOrder = [...players];
+        const hands: Record<string, PlayerHand> = {};
+
+        // Deal 2 cards to each player
+        for (const pid of turnOrder) {
+            const cards: Card[] = [];
+            let card: Card;
+            [card, deck] = drawCard(deck); cards.push(card);
+            [card, deck] = drawCard(deck); cards.push(card);
+            hands[pid] = {
+                cards,
+                status: isNaturalBlackjack(cards) ? "blackjack" : "playing",
+                result: null,
+                resultReason: null,
+            };
+        }
+
+        // Deal 2 cards to dealer
+        const dealerHand: Card[] = [];
+        let card: Card;
+        [card, deck] = drawCard(deck); dealerHand.push(card);
+        [card, deck] = drawCard(deck); dealerHand.push(card);
+
+        let state: BlackjackState = {
+            deck,
+            dealerHand,
+            hands,
+            turnOrder,
+            activePlayerIndex: 0,
+            phase: "player_turns",
+        };
+
+        // Skip to first player that needs to act (skip natural blackjacks)
+        const firstActive = findNextActiveIndex(state, -1);
+        state.activePlayerIndex = firstActive;
+
+        // If no players need to act (all natural blackjacks), go straight to resolution
+        if (firstActive === -1) {
+            return finishPlayerTurns(state);
+        }
+
+        return state;
+    },
+
+    handleAction(state: BlackjackState, action: BlackjackAction, playerId: string): GameResult<BlackjackState> {
+        if (state.phase === "resolved") return { ok: false, error: "Game is already over" };
+
+        const activeId = state.turnOrder[state.activePlayerIndex];
+        if (playerId !== activeId) return { ok: false, error: "It's not your turn" };
+
+        const hand = state.hands[playerId];
+        if (!hand || hand.status !== "playing") return { ok: false, error: "You cannot act" };
+
+        switch (action.type) {
+            case "hit": {
+                const [card, remaining] = drawCard(state.deck);
+                const newCards = [...hand.cards, card];
+                const bust = isBust(newCards);
+                const got21 = handValue(newCards) === 21;
+
+                const newHand: PlayerHand = {
+                    ...hand,
+                    cards: newCards,
+                    status: bust ? "bust" : got21 ? "stood" : "playing",
+                    result: bust ? "lose" : null,
+                    resultReason: bust ? "Player busts" : null,
+                };
+
+                const newHands = { ...state.hands, [playerId]: newHand };
+                let newState: BlackjackState = { ...state, deck: remaining, hands: newHands };
+
+                // Advance turn if bust or auto-stood on 21
+                if (bust || got21) {
+                    const nextIdx = findNextActiveIndex(newState, state.activePlayerIndex);
+                    if (nextIdx === -1) {
+                        return { ok: true, state: finishPlayerTurns(newState) };
+                    }
+                    newState = { ...newState, activePlayerIndex: nextIdx };
+                }
+
+                return { ok: true, state: newState };
+            }
+
+            case "stand": {
+                const newHand: PlayerHand = { ...hand, status: "stood" };
+                const newHands = { ...state.hands, [playerId]: newHand };
+                let newState: BlackjackState = { ...state, hands: newHands };
+
+                const nextIdx = findNextActiveIndex(newState, state.activePlayerIndex);
+                if (nextIdx === -1) {
+                    return { ok: true, state: finishPlayerTurns(newState) };
+                }
+                newState = { ...newState, activePlayerIndex: nextIdx };
+
+                return { ok: true, state: newState };
+            }
+
+            default:
+                return { ok: false, error: "Unknown action type" };
+        }
+    },
+
+    getPlayerView(state: BlackjackState, playerId: string): BlackjackPlayerView {
+        const visibleDealer = dealerVisibleHand(state);
+        const handsView: Record<string, PlayerHandView> = {};
+        for (const [id, hand] of Object.entries(state.hands)) {
+            handsView[id] = toHandView(hand);
+        }
+
+        const activeId = state.activePlayerIndex >= 0 ? state.turnOrder[state.activePlayerIndex] ?? null : null;
+
+        return {
+            dealerHand: visibleDealer,
+            dealerVisibleValue: cardValue(state.dealerHand[0]!.rank),
+            dealerFullValue: state.phase === "resolved" ? handValue(state.dealerHand) : null,
+            hands: handsView,
+            turnOrder: state.turnOrder,
+            activePlayerId: activeId,
+            myPlayerId: playerId,
+            phase: state.phase,
+            canAct: activeId === playerId && state.phase === "player_turns",
+        };
+    },
+
+    getSpectatorView(state: BlackjackState): BlackjackSpectatorView {
+        const visibleDealer = dealerVisibleHand(state);
+        const handsView: Record<string, PlayerHandView> = {};
+        for (const [id, hand] of Object.entries(state.hands)) {
+            handsView[id] = toHandView(hand);
+        }
+
+        const activeId = state.activePlayerIndex >= 0 ? state.turnOrder[state.activePlayerIndex] ?? null : null;
+
+        return {
+            dealerHand: visibleDealer,
+            dealerVisibleValue: cardValue(state.dealerHand[0]!.rank),
+            dealerFullValue: state.phase === "resolved" ? handValue(state.dealerHand) : null,
+            hands: handsView,
+            turnOrder: state.turnOrder,
+            activePlayerId: activeId,
+            phase: state.phase,
+        };
+    },
+
+    isGameOver(state: BlackjackState): GameOverResult | null {
+        if (state.phase !== "resolved") return null;
+
+        const payouts: Record<string, number> = {};
+
+        for (const [id, hand] of Object.entries(state.hands)) {
+            switch (hand.result) {
+                case "blackjack":
+                    payouts[id] = 2.5; // 3:2 payout
+                    break;
+                case "win":
+                    payouts[id] = 2; // 1:1 payout
+                    break;
+                case "push":
+                    payouts[id] = 1; // refund
+                    break;
+                // "lose" / null → no payout
+            }
+        }
+
+        // Find a "winner" for the room summary — pick any winning player, or null
+        const winner = Object.entries(state.hands).find(
+            ([, h]) => h.result === "blackjack" || h.result === "win"
+        )?.[0] ?? null;
+
+        const wins = Object.values(state.hands).filter(h => h.result === "win" || h.result === "blackjack").length;
+        const losses = Object.values(state.hands).filter(h => h.result === "lose").length;
+        const pushes = Object.values(state.hands).filter(h => h.result === "push").length;
+        const parts: string[] = [];
+        if (wins > 0) parts.push(`${wins} win${wins > 1 ? "s" : ""}`);
+        if (losses > 0) parts.push(`${losses} loss${losses > 1 ? "es" : ""}`);
+        if (pushes > 0) parts.push(`${pushes} push${pushes > 1 ? "es" : ""}`);
+
+        return {
+            winner,
+            reason: parts.join(", ") || "Game over",
+            payouts,
+        };
+    },
+
+    onPlayerDisconnect(state: BlackjackState, playerId: string): BlackjackState {
+        if (state.phase === "resolved") return state;
+
+        const hand = state.hands[playerId];
+        if (!hand || hand.status !== "playing") return state;
+
+        // Mark disconnected player as bust
+        const newHands = {
+            ...state.hands,
+            [playerId]: { ...hand, status: "bust" as const, result: "lose" as const, resultReason: "Disconnected" },
+        };
+        let newState: BlackjackState = { ...state, hands: newHands };
+
+        // Check if the disconnected player was the active player
+        const activeId = state.turnOrder[state.activePlayerIndex];
+        if (activeId === playerId) {
+            const nextIdx = findNextActiveIndex(newState, state.activePlayerIndex);
+            if (nextIdx === -1) {
+                return finishPlayerTurns(newState);
+            }
+            newState = { ...newState, activePlayerIndex: nextIdx };
+        }
+
+        // Check if all players are now done
+        const anyPlaying = Object.values(newState.hands).some(h => h.status === "playing");
+        if (!anyPlaying && newState.phase === "player_turns") {
+            return finishPlayerTurns(newState);
+        }
+
+        return newState;
+    },
+};

shared/games/blackjack/blackjack.types.ts

@@ -0,0 +1,65 @@
+export type Suit = "hearts" | "diamonds" | "clubs" | "spades";
+export type Rank = "A" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" | "10" | "J" | "Q" | "K";
+
+export interface Card {
+    suit: Suit;
+    rank: Rank;
+}
+
+// ── Per-player hand state ──
+
+export interface PlayerHand {
+    cards: Card[];
+    status: "playing" | "stood" | "bust" | "blackjack";
+    result: "win" | "blackjack" | "push" | "lose" | null;
+    resultReason: string | null;
+}
+
+// ── Game state ──
+
+export interface BlackjackState {
+    deck: Card[];
+    dealerHand: Card[];
+    hands: Record<string, PlayerHand>;
+    turnOrder: string[];
+    activePlayerIndex: number; // index into turnOrder, -1 when no active player
+    phase: "player_turns" | "resolved";
+}
+
+// ── Actions ──
+
+export type BlackjackAction =
+    | { type: "hit" }
+    | { type: "stand" };
+
+// ── Views ──
+
+export interface PlayerHandView {
+    cards: Card[];
+    value: number;
+    status: "playing" | "stood" | "bust" | "blackjack";
+    result: "win" | "blackjack" | "push" | "lose" | null;
+    resultReason: string | null;
+}
+
+export interface BlackjackPlayerView {
+    dealerHand: Card[];
+    dealerVisibleValue: number;
+    dealerFullValue: number | null;
+    hands: Record<string, PlayerHandView>;
+    turnOrder: string[];
+    activePlayerId: string | null;
+    myPlayerId: string;
+    phase: "player_turns" | "resolved";
+    canAct: boolean;
+}
+
+export interface BlackjackSpectatorView {
+    dealerHand: Card[];
+    dealerVisibleValue: number;
+    dealerFullValue: number | null;
+    hands: Record<string, PlayerHandView>;
+    turnOrder: string[];
+    activePlayerId: string | null;
+    phase: "player_turns" | "resolved";
+}