diff --git a/src/matchmaking/matchmaking-lobby.service.spec.ts b/src/matchmaking/matchmaking-lobby.service.spec.ts
new file mode 100644
index 00000000..9e0c1367
--- /dev/null
+++ b/src/matchmaking/matchmaking-lobby.service.spec.ts
@@ -0,0 +1,282 @@
+import { Logger } from "@nestjs/common";
+import Redis from "ioredis";
+import { e_match_types_enum } from "generated";
+
+import { MatchmakingLobbyService } from "./matchmaking-lobby.service";
+import { HasuraService } from "../hasura/hasura.service";
+import { MatchmakeService } from "./matchmake.service";
+import { PlayerLobby } from "./types/PlayerLobby";
+import { JoinQueueError } from "./utilities/joinQueueError";
+import { User } from "../auth/types/User";
+import { ExpectedPlayers } from "../discord-bot/enums/ExpectedPlayers";
+
+/**
+ * The party-size rule: a lobby may queue when it fits in one lineup (<= half
+ * the match) or fills the whole match exactly. Everything else is unplaceable.
+ */
+describe("MatchmakingLobbyService.verifyLobby", () => {
+  let service: MatchmakingLobbyService;
+  let mockHasura: jest.Mocked<HasuraService>;
+
+  const captainSteamId = "steam-id-1";
+  const captain = { steam_id: captainSteamId } as User;
+
+  // Premier and Faceit are demo-import only — they are never matchmade.
+  const matchmakingTypes: e_match_types_enum[] = [
+    "Duel",
+    "Wingman",
+    "Competitive",
+  ];
+
+  const buildLobby = (playerCount: number): PlayerLobby => ({
+    id: "lobby-1",
+    players: Array.from({ length: playerCount }, (_, index) => ({
+      captain: index === 0,
+      name: `player-${index + 1}`,
+      steam_id: index === 0 ? captainSteamId : `steam-id-${index + 1}`,
+      is_banned: false,
+      matchmaking_cooldown: false,
+    })),
+  });
+
+  const canQueue = async (type: e_match_types_enum, partySize: number) => {
+    try {
+      return await service.verifyLobby(buildLobby(partySize), captain, type);
+    } catch (error) {
+      if (error instanceof JoinQueueError) {
+        return false;
+      }
+      throw error;
+    }
+  };
+
+  beforeEach(() => {
+    // verifyPlayer runs per lobby member once the size check passes — every
+    // player comes back clean so only the party-size rule can fail.
+    mockHasura = {
+      query: jest.fn().mockImplementation(({ players_by_pk }) => ({
+        players_by_pk: {
+          name: "player",
+          steam_id: players_by_pk.__args.steam_id,
+          is_banned: false,
+          matchmaking_cooldown: false,
+          current_lobby_id: "lobby-1",
+          is_in_another_match: false,
+        },
+      })),
+    } as any;
+
+    const mockRedisManager = {
+      getConnection: jest.fn().mockReturnValue({} as Redis),
+    } as any;
+
+    service = new MatchmakingLobbyService(
+      new Logger("Test"),
+      mockHasura,
+      mockRedisManager,
+      {} as MatchmakeService,
+    );
+  });
+
+  it("rejects a player who is not the lobby captain", async () => {
+    await expect(
+      service.verifyLobby(
+        buildLobby(2),
+        { steam_id: "steam-id-2" } as User,
+        "Competitive",
+      ),
+    ).rejects.toThrow("you are not the captain of this lobby");
+  });
+
+  describe("Duel (2 players, 1v1)", () => {
+    const type: e_match_types_enum = "Duel";
+
+    it("accepts a solo player — fills one lineup", async () => {
+      await expect(canQueue(type, 1)).resolves.toBe(true);
+    });
+
+    it("accepts a full party of 2 — both lineups, split in-house", async () => {
+      await expect(canQueue(type, 2)).resolves.toBe(true);
+    });
+
+    it.each([3, 4, 5, 10, 11])("rejects a party of %i", async (size) => {
+      await expect(canQueue(type, size)).resolves.toBe(false);
+    });
+
+    it("explains the requirement", async () => {
+      await expect(
+        service.verifyLobby(buildLobby(3), captain, type),
+      ).rejects.toThrow(
+        "To join a Duel match, your lobby must have 1 or fewer players, or exactly 2 players. You have 3.",
+      );
+    });
+  });
+
+  describe("Wingman (4 players, 2v2)", () => {
+    const type: e_match_types_enum = "Wingman";
+
+    it.each([1, 2])("accepts a party of %i — fits one lineup", async (size) => {
+      await expect(canQueue(type, size)).resolves.toBe(true);
+    });
+
+    it("accepts a full party of 4", async () => {
+      await expect(canQueue(type, 4)).resolves.toBe(true);
+    });
+
+    it("rejects a party of 3 — too big for one lineup, too small for two", async () => {
+      await expect(canQueue(type, 3)).resolves.toBe(false);
+    });
+
+    it.each([5, 6, 10, 11])("rejects a party of %i", async (size) => {
+      await expect(canQueue(type, size)).resolves.toBe(false);
+    });
+
+    it("explains the requirement", async () => {
+      await expect(
+        service.verifyLobby(buildLobby(3), captain, type),
+      ).rejects.toThrow(
+        "To join a Wingman match, your lobby must have 2 or fewer players, or exactly 4 players. You have 3.",
+      );
+    });
+  });
+
+  describe("Competitive (10 players, 5v5)", () => {
+    const type: e_match_types_enum = "Competitive";
+
+    it.each([1, 2, 3, 4, 5])(
+      "accepts a party of %i — fits one lineup",
+      async (size) => {
+        await expect(canQueue(type, size)).resolves.toBe(true);
+      },
+    );
+
+    it("accepts a full party of 10", async () => {
+      await expect(canQueue(type, 10)).resolves.toBe(true);
+    });
+
+    it.each([6, 7, 8, 9])(
+      "rejects a party of %i — between one lineup and a full match",
+      async (size) => {
+        await expect(canQueue(type, size)).resolves.toBe(false);
+      },
+    );
+
+    it.each([11, 12, 15, 20])(
+      "rejects a party of %i — larger than the match itself",
+      async (size) => {
+        await expect(canQueue(type, size)).resolves.toBe(false);
+      },
+    );
+
+    it("explains the requirement", async () => {
+      await expect(
+        service.verifyLobby(buildLobby(7), captain, type),
+      ).rejects.toThrow(
+        "To join a Competitive match, your lobby must have 5 or fewer players, or exactly 10 players. You have 7.",
+      );
+    });
+
+    it("reports the party size when it is over the full match size", async () => {
+      await expect(
+        service.verifyLobby(buildLobby(11), captain, type),
+      ).rejects.toThrow("You have 11.");
+    });
+  });
+
+  describe("the party sizes from the queue matrix", () => {
+    const queueableTypes = async (size: number) => {
+      const queueable: e_match_types_enum[] = [];
+      for (const type of matchmakingTypes) {
+        if (await canQueue(type, size)) {
+          queueable.push(type);
+        }
+      }
+      return queueable;
+    };
+
+    it("1 — every mode", async () => {
+      await expect(queueableTypes(1)).resolves.toEqual([
+        "Duel",
+        "Wingman",
+        "Competitive",
+      ]);
+    });
+
+    it("2 — every mode (full Duel, half a Wingman)", async () => {
+      await expect(queueableTypes(2)).resolves.toEqual([
+        "Duel",
+        "Wingman",
+        "Competitive",
+      ]);
+    });
+
+    it("3 — Competitive only", async () => {
+      await expect(queueableTypes(3)).resolves.toEqual(["Competitive"]);
+    });
+
+    it("4 — Wingman and Competitive", async () => {
+      await expect(queueableTypes(4)).resolves.toEqual([
+        "Wingman",
+        "Competitive",
+      ]);
+    });
+
+    it("5 — Competitive only", async () => {
+      await expect(queueableTypes(5)).resolves.toEqual(["Competitive"]);
+    });
+
+    it.each([6, 7, 8, 9])("%i — nothing is queueable", async (size) => {
+      await expect(queueableTypes(size)).resolves.toEqual([]);
+    });
+
+    it("10 — Competitive only", async () => {
+      await expect(queueableTypes(10)).resolves.toEqual(["Competitive"]);
+    });
+
+    it.each([11, 12, 20])(
+      "%i — nothing is queueable, over every match size",
+      async (size) => {
+        await expect(queueableTypes(size)).resolves.toEqual([]);
+      },
+    );
+  });
+
+  it("stays in sync with ExpectedPlayers for every matchmaking type", async () => {
+    for (const type of matchmakingTypes) {
+      const expected = ExpectedPlayers[type];
+      const half = expected / 2;
+
+      await expect(canQueue(type, half)).resolves.toBe(true);
+      await expect(canQueue(type, half + 1)).resolves.toBe(
+        half + 1 === expected,
+      );
+      await expect(canQueue(type, expected)).resolves.toBe(true);
+      await expect(canQueue(type, expected + 1)).resolves.toBe(false);
+    }
+  });
+
+  it("does not run per-player verification when the party size is invalid", async () => {
+    await expect(
+      service.verifyLobby(buildLobby(7), captain, "Competitive"),
+    ).rejects.toThrow(JoinQueueError);
+
+    expect(mockHasura.query).not.toHaveBeenCalled();
+  });
+
+  it("still rejects a valid-sized party when a member is banned", async () => {
+    mockHasura.query.mockResolvedValueOnce({
+      players_by_pk: {
+        name: "banned-player",
+        steam_id: captainSteamId,
+        is_banned: true,
+        matchmaking_cooldown: false,
+        current_lobby_id: "lobby-1",
+        is_in_another_match: false,
+      },
+    } as any);
+
+    await expect(
+      service.verifyLobby(buildLobby(5), captain, "Competitive"),
+    ).rejects.toThrow("banned-player is banned");
+  });
+});
diff --git a/src/matchmaking/matchmaking-lobby.service.ts b/src/matchmaking/matchmaking-lobby.service.ts
index 0caa28ac..8bfd23b7 100644
--- a/src/matchmaking/matchmaking-lobby.service.ts
+++ b/src/matchmaking/matchmaking-lobby.service.ts
@@ -15,6 +15,7 @@ import {
   getMatchmakingLobbyDetailsCacheKey,
 } from "./utilities/cacheKeys";
 import { JoinQueueError } from "./utilities/joinQueueError";
+import { ExpectedPlayers } from "src/discord-bot/enums/ExpectedPlayers";
 
 @Injectable()
 export class MatchmakingLobbyService {
@@ -117,30 +118,10 @@ export class MatchmakingLobbyService {
       throw new JoinQueueError(`you are not the captain of this lobby`);
     }
 
-    const totalPlayers = lobby.players.length;
+    const partySizeError = this.getPartySizeError(type, lobby.players.length);
 
-    switch (type) {
-      case "Competitive":
-        if (totalPlayers > 5 && totalPlayers !== 10) {
-          throw new JoinQueueError(
-            `To join a Competitive match, with a lobby greater than 5 players, you must have 10 players in your lobby`,
-          );
-        }
-        break;
-      case "Wingman":
-        if (totalPlayers > 2 && totalPlayers !== 4) {
-          throw new JoinQueueError(
-            `To join a Wingman match, with a lobby greater than 2 players, you must have 4 players in your lobby`,
-          );
-        }
-        break;
-      case "Duel":
-        if (totalPlayers > 1 && totalPlayers !== 2) {
-          throw new JoinQueueError(
-            `To join a Duel match, with a lobby greater than 1 player you must have 2 players in your lobby`,
-          );
-        }
-        break;
+    if (partySizeError) {
+      throw new JoinQueueError(partySizeError);
     }
 
     for (const player of lobby.players) {
@@ -424,6 +405,37 @@ export class MatchmakingLobbyService {
     return players_by_pk.current_lobby_id || steamId;
   }
 
+  /**
+   * A party may queue a match type when it either fits inside a single lineup
+   * (half the match, matchmaking fills the other half) or fills the entire
+   * match on its own (both lineups, split in-house). Anything between those two
+   * — or anything above the full match size — cannot be placed.
+   *
+   * Duel (2): 1 or 2 · Wingman (4): 1-2 or 4 · Competitive (10): 1-5 or 10
+   */
+  private canPartyQueue(type: e_match_types_enum, partySize: number): boolean {
+    const expected = ExpectedPlayers[type];
+
+    if (!expected) {
+      return true;
+    }
+
+    return partySize <= expected / 2 || partySize === expected;
+  }
+
+  private getPartySizeError(
+    type: e_match_types_enum,
+    partySize: number,
+  ): string | undefined {
+    if (this.canPartyQueue(type, partySize)) {
+      return;
+    }
+
+    const expected = ExpectedPlayers[type];
+
+    return `To join a ${type} match, your lobby must have ${expected / 2} or fewer players, or exactly ${expected} players. You have ${partySize}.`;
+  }
+
   private async verifyPlayer(
     lobbyId: string,
     steamId: string,