diff --git a/src/room-manager.js b/src/room-manager.js
index 6d870ea..865f352 100644
--- a/src/room-manager.js
+++ b/src/room-manager.js
@@ -112,8 +112,9 @@ export class RoomManager {
     if (roomId == null) throw new TypeError("roomId is required");
 
     const room = this._rooms.get(roomId);
-    if (room) {
+    if (room && room.has(clientId)) {
       room.delete(clientId);
+      this._totalMembers--;
       this._cleanupRoom(roomId);
     }
 
@@ -272,6 +273,7 @@ export class RoomManager {
 
     const rooms = this._clientRooms.get(clientId);
     if (rooms) {
+      this._totalMembers -= rooms.size;
       for (const roomId of rooms) {
         const room = this._rooms.get(roomId);
         if (room) {
@@ -324,4 +326,17 @@ export class RoomManager {
   get clientCount() {
     return this._clientRooms.size;
   }
+
+  /**
+   * RoomManager statistics and metrics.
+   * @type {{ roomCount: number, clientCount: number, totalMembers: number, circuitBreakerState: string }}
+   */
+  get stats() {
+    return {
+      roomCount: this._rooms.size,
+      clientCount: this._clientRooms.size,
+      totalMembers: this._totalMembers,
+      circuitBreakerState: this._circuitBreakerState,
+    };
+  }
 }
diff --git a/tests/room-manager-dos.test.js b/tests/room-manager-dos.test.js
new file mode 100644
index 0000000..049bd90
--- /dev/null
+++ b/tests/room-manager-dos.test.js
@@ -0,0 +1,228 @@
+import { describe, it, expect, vi, afterEach } from "vitest";
+import { RoomManager } from "../src/room-manager.js";
+
+describe("RoomManager — DoS protection and limits", () => {
+  let rooms;
+  const fakeWs = { readyState: 1, send: () => {} };
+
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  describe("Constructor defaults & stats getter", () => {
+    it("provides sane default stats initially", () => {
+      rooms = new RoomManager();
+      expect(rooms.stats).toEqual({
+        roomCount: 0,
+        clientCount: 0,
+        totalMembers: 0,
+        circuitBreakerState: "CLOSED",
+      });
+    });
+
+    it("accurately tracks totalMembers across multiple joins and leaves", () => {
+      rooms = new RoomManager();
+      rooms.join("c1", "r1", fakeWs);
+      rooms.join("c1", "r2", fakeWs);
+      rooms.join("c2", "r1", fakeWs);
+
+      expect(rooms.stats).toEqual({
+        roomCount: 2,
+        clientCount: 2,
+        totalMembers: 3,
+        circuitBreakerState: "CLOSED",
+      });
+
+      rooms.leave("c1", "r1");
+      expect(rooms.stats.totalMembers).toBe(2);
+
+      rooms.disconnect("c1");
+      expect(rooms.stats.totalMembers).toBe(1);
+      expect(rooms.stats.clientCount).toBe(1);
+
+      rooms.disconnect("c2");
+      expect(rooms.stats.totalMembers).toBe(0);
+      expect(rooms.stats.clientCount).toBe(0);
+      expect(rooms.stats.roomCount).toBe(0);
+    });
+  });
+
+  describe("Per-client room limit (maxRoomsPerClient)", () => {
+    it("allows joining up to maxRoomsPerClient rooms", () => {
+      rooms = new RoomManager({ maxRoomsPerClient: 3 });
+      expect(rooms.join("c1", "r1", fakeWs)).toBeUndefined();
+      expect(rooms.join("c1", "r2", fakeWs)).toBeUndefined();
+      expect(rooms.join("c1", "r3", fakeWs)).toBeUndefined();
+      expect(rooms.getClientRooms("c1").size).toBe(3);
+    });
+
+    it("rejects joins when maxRoomsPerClient is exceeded", () => {
+      rooms = new RoomManager({ maxRoomsPerClient: 2 });
+      rooms.join("c1", "r1", fakeWs);
+      rooms.join("c1", "r2", fakeWs);
+
+      const err = rooms.join("c1", "r3", fakeWs);
+      expect(err).toEqual({
+        type: "error",
+        payload: {
+          code: "ROOM_LIMIT_EXCEEDED",
+          message: "Client room limit exceeded (2)",
+        },
+      });
+      expect(rooms.getClientRooms("c1").size).toBe(2);
+    });
+
+    it("allows re-joining an already joined room even if at maxRoomsPerClient", () => {
+      rooms = new RoomManager({ maxRoomsPerClient: 2 });
+      rooms.join("c1", "r1", fakeWs);
+      rooms.join("c1", "r2", fakeWs);
+
+      const newWs = { readyState: 1, send: () => {} };
+      expect(rooms.join("c1", "r1", newWs)).toBeUndefined();
+      expect(rooms.getClientRooms("c1").size).toBe(2);
+    });
+  });
+
+  describe("Per-room member limit (maxMembersPerRoom)", () => {
+    it("allows joining a room up to maxMembersPerRoom", () => {
+      rooms = new RoomManager({ maxMembersPerRoom: 2 });
+      expect(rooms.join("c1", "r1", fakeWs)).toBeUndefined();
+      expect(rooms.join("c2", "r1", fakeWs)).toBeUndefined();
+      expect(rooms.getRoomSize("r1")).toBe(2);
+    });
+
+    it("rejects new members when maxMembersPerRoom is exceeded", () => {
+      rooms = new RoomManager({ maxMembersPerRoom: 2 });
+      rooms.join("c1", "r1", fakeWs);
+      rooms.join("c2", "r1", fakeWs);
+
+      const err = rooms.join("c3", "r1", fakeWs);
+      expect(err).toEqual({
+        type: "error",
+        payload: {
+          code: "ROOM_FULL",
+          message: "Room member limit reached (2)",
+        },
+      });
+      expect(rooms.getRoomSize("r1")).toBe(2);
+    });
+
+    it("allows existing member to update socket even if room is full", () => {
+      rooms = new RoomManager({ maxMembersPerRoom: 2 });
+      rooms.join("c1", "r1", fakeWs);
+      rooms.join("c2", "r1", fakeWs);
+
+      const updatedWs = { readyState: 1, send: () => {} };
+      expect(rooms.join("c1", "r1", updatedWs)).toBeUndefined();
+      expect(rooms.getRoomSize("r1")).toBe(2);
+    });
+  });
+
+  describe("Total room count ceiling (maxRooms)", () => {
+    it("allows creating new rooms up to maxRooms", () => {
+      rooms = new RoomManager({ maxRooms: 2 });
+      expect(rooms.join("c1", "r1", fakeWs)).toBeUndefined();
+      expect(rooms.join("c2", "r2", fakeWs)).toBeUndefined();
+      expect(rooms.roomCount).toBe(2);
+    });
+
+    it("rejects first join to a new room when maxRooms is reached", () => {
+      rooms = new RoomManager({ maxRooms: 2 });
+      rooms.join("c1", "r1", fakeWs);
+      rooms.join("c2", "r2", fakeWs);
+
+      const err = rooms.join("c3", "r3", fakeWs);
+      expect(err).toEqual({
+        type: "error",
+        payload: {
+          code: "MAX_ROOMS_REACHED",
+          message: "Maximum room count ceiling reached (2)",
+        },
+      });
+      expect(rooms.roomCount).toBe(2);
+    });
+
+    it("allows joins to existing rooms when maxRooms is reached", () => {
+      rooms = new RoomManager({ maxRooms: 2 });
+      rooms.join("c1", "r1", fakeWs);
+      rooms.join("c2", "r2", fakeWs);
+
+      expect(rooms.join("c3", "r1", fakeWs)).toBeUndefined();
+      expect(rooms.getRoomSize("r1")).toBe(2);
+    });
+  });
+
+  describe("Resource-pressure circuit breaker", () => {
+    it("opens circuit breaker when heapUsed exceeds memoryThresholdBytes", () => {
+      const memorySpy = vi.spyOn(process, "memoryUsage");
+      memorySpy.mockReturnValue({ heapUsed: 1000, heapTotal: 2000, rss: 3000, external: 0, arrayBuffers: 0 });
+
+      rooms = new RoomManager({
+        circuitBreaker: {
+          enabled: true,
+          memoryThresholdBytes: 500,
+          recoveryThresholdBytes: 300,
+        },
+      });
+
+      const err = rooms.join("c1", "r1", fakeWs);
+      expect(err).toEqual({
+        type: "error",
+        payload: {
+          code: "CIRCUIT_BREAKER_OPEN",
+          message: "Circuit breaker is open due to high resource pressure",
+        },
+      });
+      expect(rooms.stats.circuitBreakerState).toBe("OPEN");
+      expect(rooms.roomCount).toBe(0);
+    });
+
+    it("recovers and closes circuit breaker when heapUsed drops below recoveryThresholdBytes", () => {
+      const memorySpy = vi.spyOn(process, "memoryUsage");
+
+      rooms = new RoomManager({
+        circuitBreaker: {
+          enabled: true,
+          memoryThresholdBytes: 500,
+          recoveryThresholdBytes: 300,
+        },
+      });
+
+      // Trip breaker
+      memorySpy.mockReturnValue({ heapUsed: 600, heapTotal: 1000, rss: 2000, external: 0, arrayBuffers: 0 });
+      const err = rooms.join("c1", "r1", fakeWs);
+      expect(err?.payload?.code).toBe("CIRCUIT_BREAKER_OPEN");
+      expect(rooms.stats.circuitBreakerState).toBe("OPEN");
+
+      // Memory pressure drops to 250 (< 300 recovery threshold)
+      memorySpy.mockReturnValue({ heapUsed: 250, heapTotal: 1000, rss: 2000, external: 0, arrayBuffers: 0 });
+      const res = rooms.join("c1", "r1", fakeWs);
+      expect(res).toBeUndefined();
+      expect(rooms.stats.circuitBreakerState).toBe("CLOSED");
+      expect(rooms.roomCount).toBe(1);
+    });
+
+    it("remains open if heapUsed is between recovery and memory thresholds", () => {
+      const memorySpy = vi.spyOn(process, "memoryUsage");
+
+      rooms = new RoomManager({
+        circuitBreaker: {
+          enabled: true,
+          memoryThresholdBytes: 500,
+          recoveryThresholdBytes: 300,
+        },
+      });
+
+      // Trip breaker
+      memorySpy.mockReturnValue({ heapUsed: 600, heapTotal: 1000, rss: 2000, external: 0, arrayBuffers: 0 });
+      rooms.join("c1", "r1", fakeWs);
+      expect(rooms.stats.circuitBreakerState).toBe("OPEN");
+
+      // Memory drops to 400 (between 300 and 500)
+      memorySpy.mockReturnValue({ heapUsed: 400, heapTotal: 1000, rss: 2000, external: 0, arrayBuffers: 0 });
+      const err = rooms.join("c1", "r1", fakeWs);
+      expect(err?.payload?.code).toBe("CIRCUIT_BREAKER_OPEN");
+      expect(rooms.stats.circuitBreakerState).toBe("OPEN");
+    });
+  });
+});