diff --git a/internal/hub/hub_integration_test.go b/internal/hub/hub_integration_test.go
index 25c6bae..2860517 100644
--- a/internal/hub/hub_integration_test.go
+++ b/internal/hub/hub_integration_test.go
@@ -24,7 +24,7 @@ func setupTestServer(t *testing.T) (*httptest.Server, *Hub) {
 	return server, h
 }
 
-// helper: dial a WebSocket connection to the test server
+// helper: dial a WebSocket connection to the test server (default room)
 func dialWS(t *testing.T, server *httptest.Server) *websocket.Conn {
 	t.Helper()
 	wsURL := "ws" + strings.TrimPrefix(server.URL, "http")
@@ -35,6 +35,17 @@ func dialWS(t *testing.T, server *httptest.Server) *websocket.Conn {
 	return conn
 }
 
+// helper: dial a WebSocket connection to a specific room
+func dialWSWithRoom(t *testing.T, server *httptest.Server, room string) *websocket.Conn {
+	t.Helper()
+	wsURL := "ws" + strings.TrimPrefix(server.URL, "http") + "?room=" + room
+	conn, _, err := websocket.DefaultDialer.Dial(wsURL, nil)
+	if err != nil {
+		t.Fatalf("Failed to dial WebSocket (room=%q): %v", room, err)
+	}
+	return conn
+}
+
 // helper: wait until hub reaches expected client count or timeout
 func waitForClients(t *testing.T, h *Hub, expected int, timeout time.Duration) {
 	t.Helper()
@@ -356,3 +367,137 @@ func TestIntegration_LargeMessage(t *testing.T) {
 		t.Errorf("Expected message length %d, got %d", 64*1024, len(msg))
 	}
 }
+
+func TestIntegration_RoomIsolation_MessagesOnlyGoToSameRoom(t *testing.T) {
+	server, h := setupTestServer(t)
+	defer server.Close()
+	defer h.Shutdown()
+
+	// Connect 2 clients to room-a, 1 client to room-b
+	connA1 := dialWSWithRoom(t, server, "room-a")
+	defer connA1.Close()
+	connA2 := dialWSWithRoom(t, server, "room-a")
+	defer connA2.Close()
+	connB1 := dialWSWithRoom(t, server, "room-b")
+	defer connB1.Close()
+
+	waitForClients(t, h, 3, time.Second)
+
+	// Send a message from client A1
+	testMsg := "hello room-a"
+	if err := connA1.WriteMessage(websocket.TextMessage, []byte(testMsg)); err != nil {
+		t.Fatalf("Failed to send message: %v", err)
+	}
+
+	// Both room-a clients should receive the message
+	for i, conn := range []*websocket.Conn{connA1, connA2} {
+		conn.SetReadDeadline(time.Now().Add(time.Second))
+		_, msg, err := conn.ReadMessage()
+		if err != nil {
+			t.Fatalf("Room-a client %d failed to read message: %v", i+1, err)
+		}
+		if string(msg) != testMsg {
+			t.Errorf("Room-a client %d expected %q, got %q", i+1, testMsg, string(msg))
+		}
+	}
+
+	// Room-b client should NOT receive the message
+	connB1.SetReadDeadline(time.Now().Add(100 * time.Millisecond))
+	_, _, err := connB1.ReadMessage()
+	if err == nil {
+		t.Fatal("Room-b client should NOT have received a message from room-a")
+	}
+	// Timeout error is expected — message was correctly not delivered
+}
+
+func TestIntegration_RoomIsolation_MultipleRoomsIndependent(t *testing.T) {
+	server, h := setupTestServer(t)
+	defer server.Close()
+	defer h.Shutdown()
+
+	// Connect clients to two different rooms
+	connA := dialWSWithRoom(t, server, "alpha")
+	defer connA.Close()
+	connB := dialWSWithRoom(t, server, "beta")
+	defer connB.Close()
+
+	waitForClients(t, h, 2, time.Second)
+
+	// Send message from room alpha
+	msgAlpha := "alpha message"
+	if err := connA.WriteMessage(websocket.TextMessage, []byte(msgAlpha)); err != nil {
+		t.Fatalf("Failed to send alpha message: %v", err)
+	}
+
+	// Room alpha client receives its own message
+	connA.SetReadDeadline(time.Now().Add(time.Second))
+	_, msg, err := connA.ReadMessage()
+	if err != nil {
+		t.Fatalf("Alpha client failed to read: %v", err)
+	}
+	if string(msg) != msgAlpha {
+		t.Errorf("Alpha client expected %q, got %q", msgAlpha, string(msg))
+	}
+
+	// Send message from room beta
+	msgBeta := "beta message"
+	if err := connB.WriteMessage(websocket.TextMessage, []byte(msgBeta)); err != nil {
+		t.Fatalf("Failed to send beta message: %v", err)
+	}
+
+	// Room beta client receives its own message
+	connB.SetReadDeadline(time.Now().Add(time.Second))
+	_, msg, err = connB.ReadMessage()
+	if err != nil {
+		t.Fatalf("Beta client failed to read: %v", err)
+	}
+	if string(msg) != msgBeta {
+		t.Errorf("Beta client expected %q, got %q", msgBeta, string(msg))
+	}
+
+	// Verify alpha did NOT receive beta's message
+	connA.SetReadDeadline(time.Now().Add(100 * time.Millisecond))
+	_, _, err = connA.ReadMessage()
+	if err == nil {
+		t.Fatal("Alpha client should NOT have received beta's message")
+	}
+
+	// Verify beta did NOT receive alpha's message
+	connB.SetReadDeadline(time.Now().Add(100 * time.Millisecond))
+	_, _, err = connB.ReadMessage()
+	if err == nil {
+		t.Fatal("Beta client should NOT have received alpha's message")
+	}
+}
+
+func TestIntegration_BroadcastToEmptyRoom(t *testing.T) {
+	server, h := setupTestServer(t)
+	defer server.Close()
+	defer h.Shutdown()
+
+	// Connect a client to room-a and a separate client to verify hub is functional
+	connA := dialWSWithRoom(t, server, "room-a")
+	defer connA.Close()
+
+	waitForClients(t, h, 1, time.Second)
+
+	// Send directly to broadcast channel targeting a non-existent room.
+	// This should be handled gracefully (no panic, no delivery).
+	h.broadcast <- broadcastMsg{room: "non-existent", data: []byte("ghost message")}
+
+	// Give the hub time to process
+	time.Sleep(50 * time.Millisecond)
+
+	// Now send a real message to room-a to confirm hub is still functional
+	h.broadcast <- broadcastMsg{room: "room-a", data: []byte("real message")}
+
+	connA.SetReadDeadline(time.Now().Add(time.Second))
+	_, msg, err := connA.ReadMessage()
+	if err != nil {
+		t.Fatalf("Room-a client failed to read real message after empty-room broadcast: %v", err)
+	}
+	if string(msg) != "real message" {
+		t.Errorf("Expected %q, got %q", "real message", string(msg))
+	}
+}
+
diff --git a/internal/hub/hub_test.go b/internal/hub/hub_test.go
index dd4f6df..e455e93 100644
--- a/internal/hub/hub_test.go
+++ b/internal/hub/hub_test.go
@@ -245,3 +245,38 @@ func TestUnregisterUnknownConnNoPanic(t *testing.T) {
 		t.Errorf("Expected 0 clients, got %d", count)
 	}
 }
+
+func TestBroadcastRoomIsolation(t *testing.T) {
+	h := New(newTestLogger())
+	go h.Run()
+	defer h.Shutdown()
+
+	connA := dialTestHub(t, h, "room-a")
+	defer connA.Close()
+	connB := dialTestHub(t, h, "room-b")
+	defer connB.Close()
+
+	// Allow registers to be processed
+	time.Sleep(50 * time.Millisecond)
+
+	// Send message to room-a via broadcast channel
+	h.broadcast <- broadcastMsg{room: "room-a", data: []byte("for-a-only")}
+
+	// Room-a client should receive the message
+	connA.SetReadDeadline(time.Now().Add(time.Second))
+	_, msg, err := connA.ReadMessage()
+	if err != nil {
+		t.Fatalf("Room-a client failed to read: %v", err)
+	}
+	if string(msg) != "for-a-only" {
+		t.Errorf("Expected %q, got %q", "for-a-only", string(msg))
+	}
+
+	// Room-b client should NOT receive the message (short timeout)
+	connB.SetReadDeadline(time.Now().Add(100 * time.Millisecond))
+	_, _, err = connB.ReadMessage()
+	if err == nil {
+		t.Fatal("Room-b client should NOT have received room-a's message")
+	}
+}
+