diff --git a/pkg/coordinator/hub.go b/pkg/coordinator/hub.go
index 8b33b00d..f4a1398c 100644
--- a/pkg/coordinator/hub.go
+++ b/pkg/coordinator/hub.go
@@ -68,7 +68,14 @@ func (h *Hub) handleUserConnection() http.HandlerFunc {
 			h.log.Info().Msg("no free workers")
 			return
 		}
-		user.Bind(worker)
+
+		bound := user.Bind(worker)
+		if !bound {
+			user.Notify(api.ErrNoFreeSlots, "")
+			h.log.Info().Msg("no free slots")
+			return
+		}
+
 		h.users.Add(user)
 
 		apps := worker.AppNames()
diff --git a/pkg/coordinator/user.go b/pkg/coordinator/user.go
index a6865d75..5d1e7ed5 100644
--- a/pkg/coordinator/user.go
+++ b/pkg/coordinator/user.go
@@ -28,9 +28,10 @@ func NewUser(sock *com.Connection, log *logger.Logger) *User {
 	}
 }
 
-func (u *User) Bind(w *Worker) {
+func (u *User) Bind(w *Worker) bool {
 	u.w = w
-	u.w.Reserve()
+
+	return u.w.TryReserve()
 }
 
 func (u *User) Disconnect() {
diff --git a/pkg/coordinator/worker.go b/pkg/coordinator/worker.go
index a00ad226..f4b2b2d0 100644
--- a/pkg/coordinator/worker.go
+++ b/pkg/coordinator/worker.go
@@ -135,6 +135,11 @@ func (w *Worker) AppNames() []api.GameInfo {
 }
 
 func (w *Worker) AddSession(id string) {
+	// sessions can be uninitialized until the coordinator pushes them to the worker
+	if w.Sessions == nil {
+		return
+	}
+
 	w.Sessions[id] = struct{}{}
 }
 
@@ -159,13 +164,40 @@ type slotted int32
 // there are no players in the room (worker).
 func (s *slotted) HasSlot() bool { return atomic.LoadInt32((*int32)(s)) == 0 }
 
-// Reserve increments user counter of the worker.
-func (s *slotted) Reserve() { atomic.AddInt32((*int32)(s), 1) }
+// TryReserve reserves the slot only when it's free.
+func (s *slotted) TryReserve() bool {
+	for {
+		current := atomic.LoadInt32((*int32)(s))
+		if current != 0 {
+			return false
+		}
+		if atomic.CompareAndSwapInt32((*int32)(s), 0, 1) {
+			return true
+		}
+	}
+}
 
 // UnReserve decrements user counter of the worker.
 func (s *slotted) UnReserve() {
-	if atomic.AddInt32((*int32)(s), -1) < 0 {
-		atomic.StoreInt32((*int32)(s), 0)
+	for {
+		current := atomic.LoadInt32((*int32)(s))
+		if current <= 0 {
+			// reset to zero
+			if current < 0 {
+				if atomic.CompareAndSwapInt32((*int32)(s), current, 0) {
+					return
+				}
+				continue
+			}
+
+			return
+		}
+
+		// Regular decrement for positive values
+		newVal := current - 1
+		if atomic.CompareAndSwapInt32((*int32)(s), current, newVal) {
+			return
+		}
 	}
 }
 
diff --git a/pkg/coordinator/worker_test.go b/pkg/coordinator/worker_test.go
new file mode 100644
index 00000000..a08b8b71
--- /dev/null
+++ b/pkg/coordinator/worker_test.go
@@ -0,0 +1,193 @@
+package coordinator
+
+import (
+	"sync"
+	"sync/atomic"
+	"testing"
+)
+
+func TestSlotted(t *testing.T) {
+	t.Run("UnReserve", func(t *testing.T) {
+		t.Run("BasicDecrement", testUnReserveBasic)
+		t.Run("PreventUnderflow", testUnReserveUnderflow)
+		t.Run("ConcurrentDecrement", testUnReserveConcurrent)
+	})
+
+	t.Run("TryReserve", func(t *testing.T) {
+		t.Run("SuccessWhenZero", testTryReserveSuccess)
+		t.Run("FailWhenNonZero", testTryReserveFailure)
+		t.Run("ConcurrentReservations", testTryReserveConcurrent)
+	})
+
+	t.Run("Integration", func(t *testing.T) {
+		t.Run("ReserveUnreserveFlow", testReserveUnreserveFlow)
+		t.Run("FreeSlots", testFreeSlots)
+		t.Run("HasSlot", testHasSlot)
+	})
+}
+
+func testUnReserveBasic(t *testing.T) {
+	t.Parallel()
+	var s slotted
+
+	// Initial state
+	if atomic.LoadInt32((*int32)(&s)) != 0 {
+		t.Fatal("initial state not zero")
+	}
+
+	// Test normal decrement
+	s.TryReserve() // 0 -> 1
+	s.UnReserve()
+	if atomic.LoadInt32((*int32)(&s)) != 0 {
+		t.Error("failed to decrement to zero")
+	}
+
+	// Test multiple decrements
+	s.TryReserve() // 0 -> 1
+	s.TryReserve() // 1 -> 2
+	s.UnReserve()
+	s.UnReserve()
+	if atomic.LoadInt32((*int32)(&s)) != 0 {
+		t.Error("failed to decrement multiple times")
+	}
+}
+
+func testUnReserveUnderflow(t *testing.T) {
+	t.Parallel()
+	var s slotted
+
+	t.Run("PreventNewUnderflow", func(t *testing.T) {
+		s.UnReserve() // Start at 0
+		if atomic.LoadInt32((*int32)(&s)) != 0 {
+			t.Error("should remain at 0 when unreserving from 0")
+		}
+	})
+
+	t.Run("FixExistingNegative", func(t *testing.T) {
+		atomic.StoreInt32((*int32)(&s), -5)
+		s.UnReserve()
+		if current := atomic.LoadInt32((*int32)(&s)); current != 0 {
+			t.Errorf("should fix negative value to 0, got %d", current)
+		}
+	})
+}
+
+func testUnReserveConcurrent(t *testing.T) {
+	t.Parallel()
+
+	var s slotted
+	const workers = 100
+	var wg sync.WaitGroup
+
+	atomic.StoreInt32((*int32)(&s), int32(workers))
+	wg.Add(workers)
+
+	for i := 0; i < workers; i++ {
+		go func() {
+			defer wg.Done()
+			s.UnReserve()
+		}()
+	}
+
+	wg.Wait()
+
+	if current := atomic.LoadInt32((*int32)(&s)); current != 0 {
+		t.Errorf("unexpected final value: %d (want 0)", current)
+	}
+}
+
+func testTryReserveSuccess(t *testing.T) {
+	t.Parallel()
+	var s slotted
+
+	if !s.TryReserve() {
+		t.Error("should succeed when zero")
+	}
+	if atomic.LoadInt32((*int32)(&s)) != 1 {
+		t.Error("failed to increment")
+	}
+}
+
+func testTryReserveFailure(t *testing.T) {
+	t.Parallel()
+	var s slotted
+
+	atomic.StoreInt32((*int32)(&s), 1)
+	if s.TryReserve() {
+		t.Error("should fail when non-zero")
+	}
+}
+
+func testTryReserveConcurrent(t *testing.T) {
+	t.Parallel()
+	var s slotted
+	const workers = 100
+	var success int32
+	var wg sync.WaitGroup
+
+	wg.Add(workers)
+	for i := 0; i < workers; i++ {
+		go func() {
+			defer wg.Done()
+			if s.TryReserve() {
+				atomic.AddInt32(&success, 1)
+			}
+		}()
+	}
+
+	wg.Wait()
+
+	if success != 1 {
+		t.Errorf("unexpected success count: %d (want 1)", success)
+	}
+	if atomic.LoadInt32((*int32)(&s)) != 1 {
+		t.Error("counter not properly incremented")
+	}
+}
+
+func testReserveUnreserveFlow(t *testing.T) {
+	t.Parallel()
+	var s slotted
+
+	// Successful reservation
+	if !s.TryReserve() {
+		t.Fatal("failed initial reservation")
+	}
+
+	// Second reservation should fail
+	if s.TryReserve() {
+		t.Error("unexpected successful second reservation")
+	}
+
+	// Unreserve and try again
+	s.UnReserve()
+	if !s.TryReserve() {
+		t.Error("failed reservation after unreserve")
+	}
+}
+
+func testFreeSlots(t *testing.T) {
+	t.Parallel()
+	var s slotted
+
+	// Set to arbitrary value
+	atomic.StoreInt32((*int32)(&s), 5)
+	s.FreeSlots()
+	if atomic.LoadInt32((*int32)(&s)) != 0 {
+		t.Error("FreeSlots failed to reset counter")
+	}
+}
+
+func testHasSlot(t *testing.T) {
+	t.Parallel()
+	var s slotted
+
+	if !s.HasSlot() {
+		t.Error("should have slot when zero")
+	}
+
+	s.TryReserve()
+	if s.HasSlot() {
+		t.Error("shouldn't have slot when reserved")
+	}
+}