diff --git a/locks_test.go b/locks_test.go
new file mode 100644
index 0000000..57ec5bb
--- /dev/null
+++ b/locks_test.go
@@ -0,0 +1,63 @@
+package main
+
+import (
+	"context"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+// TestLockController_PromotesUnderContention — N goroutines hit the same
+// author simultaneously. The controller should observe the queue depth and
+// move the scope from Coarse to Medium.
+func TestLockController_PromotesUnderContention(t *testing.T) {
+	s, _, _ := newTestStore(t)
+	cp := &CreatePost{AuthorID: "scott", Body: "lock test"}
+	must(t, Submit(context.Background(), s, cp))
+
+	const N = 80
+	var wg sync.WaitGroup
+	var ran atomic.Int64
+	for i := 0; i < N; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			m := &AddComment{PostID: cp.ResolvedPostID(), AuthorID: "hermes", Body: "y"}
+			if err := Submit(context.Background(), s, m); err == nil {
+				ran.Add(1)
+			}
+		}()
+	}
+	wg.Wait()
+
+	st := s.lockCtrl.getOrCreate("hermes")
+	// After 80 concurrent submissions, scope may have promoted. We don't
+	// assert the exact scope (depends on timing); we just assert no deadlock
+	// and all ran.
+	if ran.Load() != N {
+		t.Fatalf("expected %d to run, got %d", N, ran.Load())
+	}
+	t.Logf("final scope for hermes: %s (queueDepth=%d)", st.scope, st.queueDepth)
+}
+
+// TestLockController_DemotesOnIdle — after a quiet period, scope should
+// settle back to Coarse. Trigger Tick() manually to simulate the periodic
+// controller.
+func TestLockController_DemotesOnIdle(t *testing.T) {
+	s, _, _ := newTestStore(t)
+	cp := &CreatePost{AuthorID: "scott", Body: "demote test"}
+	must(t, Submit(context.Background(), s, cp))
+
+	// Manually set state to Fine, then tick to decay queue depth and demote.
+	st := s.lockCtrl.getOrCreate("hermes")
+	st.scope = ScopeFine
+	st.queueDepth = 5 // below fineToMediumBelow=100
+	st.belowSince = time.Now().Add(-time.Hour) // way past the demotion window
+
+	s.lockCtrl.Tick()
+	chosen := s.lockCtrl.chooseScope(st)
+	if chosen == ScopeFine {
+		t.Fatalf("expected demotion from Fine, still Fine")
+	}
+}