diff --git a/internal/cluster/cluster_test.go b/internal/cluster/cluster_test.go
new file mode 100644
index 0000000..5f062f9
--- /dev/null
+++ b/internal/cluster/cluster_test.go
@@ -0,0 +1,395 @@
+package cluster
+
+import (
+	"context"
+	"encoding/json"
+	"go-upkeep/internal/models"
+	"go-upkeep/internal/monitor"
+	"net/http"
+	"net/http/httptest"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+// --- Mock Store (minimal, for monitor.NewEngine) ---
+
+type mockStore struct {
+	mu    sync.Mutex
+	sites []models.Site
+}
+
+func (m *mockStore) Init() error                                              { return nil }
+func (m *mockStore) GetSites() ([]models.Site, error)                         { return m.sites, nil }
+func (m *mockStore) AddSite(models.Site) error                                { return nil }
+func (m *mockStore) UpdateSite(models.Site) error                             { return nil }
+func (m *mockStore) UpdateSitePaused(int, bool) error                         { return nil }
+func (m *mockStore) DeleteSite(int) error                                     { return nil }
+func (m *mockStore) GetAllAlerts() ([]models.AlertConfig, error)              { return nil, nil }
+func (m *mockStore) GetAlert(int) (models.AlertConfig, error)                 { return models.AlertConfig{}, nil }
+func (m *mockStore) AddAlert(string, string, map[string]string) error         { return nil }
+func (m *mockStore) UpdateAlert(int, string, string, map[string]string) error { return nil }
+func (m *mockStore) DeleteAlert(int) error                                    { return nil }
+func (m *mockStore) GetAllUsers() ([]models.User, error)                      { return nil, nil }
+func (m *mockStore) AddUser(string, string, string) error                     { return nil }
+func (m *mockStore) UpdateUser(int, string, string, string) error             { return nil }
+func (m *mockStore) DeleteUser(int) error                                     { return nil }
+func (m *mockStore) SaveCheck(int, int64, bool) error                         { return nil }
+func (m *mockStore) SaveCheckFromNode(int, string, int64, bool) error         { return nil }
+func (m *mockStore) LoadAllHistory(int) (map[int][]models.CheckRecord, error) { return nil, nil }
+func (m *mockStore) ExportData() (models.Backup, error)                       { return models.Backup{}, nil }
+func (m *mockStore) ImportData(models.Backup) error                           { return nil }
+func (m *mockStore) GetSiteByName(string) (models.Site, error)                { return models.Site{}, nil }
+func (m *mockStore) GetAlertByName(string) (models.AlertConfig, error) {
+	return models.AlertConfig{}, nil
+}
+func (m *mockStore) AddSiteReturningID(models.Site) (int, error) { return 0, nil }
+func (m *mockStore) AddAlertReturningID(string, string, map[string]string) (int, error) {
+	return 0, nil
+}
+func (m *mockStore) RegisterNode(models.ProbeNode) error      { return nil }
+func (m *mockStore) GetNode(string) (models.ProbeNode, error) { return models.ProbeNode{}, nil }
+func (m *mockStore) GetAllNodes() ([]models.ProbeNode, error) { return nil, nil }
+func (m *mockStore) UpdateNodeLastSeen(string) error          { return nil }
+func (m *mockStore) DeleteNode(string) error                  { return nil }
+func (m *mockStore) SaveLog(string) error                     { return nil }
+func (m *mockStore) LoadLogs(int) ([]string, error)           { return nil, nil }
+func (m *mockStore) GetActiveMaintenanceWindows() ([]models.MaintenanceWindow, error) {
+	return nil, nil
+}
+func (m *mockStore) GetAllMaintenanceWindows(int) ([]models.MaintenanceWindow, error) {
+	return nil, nil
+}
+func (m *mockStore) AddMaintenanceWindow(models.MaintenanceWindow) error { return nil }
+func (m *mockStore) EndMaintenanceWindow(int) error                      { return nil }
+func (m *mockStore) DeleteMaintenanceWindow(int) error                   { return nil }
+func (m *mockStore) IsMonitorInMaintenance(int) (bool, error)            { return false, nil }
+func (m *mockStore) GetPreference(string) (string, error)                { return "", nil }
+func (m *mockStore) SetPreference(string, string) error                  { return nil }
+func (m *mockStore) Close() error                                        { return nil }
+
+// --- Cluster Start Tests ---
+
+func TestStart_LeaderMode(t *testing.T) {
+	eng := monitor.NewEngine(&mockStore{})
+	eng.SetActive(false)
+
+	ctx := context.Background()
+	Start(ctx, Config{Mode: "leader"}, eng)
+
+	if !eng.IsActive() {
+		t.Error("leader mode should set engine active")
+	}
+}
+
+func TestStart_FollowerMode(t *testing.T) {
+	eng := monitor.NewEngine(&mockStore{})
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	Start(ctx, Config{Mode: "follower", PeerURL: "http://localhost:9999"}, eng)
+	time.Sleep(50 * time.Millisecond)
+
+	if eng.IsActive() {
+		t.Error("follower mode should set engine inactive")
+	}
+}
+
+// --- Follower Loop Tests ---
+
+func TestFollowerLoop_FailoverOnLeaderDown(t *testing.T) {
+	eng := monitor.NewEngine(&mockStore{})
+	eng.SetActive(false)
+
+	// Server always returns 503
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(503)
+	}))
+	defer srv.Close()
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	go runFollowerLoop(ctx, Config{PeerURL: srv.URL, SharedKey: "key"}, eng)
+
+	// Follower checks every 5s, needs 3 failures → ~15s minimum
+	// But we can't wait that long in a test. The loop sleeps 5s between checks.
+	// We'll wait up to 20s for failover.
+	deadline := time.After(20 * time.Second)
+	for {
+		if eng.IsActive() {
+			return // success
+		}
+		select {
+		case <-deadline:
+			t.Fatal("expected failover to ACTIVE after 3 failures")
+		case <-time.After(500 * time.Millisecond):
+		}
+	}
+}
+
+func TestFollowerLoop_RecoveryOnLeaderReturn(t *testing.T) {
+	eng := monitor.NewEngine(&mockStore{})
+	eng.SetActive(true) // simulate already failed over
+
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(200)
+		w.Write([]byte("OK"))
+	}))
+	defer srv.Close()
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	go runFollowerLoop(ctx, Config{PeerURL: srv.URL}, eng)
+
+	deadline := time.After(10 * time.Second)
+	for {
+		if !eng.IsActive() {
+			return // success — switched back to passive
+		}
+		select {
+		case <-deadline:
+			t.Fatal("expected switch back to PASSIVE when leader returns")
+		case <-time.After(500 * time.Millisecond):
+		}
+	}
+}
+
+func TestFollowerLoop_SendsSecret(t *testing.T) {
+	var mu sync.Mutex
+	var receivedSecret string
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		mu.Lock()
+		receivedSecret = r.Header.Get("X-Upkeep-Secret")
+		mu.Unlock()
+		w.WriteHeader(200)
+		w.Write([]byte("OK"))
+	}))
+	defer srv.Close()
+
+	eng := monitor.NewEngine(&mockStore{})
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	go runFollowerLoop(ctx, Config{PeerURL: srv.URL, SharedKey: "test-secret"}, eng)
+
+	deadline := time.After(10 * time.Second)
+	for {
+		mu.Lock()
+		got := receivedSecret
+		mu.Unlock()
+		if got == "test-secret" {
+			return
+		}
+		select {
+		case <-deadline:
+			t.Fatalf("expected secret 'test-secret', got %q", got)
+		case <-time.After(500 * time.Millisecond):
+		}
+	}
+}
+
+func TestFollowerLoop_CancelContext(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(200)
+	}))
+	defer srv.Close()
+
+	eng := monitor.NewEngine(&mockStore{})
+	ctx, cancel := context.WithCancel(context.Background())
+
+	done := make(chan struct{})
+	go func() {
+		runFollowerLoop(ctx, Config{PeerURL: srv.URL}, eng)
+		close(done)
+	}()
+
+	cancel()
+	select {
+	case <-done:
+	case <-time.After(3 * time.Second):
+		t.Fatal("expected follower loop to exit on context cancel")
+	}
+}
+
+// --- Probe Tests ---
+
+func TestProbeRegister_Success(t *testing.T) {
+	var received map[string]string
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		json.NewDecoder(r.Body).Decode(&received)
+		w.WriteHeader(200)
+	}))
+	defer srv.Close()
+
+	err := probeRegister(context.Background(), srv.Client(), ProbeConfig{
+		NodeID: "n1", NodeName: "US East", Region: "us-east", LeaderURL: srv.URL, SharedKey: "key",
+	})
+	if err != nil {
+		t.Fatalf("register: %v", err)
+	}
+	if received["id"] != "n1" {
+		t.Errorf("expected id n1, got %s", received["id"])
+	}
+}
+
+func TestProbeRegister_Failure(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(401)
+	}))
+	defer srv.Close()
+
+	err := probeRegister(context.Background(), srv.Client(), ProbeConfig{
+		LeaderURL: srv.URL,
+	})
+	if err == nil {
+		t.Error("expected error on 401")
+	}
+}
+
+func TestProbeFetchAssignments_Success(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		json.NewEncoder(w).Encode(map[string][]models.Site{
+			"sites": {{ID: 1, Name: "s1", Type: "http", URL: "http://example.com"}},
+		})
+	}))
+	defer srv.Close()
+
+	sites, err := probeFetchAssignments(context.Background(), srv.Client(), ProbeConfig{
+		NodeID: "n1", LeaderURL: srv.URL, SharedKey: "key",
+	})
+	if err != nil {
+		t.Fatalf("fetch: %v", err)
+	}
+	if len(sites) != 1 {
+		t.Errorf("expected 1 site, got %d", len(sites))
+	}
+}
+
+func TestProbeFetchAssignments_Unauthorized(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(401)
+	}))
+	defer srv.Close()
+
+	_, err := probeFetchAssignments(context.Background(), srv.Client(), ProbeConfig{
+		LeaderURL: srv.URL,
+	})
+	if err == nil {
+		t.Error("expected error on 401")
+	}
+}
+
+func TestProbeExecuteChecks(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(200)
+	}))
+	defer srv.Close()
+
+	sites := []models.Site{
+		{ID: 1, Type: "http", URL: srv.URL},
+		{ID: 2, Type: "http", URL: srv.URL},
+	}
+
+	strict := &http.Client{}
+	insecure := &http.Client{}
+	results := probeExecuteChecks(context.Background(), sites, strict, insecure)
+
+	if len(results) != 2 {
+		t.Fatalf("expected 2 results, got %d", len(results))
+	}
+	for _, r := range results {
+		if !r.IsUp {
+			t.Errorf("site %d expected UP", r.SiteID)
+		}
+	}
+}
+
+func TestProbeExecuteChecks_Concurrency(t *testing.T) {
+	var concurrent int64
+	var maxConcurrent int64
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		cur := atomic.AddInt64(&concurrent, 1)
+		for {
+			old := atomic.LoadInt64(&maxConcurrent)
+			if cur <= old || atomic.CompareAndSwapInt64(&maxConcurrent, old, cur) {
+				break
+			}
+		}
+		time.Sleep(50 * time.Millisecond)
+		atomic.AddInt64(&concurrent, -1)
+		w.WriteHeader(200)
+	}))
+	defer srv.Close()
+
+	var sites []models.Site
+	for i := 0; i < 20; i++ {
+		sites = append(sites, models.Site{ID: i + 1, Type: "http", URL: srv.URL})
+	}
+
+	results := probeExecuteChecks(context.Background(), sites, &http.Client{}, &http.Client{})
+	if len(results) != 20 {
+		t.Errorf("expected 20 results, got %d", len(results))
+	}
+	mc := atomic.LoadInt64(&maxConcurrent)
+	if mc > 10 {
+		t.Errorf("expected max 10 concurrent, got %d", mc)
+	}
+}
+
+func TestProbeReportResults_Success(t *testing.T) {
+	var received struct {
+		NodeID  string            `json:"node_id"`
+		Results []probeResultItem `json:"results"`
+	}
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		json.NewDecoder(r.Body).Decode(&received)
+		w.WriteHeader(200)
+	}))
+	defer srv.Close()
+
+	err := probeReportResults(context.Background(), srv.Client(), ProbeConfig{
+		NodeID: "n1", LeaderURL: srv.URL, SharedKey: "key",
+	}, []probeResultItem{{SiteID: 1, LatencyNs: 5000000, IsUp: true}})
+
+	if err != nil {
+		t.Fatalf("report: %v", err)
+	}
+	if received.NodeID != "n1" {
+		t.Errorf("expected n1, got %s", received.NodeID)
+	}
+	if len(received.Results) != 1 {
+		t.Errorf("expected 1 result, got %d", len(received.Results))
+	}
+}
+
+func TestProbeReportResults_Failure(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(500)
+	}))
+	defer srv.Close()
+
+	err := probeReportResults(context.Background(), srv.Client(), ProbeConfig{
+		LeaderURL: srv.URL,
+	}, []probeResultItem{{SiteID: 1}})
+
+	if err == nil {
+		t.Error("expected error on 500")
+	}
+}
+
+// --- sleepCtx ---
+
+func TestSleepCtx_Cancel(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+
+	start := time.Now()
+	sleepCtx(ctx, 10*time.Second)
+	if time.Since(start) > time.Second {
+		t.Error("expected immediate return on canceled context")
+	}
+}