diff --git a/backend/internal/services/notification_service.go b/backend/internal/services/notification_service.go
index abfe680e..6466856a 100644
--- a/backend/internal/services/notification_service.go
+++ b/backend/internal/services/notification_service.go
@@ -33,14 +33,29 @@ type NotificationService struct {
 	validateSlackURL   func(string) error
 }
 
-func NewNotificationService(db *gorm.DB, mailService MailServiceInterface) *NotificationService {
-	return &NotificationService{
+// NotificationServiceOption configures a NotificationService at construction time.
+type NotificationServiceOption func(*NotificationService)
+
+// WithSlackURLValidator overrides the Slack webhook URL validator. Intended for use
+// in tests that need to bypass real URL validation without mutating shared state.
+func WithSlackURLValidator(fn func(string) error) NotificationServiceOption {
+	return func(s *NotificationService) {
+		s.validateSlackURL = fn
+	}
+}
+
+func NewNotificationService(db *gorm.DB, mailService MailServiceInterface, opts ...NotificationServiceOption) *NotificationService {
+	s := &NotificationService{
 		DB:                 db,
 		httpWrapper:        notifications.NewNotifyHTTPWrapper(),
 		mailService:        mailService,
 		telegramAPIBaseURL: "https://api.telegram.org",
 		validateSlackURL:   validateSlackWebhookURL,
 	}
+	for _, opt := range opts {
+		opt(s)
+	}
+	return s
 }
 
 var discordWebhookRegex = regexp.MustCompile(`^https://discord(?:app)?\.com/api/webhooks/(\d+)/([a-zA-Z0-9_-]+)`)
diff --git a/backend/internal/services/notification_service_json_test.go b/backend/internal/services/notification_service_json_test.go
index c69f21fd..7c84a3e3 100644
--- a/backend/internal/services/notification_service_json_test.go
+++ b/backend/internal/services/notification_service_json_test.go
@@ -193,8 +193,7 @@ func TestSendJSONPayload_Slack(t *testing.T) {
 	db, err := gorm.Open(sqlite.Open("file::memory:"), &gorm.Config{})
 	require.NoError(t, err)
 
-	svc := NewNotificationService(db, nil)
-	svc.validateSlackURL = func(rawURL string) error { return nil }
+	svc := NewNotificationService(db, nil, WithSlackURLValidator(func(string) error { return nil }))
 
 	provider := models.NotificationProvider{
 		Type:     "slack",
diff --git a/backend/internal/services/notification_service_test.go b/backend/internal/services/notification_service_test.go
index d41a2e6e..20a645bd 100644
--- a/backend/internal/services/notification_service_test.go
+++ b/backend/internal/services/notification_service_test.go
@@ -1453,8 +1453,7 @@ func TestSendJSONPayload_ServiceSpecificValidation(t *testing.T) {
 	})
 
 	t.Run("slack_requires_text_or_blocks", func(t *testing.T) {
-		defer func() { svc.validateSlackURL = validateSlackWebhookURL }()
-		svc.validateSlackURL = func(rawURL string) error { return nil }
+		subSvc := NewNotificationService(db, nil, WithSlackURLValidator(func(string) error { return nil }))
 
 		provider := models.NotificationProvider{
 			Type:     "slack",
@@ -1470,7 +1469,7 @@ func TestSendJSONPayload_ServiceSpecificValidation(t *testing.T) {
 			"EventType": "test",
 		}
 
-		err := svc.sendJSONPayload(context.Background(), provider, data)
+		err := subSvc.sendJSONPayload(context.Background(), provider, data)
 		require.Error(t, err)
 		assert.Contains(t, err.Error(), "slack payload requires 'text' or 'blocks' field")
 	})
@@ -1480,9 +1479,7 @@ func TestSendJSONPayload_ServiceSpecificValidation(t *testing.T) {
 			w.WriteHeader(http.StatusOK)
 		}))
 		defer server.Close()
-
-		defer func() { svc.validateSlackURL = validateSlackWebhookURL }()
-		svc.validateSlackURL = func(rawURL string) error { return nil }
+		subSvc := NewNotificationService(db, nil, WithSlackURLValidator(func(string) error { return nil }))
 
 		provider := models.NotificationProvider{
 			Type:     "slack",
@@ -1498,7 +1495,7 @@ func TestSendJSONPayload_ServiceSpecificValidation(t *testing.T) {
 			"EventType": "test",
 		}
 
-		err := svc.sendJSONPayload(context.Background(), provider, data)
+		err := subSvc.sendJSONPayload(context.Background(), provider, data)
 		require.NoError(t, err)
 	})
 
@@ -1507,9 +1504,7 @@ func TestSendJSONPayload_ServiceSpecificValidation(t *testing.T) {
 			w.WriteHeader(http.StatusOK)
 		}))
 		defer server.Close()
-
-		defer func() { svc.validateSlackURL = validateSlackWebhookURL }()
-		svc.validateSlackURL = func(rawURL string) error { return nil }
+		subSvc := NewNotificationService(db, nil, WithSlackURLValidator(func(string) error { return nil }))
 
 		provider := models.NotificationProvider{
 			Type:     "slack",
@@ -1525,7 +1520,7 @@ func TestSendJSONPayload_ServiceSpecificValidation(t *testing.T) {
 			"EventType": "test",
 		}
 
-		err := svc.sendJSONPayload(context.Background(), provider, data)
+		err := subSvc.sendJSONPayload(context.Background(), provider, data)
 		require.NoError(t, err)
 	})
 
@@ -3306,8 +3301,7 @@ func TestNotificationService_TestProvider_Slack(t *testing.T) {
 	}))
 	defer server.Close()
 
-	svc := NewNotificationService(db, nil)
-	svc.validateSlackURL = func(rawURL string) error { return nil }
+	svc := NewNotificationService(db, nil, WithSlackURLValidator(func(string) error { return nil }))
 
 	provider := models.NotificationProvider{
 		Type:     "slack",
@@ -3337,8 +3331,7 @@ func TestNotificationService_SendExternal_Slack(t *testing.T) {
 	}))
 	defer server.Close()
 
-	svc := NewNotificationService(db, nil)
-	svc.validateSlackURL = func(rawURL string) error { return nil }
+	svc := NewNotificationService(db, nil, WithSlackURLValidator(func(string) error { return nil }))
 
 	provider := models.NotificationProvider{
 		Name:             "Slack E2E",
@@ -3374,8 +3367,7 @@ func TestNotificationService_Slack_PayloadNormalizesMessageToText(t *testing.T)
 	}))
 	defer server.Close()
 
-	svc := NewNotificationService(db, nil)
-	svc.validateSlackURL = func(rawURL string) error { return nil }
+	svc := NewNotificationService(db, nil, WithSlackURLValidator(func(string) error { return nil }))
 
 	provider := models.NotificationProvider{
 		Type:     "slack",
@@ -3402,8 +3394,7 @@ func TestNotificationService_Slack_PayloadNormalizesMessageToText(t *testing.T)
 func TestNotificationService_Slack_PayloadRequiresTextOrBlocks(t *testing.T) {
 	db := setupNotificationTestDB(t)
 
-	svc := NewNotificationService(db, nil)
-	svc.validateSlackURL = func(rawURL string) error { return nil }
+	svc := NewNotificationService(db, nil, WithSlackURLValidator(func(string) error { return nil }))
 
 	provider := models.NotificationProvider{
 		Type:     "slack",