diff --git a/backend/internal/repository/ent.go b/backend/internal/repository/ent.go
index 8005f114..d7d574e8 100644
--- a/backend/internal/repository/ent.go
+++ b/backend/internal/repository/ent.go
@@ -65,5 +65,18 @@ func InitEnt(cfg *config.Config) (*ent.Client, *sql.DB, error) {
 
 	// 创建 Ent 客户端，绑定到已配置的数据库驱动。
 	client := ent.NewClient(ent.Driver(drv))
+
+	// SIMPLE 模式：启动时补齐各平台默认分组。
+	// - anthropic/openai/gemini: 确保存在 <platform>-default
+	// - antigravity: 仅要求存在 >=2 个未软删除分组（用于 claude/gemini 混合调度场景）
+	if cfg.RunMode == config.RunModeSimple {
+		seedCtx, seedCancel := context.WithTimeout(context.Background(), 30*time.Second)
+		defer seedCancel()
+		if err := ensureSimpleModeDefaultGroups(seedCtx, client); err != nil {
+			_ = client.Close()
+			return nil, nil, err
+		}
+	}
+
 	return client, drv.DB(), nil
 }
diff --git a/backend/internal/repository/simple_mode_default_groups.go b/backend/internal/repository/simple_mode_default_groups.go
new file mode 100644
index 00000000..56309184
--- /dev/null
+++ b/backend/internal/repository/simple_mode_default_groups.go
@@ -0,0 +1,82 @@
+package repository
+
+import (
+	"context"
+	"fmt"
+
+	dbent "github.com/Wei-Shaw/sub2api/ent"
+	"github.com/Wei-Shaw/sub2api/ent/group"
+	"github.com/Wei-Shaw/sub2api/internal/service"
+)
+
+func ensureSimpleModeDefaultGroups(ctx context.Context, client *dbent.Client) error {
+	if client == nil {
+		return fmt.Errorf("nil ent client")
+	}
+
+	requiredByPlatform := map[string]int{
+		service.PlatformAnthropic:   1,
+		service.PlatformOpenAI:      1,
+		service.PlatformGemini:      1,
+		service.PlatformAntigravity: 2,
+	}
+
+	for platform, minCount := range requiredByPlatform {
+		count, err := client.Group.Query().
+			Where(group.PlatformEQ(platform), group.DeletedAtIsNil()).
+			Count(ctx)
+		if err != nil {
+			return fmt.Errorf("count groups for platform %s: %w", platform, err)
+		}
+
+		if platform == service.PlatformAntigravity {
+			if count < minCount {
+				for i := count; i < minCount; i++ {
+					name := fmt.Sprintf("%s-default-%d", platform, i+1)
+					if err := createGroupIfNotExists(ctx, client, name, platform); err != nil {
+						return err
+					}
+				}
+			}
+			continue
+		}
+
+		// Non-antigravity platforms: ensure <platform>-default exists.
+		name := platform + "-default"
+		if err := createGroupIfNotExists(ctx, client, name, platform); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+func createGroupIfNotExists(ctx context.Context, client *dbent.Client, name, platform string) error {
+	exists, err := client.Group.Query().
+		Where(group.NameEQ(name), group.DeletedAtIsNil()).
+		Exist(ctx)
+	if err != nil {
+		return fmt.Errorf("check group exists %s: %w", name, err)
+	}
+	if exists {
+		return nil
+	}
+
+	_, err = client.Group.Create().
+		SetName(name).
+		SetDescription("Auto-created default group").
+		SetPlatform(platform).
+		SetStatus(service.StatusActive).
+		SetSubscriptionType(service.SubscriptionTypeStandard).
+		SetRateMultiplier(1.0).
+		SetIsExclusive(false).
+		Save(ctx)
+	if err != nil {
+		if dbent.IsConstraintError(err) {
+			// Concurrent server startups may race on creation; treat as success.
+			return nil
+		}
+		return fmt.Errorf("create default group %s: %w", name, err)
+	}
+	return nil
+}
diff --git a/backend/internal/repository/simple_mode_default_groups_integration_test.go b/backend/internal/repository/simple_mode_default_groups_integration_test.go
new file mode 100644
index 00000000..3327257b
--- /dev/null
+++ b/backend/internal/repository/simple_mode_default_groups_integration_test.go
@@ -0,0 +1,84 @@
+//go:build integration
+
+package repository
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/Wei-Shaw/sub2api/ent/group"
+	"github.com/Wei-Shaw/sub2api/internal/service"
+	"github.com/stretchr/testify/require"
+)
+
+func TestEnsureSimpleModeDefaultGroups_CreatesMissingDefaults(t *testing.T) {
+	ctx := context.Background()
+	tx := testEntTx(t)
+	client := tx.Client()
+
+	seedCtx, cancel := context.WithTimeout(ctx, 10*time.Second)
+	defer cancel()
+
+	require.NoError(t, ensureSimpleModeDefaultGroups(seedCtx, client))
+
+	assertGroupExists := func(name string) {
+		exists, err := client.Group.Query().Where(group.NameEQ(name), group.DeletedAtIsNil()).Exist(seedCtx)
+		require.NoError(t, err)
+		require.True(t, exists, "expected group %s to exist", name)
+	}
+
+	assertGroupExists(service.PlatformAnthropic + "-default")
+	assertGroupExists(service.PlatformOpenAI + "-default")
+	assertGroupExists(service.PlatformGemini + "-default")
+	assertGroupExists(service.PlatformAntigravity + "-default-1")
+	assertGroupExists(service.PlatformAntigravity + "-default-2")
+}
+
+func TestEnsureSimpleModeDefaultGroups_IgnoresSoftDeletedGroups(t *testing.T) {
+	ctx := context.Background()
+	tx := testEntTx(t)
+	client := tx.Client()
+
+	seedCtx, cancel := context.WithTimeout(ctx, 10*time.Second)
+	defer cancel()
+
+	// Create and then soft-delete an anthropic default group.
+	g, err := client.Group.Create().
+		SetName(service.PlatformAnthropic + "-default").
+		SetPlatform(service.PlatformAnthropic).
+		SetStatus(service.StatusActive).
+		SetSubscriptionType(service.SubscriptionTypeStandard).
+		SetRateMultiplier(1.0).
+		SetIsExclusive(false).
+		Save(seedCtx)
+	require.NoError(t, err)
+
+	_, err = client.Group.Delete().Where(group.IDEQ(g.ID)).Exec(seedCtx)
+	require.NoError(t, err)
+
+	require.NoError(t, ensureSimpleModeDefaultGroups(seedCtx, client))
+
+	// New active one should exist.
+	count, err := client.Group.Query().Where(group.NameEQ(service.PlatformAnthropic+"-default"), group.DeletedAtIsNil()).Count(seedCtx)
+	require.NoError(t, err)
+	require.Equal(t, 1, count)
+}
+
+func TestEnsureSimpleModeDefaultGroups_AntigravityNeedsTwoGroupsOnlyByCount(t *testing.T) {
+	ctx := context.Background()
+	tx := testEntTx(t)
+	client := tx.Client()
+
+	seedCtx, cancel := context.WithTimeout(ctx, 10*time.Second)
+	defer cancel()
+
+	mustCreateGroup(t, client, &service.Group{Name: "ag-custom-1-" + time.Now().Format(time.RFC3339Nano), Platform: service.PlatformAntigravity})
+	mustCreateGroup(t, client, &service.Group{Name: "ag-custom-2-" + time.Now().Format(time.RFC3339Nano), Platform: service.PlatformAntigravity})
+
+	require.NoError(t, ensureSimpleModeDefaultGroups(seedCtx, client))
+
+	count, err := client.Group.Query().Where(group.PlatformEQ(service.PlatformAntigravity), group.DeletedAtIsNil()).Count(seedCtx)
+	require.NoError(t, err)
+	require.GreaterOrEqual(t, count, 2)
+}