diff --git a/backend/internal/app/model_catalog.go b/backend/internal/app/model_catalog.go
index a03557374..0fa48db2a 100644
--- a/backend/internal/app/model_catalog.go
+++ b/backend/internal/app/model_catalog.go
@@ -21,10 +21,11 @@ const (
 
 // ModelCatalogResponse 是创作端模型选择的统一读模型。
 // Source=frontend 时读取 Models；Source=system 时读取 Channels。
+// 两个集合都始终序列化为数组：空目录必须发 []，缺字段会被前端判成畸形响应。
 type ModelCatalogResponse struct {
 	Source   ModelCatalogSource     `json:"source"`
-	Models   []PublicLogicalModel   `json:"models,omitempty"`
-	Channels []PublicChannelCatalog `json:"channels,omitempty"`
+	Models   []PublicLogicalModel   `json:"models"`
+	Channels []PublicChannelCatalog `json:"channels"`
 }
 
 // PublicChannelCatalog 公开的渠道目录信息（脱敏）
@@ -74,25 +75,25 @@ func (s *Service) ModelCatalog(intent *ModelRequestIntent) (*ModelCatalogRespons
 		return nil, err
 	}
 
+	// 两个集合都初始化成非 nil 空切片：空目录要发 []，不能因为“没有模型”而丢掉字段。
+	response := &ModelCatalogResponse{Models: []PublicLogicalModel{}, Channels: []PublicChannelCatalog{}}
 	if frontendEnabled {
 		models, err := s.PublicLogicalModels(intent)
 		if err != nil {
 			return nil, err
 		}
-		return &ModelCatalogResponse{
-			Source: ModelCatalogSourceFrontend,
-			Models: models,
-		}, nil
+		response.Source = ModelCatalogSourceFrontend
+		response.Models = append(response.Models, models...)
+		return response, nil
 	}
 
 	channels, err := s.publicSystemChannelCatalog(intent)
 	if err != nil {
 		return nil, err
 	}
-	return &ModelCatalogResponse{
-		Source:   ModelCatalogSourceSystem,
-		Channels: channels,
-	}, nil
+	response.Source = ModelCatalogSourceSystem
+	response.Channels = append(response.Channels, channels...)
+	return response, nil
 }
 
 // publicSystemChannelCatalog 组装普通用户可见的系统渠道读模型，不暴露密钥、Base URL 等执行凭证。
diff --git a/backend/internal/app/model_catalog_test.go b/backend/internal/app/model_catalog_test.go
new file mode 100644
index 000000000..a7fa29f9a
--- /dev/null
+++ b/backend/internal/app/model_catalog_test.go
@@ -0,0 +1,73 @@
+package app
+
+import (
+	"encoding/json"
+	"testing"
+
+	"infinite-canvas/backend/internal/model"
+	"infinite-canvas/backend/internal/repository"
+
+	"gorm.io/driver/sqlite"
+	"gorm.io/gorm"
+)
+
+// 前台模型目录为空时也必须发 models: []，不能因为 omitempty 丢掉字段：
+// 前端在 source=frontend 时强制校验 models 是数组，缺字段会被判成畸形响应；
+// 而 auth-session-hydrator 会把该异常当成未登录并清掉会话，表现为刷新后跳回登录页。
+func TestModelCatalogAlwaysSerializesCollectionsAsArrays(t *testing.T) {
+	for _, test := range []struct {
+		name            string
+		frontendEnabled bool
+		wantSource      string
+	}{
+		{name: "空的前台模型目录", frontendEnabled: true, wantSource: "frontend"},
+		{name: "空的系统渠道目录", frontendEnabled: false, wantSource: "system"},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			svc, db := newModelCatalogTestService(t)
+			if test.frontendEnabled {
+				if err := db.Create(&model.SystemSetting{Key: featureAvailabilitySettingKey, ValueJSON: `{"frontendModelsEnabled":true}`}).Error; err != nil {
+					t.Fatal(err)
+				}
+			}
+
+			catalog, err := svc.ModelCatalog(nil)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if string(catalog.Source) != test.wantSource {
+				t.Fatalf("source = %q, want %q", catalog.Source, test.wantSource)
+			}
+
+			encoded, err := json.Marshal(catalog)
+			if err != nil {
+				t.Fatal(err)
+			}
+			var payload map[string]json.RawMessage
+			if err := json.Unmarshal(encoded, &payload); err != nil {
+				t.Fatal(err)
+			}
+			for _, key := range []string{"models", "channels"} {
+				raw, present := payload[key]
+				if !present || string(raw) == "null" {
+					t.Fatalf("%s 集合没有序列化成数组：%s", key, encoded)
+				}
+			}
+		})
+	}
+}
+
+func newModelCatalogTestService(t *testing.T) (*Service, *gorm.DB) {
+	t.Helper()
+	db, err := gorm.Open(sqlite.Open(":memory:"), &gorm.Config{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if sqlDB, err := db.DB(); err == nil {
+		sqlDB.SetMaxOpenConns(1)
+	}
+	if err := db.AutoMigrate(&model.SystemSetting{}, &model.AdminAuditEvent{}, &model.ModelChannel{}, &model.ChannelModel{}, &model.LogicalModel{}); err != nil {
+		t.Fatal(err)
+	}
+	return New(repository.New(db), t.TempDir()), db
+}