diff --git a/backend/internal/logic/projects/dashboard.go b/backend/internal/logic/projects/dashboard.go
new file mode 100644
index 0000000..141428d
--- /dev/null
+++ b/backend/internal/logic/projects/dashboard.go
@@ -0,0 +1,24 @@
+package projects
+
+import "senlinai-agent/backend/internal/models"
+
+// Dashboard 保留旧概览查询给尚未迁移的 handler；所有统计仍严格限定项目所有者。
+func (s *Service) Dashboard(ownerID uint, projectID uint) (Dashboard, error) {
+	if err := ensureProjectOwner(ownerID, projectID); err != nil {
+		return Dashboard{}, err
+	}
+	dashboard := Dashboard{ProjectID: projectID}
+	if err := models.DBService.Model(&models.SenlinAgentInboxItem{}).Where("project_id = ? AND status = ?", projectID, "open").Count(&dashboard.PendingInboxCount).Error; err != nil {
+		return dashboard, err
+	}
+	if err := models.DBService.Model(&models.SenlinAgentTask{}).Where("project_id = ? AND status <> ?", projectID, "done").Count(&dashboard.OpenTaskCount).Error; err != nil {
+		return dashboard, err
+	}
+	if err := models.DBService.Model(&models.SenlinAgentNote{}).Where("project_id = ?", projectID).Count(&dashboard.RecentNoteCount).Error; err != nil {
+		return dashboard, err
+	}
+	if err := models.DBService.Model(&models.SenlinAgentAISession{}).Where("project_id = ?", projectID).Count(&dashboard.RecentSessionCount).Error; err != nil {
+		return dashboard, err
+	}
+	return dashboard, nil
+}
diff --git a/backend/internal/logic/projects/workspace.go b/backend/internal/logic/projects/workspace.go
index a1f3593..828bb25 100644
--- a/backend/internal/logic/projects/workspace.go
+++ b/backend/internal/logic/projects/workspace.go
@@ -17,27 +17,6 @@ type workspaceCounts struct {
 	cronPlans    int64
 }
 
-// Dashboard 保留旧概览查询给尚未迁移的 handler；所有统计仍严格限定项目所有者。
-func (s *Service) Dashboard(ownerID uint, projectID uint) (Dashboard, error) {
-	if err := ensureProjectOwner(ownerID, projectID); err != nil {
-		return Dashboard{}, err
-	}
-	dashboard := Dashboard{ProjectID: projectID}
-	if err := models.DBService.Model(&models.SenlinAgentInboxItem{}).Where("project_id = ? AND status = ?", projectID, "open").Count(&dashboard.PendingInboxCount).Error; err != nil {
-		return dashboard, err
-	}
-	if err := models.DBService.Model(&models.SenlinAgentTask{}).Where("project_id = ? AND status <> ?", projectID, "done").Count(&dashboard.OpenTaskCount).Error; err != nil {
-		return dashboard, err
-	}
-	if err := models.DBService.Model(&models.SenlinAgentNote{}).Where("project_id = ?", projectID).Count(&dashboard.RecentNoteCount).Error; err != nil {
-		return dashboard, err
-	}
-	if err := models.DBService.Model(&models.SenlinAgentAISession{}).Where("project_id = ?", projectID).Count(&dashboard.RecentSessionCount).Error; err != nil {
-		return dashboard, err
-	}
-	return dashboard, nil
-}
-
 // Workspace 在确认项目归属后执行首屏聚合；后续子查询只接收已授权的内部主键。
 func (s *Service) Workspace(ownerID uint, projectIdentity string) (WorkspaceDTO, error) {
 	project, err := FindOwnedProject(ownerID, projectIdentity)
@@ -191,14 +170,10 @@ func (s *Service) workspaceTasks(projectID uint, projectIdentity string) ([]Work
 		if err != nil {
 			return nil, err
 		}
-		completed := task.Status == "done"
-		var completedAt *time.Time
-		if completed {
-			completedAt = utcOptionalTime(&task.UpdatedAt)
-		}
+		// 当前模型没有真实完成时间字段，不能用更新时间伪装；CompletedAt 保持 nil。
 		item := WorkspaceTaskDTO{
 			ID: task.Identity, ProjectID: projectIdentity, Title: task.Title, Summary: task.Description,
-			Completed: completed, Owner: owner, Due: utcOptionalTime(task.DueAt), CreatedAt: task.CreatedAt.UTC(), CompletedAt: completedAt,
+			Completed: task.Status == "done", Owner: owner, Due: utcOptionalTime(task.DueAt), CreatedAt: task.CreatedAt.UTC(),
 		}
 		if tag, ok := tags[task.ID]; ok {
 			item.TagID = &tag.ID
diff --git a/backend/internal/logic/projects/workspace_test.go b/backend/internal/logic/projects/workspace_test.go
index 133dfe4..2312183 100644
--- a/backend/internal/logic/projects/workspace_test.go
+++ b/backend/internal/logic/projects/workspace_test.go
@@ -3,6 +3,7 @@ package projects
 import (
 	"encoding/json"
 	"fmt"
+	"os"
 	"testing"
 	"time"
 
@@ -63,7 +64,7 @@ func TestWorkspaceUsesIdentitiesUTCTimesAndProjectScopedTags(t *testing.T) {
 	require.Equal(t, "UI", workspace.Tasks[0].Tag)
 	require.Equal(t, nextRun.UTC(), *workspace.Tasks[0].Due)
 	require.Equal(t, createdAt.UTC(), workspace.Tasks[0].CreatedAt)
-	require.Equal(t, updatedAt.UTC(), *workspace.Tasks[0].CompletedAt)
+	require.Nil(t, workspace.Tasks[0].CompletedAt)
 
 	require.Len(t, workspace.AISessions, 1)
 	require.Equal(t, session.Identity, workspace.AISessions[0].ID)
@@ -93,6 +94,34 @@ func TestWorkspaceUsesIdentitiesUTCTimesAndProjectScopedTags(t *testing.T) {
 	require.NotContains(t, string(payload), "duration")
 }
 
+func TestWorkspaceDoneTaskDoesNotInventCompletedAt(t *testing.T) {
+	database := newTestDB(t)
+	service := NewService()
+	require.NoError(t, database.Create(&models.SenlinAgentUser{Email: "owner@example.com", DisplayName: "Owner", PasswordHash: "hash"}).Error)
+	project, err := service.CreateProject(1, "Alpha", "")
+	require.NoError(t, err)
+	createdAt := time.Date(2026, 7, 20, 1, 0, 0, 0, time.UTC)
+	updatedAt := createdAt.Add(2 * time.Hour)
+	require.NoError(t, database.Create(&models.SenlinAgentTask{
+		ProjectID: project.ID, CreatedBy: 1, Title: "Done", Status: "done", CreatedAt: createdAt, UpdatedAt: updatedAt,
+	}).Error)
+
+	workspace, err := service.Workspace(1, project.Identity)
+
+	require.NoError(t, err)
+	require.Len(t, workspace.Tasks, 1)
+	require.Nil(t, workspace.Tasks[0].CompletedAt)
+	payload, err := json.Marshal(workspace.Tasks[0])
+	require.NoError(t, err)
+	require.Contains(t, string(payload), `"completedAt":null`)
+}
+
+func TestWorkspaceFileDoesNotOwnDashboard(t *testing.T) {
+	source, err := os.ReadFile("workspace.go")
+	require.NoError(t, err)
+	require.NotContains(t, string(source), "func (s *Service) Dashboard")
+}
+
 func TestWorkspaceRejectsProjectOwnedByAnotherUserByIdentity(t *testing.T) {
 	newTestDB(t)
 	service := NewService()