diff --git a/openapi/chat/session.go b/openapi/chat/session.go
index e61088d9..e80d29c6 100644
--- a/openapi/chat/session.go
+++ b/openapi/chat/session.go
@@ -49,15 +49,24 @@ func ListChats(c *gin.Context) {
 		return
 	}
 
-	// Return result
-	response.RespondWithSuccess(c, response.StatusOK, gin.H{
-		"data":      result.Data,
-		"groups":    result.Groups,
+	// Build response based on grouping mode
+	// When group_by is set, data should be nil to avoid duplication
+	resp := gin.H{
 		"page":      result.Page,
 		"pagesize":  result.PageSize,
 		"pagecount": result.PageCount,
 		"total":     result.Total,
-	})
+	}
+
+	if len(result.Groups) > 0 {
+		// Grouped response: only include groups
+		resp["groups"] = result.Groups
+	} else {
+		// Flat response: only include data
+		resp["data"] = result.Data
+	}
+
+	response.RespondWithSuccess(c, response.StatusOK, resp)
 }
 
 // GetChat retrieves a single chat session by ID
diff --git a/openapi/tests/chat/session_test.go b/openapi/tests/chat/session_test.go
index dbfe6441..177c7be5 100644
--- a/openapi/tests/chat/session_test.go
+++ b/openapi/tests/chat/session_test.go
@@ -300,9 +300,11 @@ func TestListChatSessions(t *testing.T) {
 		err = json.NewDecoder(resp.Body).Decode(&response)
 		assert.NoError(t, err)
 
-		// Check for groups in response
+		// Check for groups in response (when group_by=time, only groups is returned, not data)
 		_, hasGroups := response["groups"]
+		_, hasData := response["data"]
 		assert.True(t, hasGroups, "Response should contain groups when group_by=time")
+		assert.False(t, hasData, "Response should NOT contain data when group_by=time (to avoid duplication)")
 		t.Logf("Successfully retrieved chat sessions with time grouping")
 	})