diff --git a/agent/store/CHAT_STORAGE_DESIGN.md b/agent/store/CHAT_STORAGE_DESIGN.md
index 4e7248a1..603831c1 100644
--- a/agent/store/CHAT_STORAGE_DESIGN.md
+++ b/agent/store/CHAT_STORAGE_DESIGN.md
@@ -1673,6 +1673,11 @@ GET /v1/chat/sessions/chat_123/messages?limit=100&offset=0&role=assistant&type=t
 | `type` | string | - | Filter by message type |
 | `limit` | int | 100 | Max messages to return (max 1000) |
 | `offset` | int | 0 | Offset for pagination |
+| `locale` | string | - | Locale for assistant info (e.g., `zh-cn`, `en-us`). Falls back to `Accept-Language` header |
+
+**Locale Resolution Priority:**
+1. Query parameter `locale`
+2. HTTP header `Accept-Language`
 
 **Response:**
 
@@ -1708,10 +1713,20 @@ GET /v1/chat/sessions/chat_123/messages?limit=100&offset=0&role=assistant&type=t
       "created_at": "2024-01-15T10:00:05Z"
     }
   ],
-  "count": 2
+  "count": 2,
+  "assistants": {
+    "weather_assistant": {
+      "assistant_id": "weather_assistant",
+      "name": "Weather Assistant",
+      "avatar": "https://example.com/weather-avatar.png",
+      "description": "Get weather information for any location"
+    }
+  }
 }
 ```
 
+**Note:** The `assistants` field contains localized assistant information (name, avatar, description) for all unique `assistant_id` values found in the messages. This allows the frontend to display assistant details without additional API calls. The locale is determined by the `locale` query parameter or `Accept-Language` header.
+
 ### Permission Filtering
 
 All endpoints respect Yao's permission system:
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")
 	})