fix(server): 对话保存防回退守卫与load同对话跳过切换前保存

crud_mixin.save_conversation 新增 allow_shrink 参数与守卫：消息数缩减且未
豁免时拒绝写入，堵住工作区级/对话级双实例旧内存回写（当日三次实锤：
load切换前保存542→506、新建前保存104→77、__del__析构103→77）。
检查点恢复作为唯一合法缩减路径显式豁免。load_conversation_by_id 加载
目标==当前对话时跳过切换前保存（接下来以磁盘为准重载）。debug trace
增加 shrink_suspect 字段覆盖旧非空回写场景。

server/conversation.py

@@ -1379,6 +1379,8 @@ def _restore_checkpoint_to_conversation(
         model_key=restore_model_key,
         has_images=restore_has_images,
         has_videos=restore_has_videos,
+        # 恢复旧检查点是唯一合法的消息数缩减路径，显式豁免防回退守卫
+        allow_shrink=True,
     )
     if not ok:
         raise VersioningError("保存恢复后的对话失败")

utils/context_manager/conversation_mixin.py

@@ -189,8 +189,16 @@ class ConversationMixin:
         Returns:
             bool: 加载是否成功
         """
-        # 先保存当前对话
-        if self.current_conversation_id and self.conversation_history:
+        # 先保存当前对话（加载目标就是当前对话时跳过：接下来会以磁盘为准重载，
+        # 若本实例内存落后于磁盘，保存反而把旧历史回写覆盖新消息——双实例并存时
+        # 这是已实锤的回退源头；其余路径的落后写入由 save_conversation 守卫拦截）
+        current_norm = (self.current_conversation_id or "").removeprefix("conv_")
+        target_norm = (conversation_id or "").removeprefix("conv_")
+        if (
+            self.current_conversation_id
+            and self.conversation_history
+            and current_norm != target_norm
+        ):
             self.save_current_conversation()
         
         # 加载指定对话：先按ID路由到对应 manager

utils/conversation_manager/crud_mixin.py

@@ -50,6 +50,7 @@ def _debug_conversation_save_trace(conversation_id: str, file_path, data: Dict):
         except Exception:
             old_len = None
         wipe_suspect = bool(old_len and new_len == 0)
+        shrink_suspect = bool(old_len and 0 < new_len < old_len)
         # 只保留项目内调用帧，排除标准库/第三方
         try:
             repo_root = Path(__file__).resolve().parents[2]
@@ -71,6 +72,7 @@ def _debug_conversation_save_trace(conversation_id: str, file_path, data: Dict):
             "old_msg_len": old_len,
             "new_msg_len": new_len,
             "wipe_suspect": wipe_suspect,
+            "shrink_suspect": shrink_suspect,
             "title": (data.get("title") if isinstance(data, dict) else None),
             "thread": threading.current_thread().name,
             "stack_tail": frames[-14:],
@@ -82,8 +84,9 @@ def _debug_conversation_save_trace(conversation_id: str, file_path, data: Dict):
         log_dir.mkdir(parents=True, exist_ok=True)
         with open(log_dir / "conversation_save_debug.log", "a", encoding="utf-8") as f:
             f.write(json.dumps(record, ensure_ascii=False) + "\n")
-        if wipe_suspect:
-            print(f"🚨 [ConvSaveDebug] 对话 {conversation_id} 消息将被空列表覆盖！栈: {frames[-14:]}")
+        if wipe_suspect or shrink_suspect:
+            kind = "空列表覆盖" if wipe_suspect else f"缩减 {old_len}->{new_len}"
+            print(f"🚨 [ConvSaveDebug] 对话 {conversation_id} 消息将被{kind}！栈: {frames[-14:]}")
     except Exception:
         pass
 
@@ -318,7 +321,8 @@ class CrudMixin:
         has_videos: Optional[bool] = None,
         project_file_tree: Optional[str] = None,
         project_statistics: Optional[Dict] = None,
-        project_snapshot_at: Optional[str] = None
+        project_snapshot_at: Optional[str] = None,
+        allow_shrink: bool = False
     ) -> bool:
         """
         保存对话（更新现有对话）
@@ -328,6 +332,7 @@ class CrudMixin:
             messages: 消息列表
             project_path: 项目路径
             thinking_mode: 思考模式
+            allow_shrink: 允许消息数缩减（仅检查点恢复等合法缩减路径使用）
         
         Returns:
             bool: 保存是否成功
@@ -339,6 +344,19 @@ class CrudMixin:
                 print(f"⚠️ 对话 {conversation_id} 不存在，无法更新")
                 return False
             
+            # 防回退守卫：消息数缩减且未显式豁免时拒绝写入。
+            # 工作区级/对话级 terminal 双实例并存时，旧实例内存历史可能落后于磁盘，
+            # 「切换/新建对话前保存当前对话」「__del__」等路径会把旧历史回写覆盖新消息。
+            old_messages = existing_data.get("messages")
+            old_len = len(old_messages) if isinstance(old_messages, list) else 0
+            new_len = len(messages) if isinstance(messages, list) else 0
+            if new_len < old_len and not allow_shrink:
+                print(
+                    f"🚨 [ConvSaveGuard] 拒绝保存 {conversation_id}: "
+                    f"消息数 {old_len} -> {new_len} 缩减且未豁免（疑似旧内存回写）"
+                )
+                return False
+            
             # 更新数据
             existing_data["messages"] = messages
             existing_data["updated_at"] = datetime.now().isoformat()