diff --git a/src/agents/common/middlewares/attachment_middleware.py b/src/agents/common/middlewares/attachment_middleware.py
index 5c6136e8..663cb0d8 100644
--- a/src/agents/common/middlewares/attachment_middleware.py
+++ b/src/agents/common/middlewares/attachment_middleware.py
@@ -18,7 +18,6 @@ class AttachmentState(AgentState):
     """扩展 AgentState 以支持附件"""
 
     attachments: NotRequired[list[dict]]
-    files: NotRequired[dict[str, str]]  # {"/attachments/xxx/file.md": content}
 
 
 def _build_attachment_prompt(attachments: Sequence[dict]) -> str | None:
diff --git a/src/services/conversation_service.py b/src/services/conversation_service.py
index 3f44d101..e0ef33f1 100644
--- a/src/services/conversation_service.py
+++ b/src/services/conversation_service.py
@@ -84,19 +84,24 @@ async def _sync_thread_attachment_state(
         if not isinstance(existing_files, dict):
             existing_files = {}
 
-        attachment_files = _build_state_files(attachments)
-        merged_files = {
-            path: file_data
-            for path, file_data in existing_files.items()
-            if isinstance(path, str) and not path.startswith("/attachments/")
+        # 仅对 /attachments 命名空间做增量更新，避免覆盖 agent 运行期生成的其它文件。
+        next_attachment_files = _build_state_files(attachments)
+        prev_attachment_paths = {
+            path
+            for path in existing_files.keys()
+            if isinstance(path, str) and path.startswith("/attachments/")
         }
-        merged_files.update(attachment_files)
+        next_attachment_paths = set(next_attachment_files.keys())
+
+        file_updates: dict[str, dict | None] = {**next_attachment_files}
+        for removed_path in prev_attachment_paths - next_attachment_paths:
+            file_updates[removed_path] = None
 
         await graph.aupdate_state(
             config=config,
             values={
                 "attachments": attachments,
-                "files": merged_files,
+                "files": file_updates,
             },
         )
     except Exception as e:
diff --git a/test/test_conversation_service_attachment_state.py b/test/test_conversation_service_attachment_state.py
index fb04048a..5155f1ee 100644
--- a/test/test_conversation_service_attachment_state.py
+++ b/test/test_conversation_service_attachment_state.py
@@ -80,8 +80,8 @@ async def test_sync_thread_attachment_state_updates_graph(monkeypatch: pytest.Mo
     assert captured["write_config"] == {"configurable": {"thread_id": "thread-1", "user_id": "u1"}}
     assert captured["write_values"]["attachments"] == attachments
     assert "/attachments/resume.md" in captured["write_values"]["files"]
-    assert "/attachments/old.md" not in captured["write_values"]["files"]
-    assert "/work/result.md" in captured["write_values"]["files"]
+    assert captured["write_values"]["files"]["/attachments/old.md"] is None
+    assert "/work/result.md" not in captured["write_values"]["files"]
 
 
 @pytest.mark.asyncio