diff --git a/src/services/conversation_service.py b/src/services/conversation_service.py
index 2039599e..3f44d101 100644
--- a/src/services/conversation_service.py
+++ b/src/services/conversation_service.py
@@ -77,11 +77,26 @@ async def _sync_thread_attachment_state(
             return
 
         graph = await agent.get_graph()
+        config = {"configurable": {"thread_id": thread_id, "user_id": str(user_id)}}
+        state = await graph.aget_state(config)
+        state_values = getattr(state, "values", {}) if state else {}
+        existing_files = state_values.get("files", {}) if isinstance(state_values, dict) else {}
+        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/")
+        }
+        merged_files.update(attachment_files)
+
         await graph.aupdate_state(
-            config={"configurable": {"thread_id": thread_id, "user_id": str(user_id)}},
+            config=config,
             values={
                 "attachments": attachments,
-                "files": _build_state_files(attachments),
+                "files": merged_files,
             },
         )
     except Exception as e:
diff --git a/test/test_conversation_service_attachment_state.py b/test/test_conversation_service_attachment_state.py
index def2a918..fb04048a 100644
--- a/test/test_conversation_service_attachment_state.py
+++ b/test/test_conversation_service_attachment_state.py
@@ -37,11 +37,23 @@ def test_build_state_files_only_parsed_and_with_content():
 @pytest.mark.asyncio
 async def test_sync_thread_attachment_state_updates_graph(monkeypatch: pytest.MonkeyPatch):
     captured: dict = {}
+    fake_state = SimpleNamespace(
+        values={
+            "files": {
+                "/attachments/old.md": {"content": ["old"]},
+                "/work/result.md": {"content": ["keep"]},
+            }
+        }
+    )
 
     class FakeGraph:
+        async def aget_state(self, config):
+            captured["read_config"] = config
+            return fake_state
+
         async def aupdate_state(self, *, config, values):
-            captured["config"] = config
-            captured["values"] = values
+            captured["write_config"] = config
+            captured["write_values"] = values
 
     class FakeAgent:
         async def get_graph(self):
@@ -64,9 +76,12 @@ async def test_sync_thread_attachment_state_updates_graph(monkeypatch: pytest.Mo
         attachments=attachments,
     )
 
-    assert captured["config"] == {"configurable": {"thread_id": "thread-1", "user_id": "u1"}}
-    assert captured["values"]["attachments"] == attachments
-    assert "/attachments/resume.md" in captured["values"]["files"]
+    assert captured["read_config"] == {"configurable": {"thread_id": "thread-1", "user_id": "u1"}}
+    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"]
 
 
 @pytest.mark.asyncio