diff --git a/nanobot/utils/helpers.py b/nanobot/utils/helpers.py
index 86b4789d..3925e785 100644
--- a/nanobot/utils/helpers.py
+++ b/nanobot/utils/helpers.py
@@ -5,6 +5,7 @@ import json
 import os
 import re
 import shutil
+import stat
 import time
 import uuid
 from contextlib import suppress
@@ -454,8 +455,13 @@ def _cleanup_tool_result_buckets(root: Path, current_bucket: Path) -> None:
 
 def _write_text_atomic(path: Path, content: str) -> None:
     tmp = path.with_name(f".{path.name}.{uuid.uuid4().hex}.tmp")
+    existing_mode: int | None = None
+    with suppress(OSError):
+        existing_mode = stat.S_IMODE(path.stat().st_mode)
     try:
         with open(tmp, "w", encoding="utf-8") as f:
+            if existing_mode is not None:
+                os.chmod(tmp, existing_mode)
             f.write(content)
             f.flush()
             os.fsync(f.fileno())
diff --git a/tests/config/test_config_atomic_save.py b/tests/config/test_config_atomic_save.py
index f2ac2367..4ebde1ea 100644
--- a/tests/config/test_config_atomic_save.py
+++ b/tests/config/test_config_atomic_save.py
@@ -3,6 +3,8 @@
 from __future__ import annotations
 
 import json
+import os
+import stat
 from pathlib import Path
 
 import pytest
@@ -18,6 +20,17 @@ def test_save_config_round_trips(tmp_path: Path) -> None:
     assert loaded.agents.defaults.model
 
 
+@pytest.mark.skipif(os.name == "nt", reason="Windows does not expose POSIX file modes")
+def test_save_config_preserves_existing_file_mode(tmp_path: Path) -> None:
+    path = tmp_path / "config.json"
+    path.write_text("{}", encoding="utf-8")
+    path.chmod(0o600)
+
+    save_config(Config(), path)
+
+    assert stat.S_IMODE(path.stat().st_mode) == 0o600
+
+
 def test_save_config_preserves_existing_file_when_write_fails(
     tmp_path: Path, monkeypatch: pytest.MonkeyPatch
 ) -> None: