diff --git a/router/main.py b/router/main.py
index 556a51b..1b23603 100644
--- a/router/main.py
+++ b/router/main.py
@@ -624,6 +624,7 @@ def _atomic_write_json_sync(path: str, data) -> None:
     """Synchronously write JSON data to path using atomic temp-file + os.replace."""
     os.makedirs(os.path.dirname(path), exist_ok=True)
     fd, tmp_path = tempfile.mkstemp(dir=os.path.dirname(path), suffix=".tmp")
+    written = False
     try:
         try:
             f = os.fdopen(fd, "w", encoding="utf-8")
@@ -634,12 +635,13 @@ def _atomic_write_json_sync(path: str, data) -> None:
         with f:
             json.dump(data, f, indent=2)
         os.replace(tmp_path, path)
-    except Exception:
-        try:
-            os.unlink(tmp_path)
-        except Exception:
-            pass
-        raise
+        written = True
+    finally:
+        if not written:
+            try:
+                os.unlink(tmp_path)
+            except OSError:
+                pass
 
 
 async def _atomic_write_json_async(path: str, data) -> None:
diff --git a/tests/test_atomic_write.py b/tests/test_atomic_write.py
index 99d220e..c944c69 100644
--- a/tests/test_atomic_write.py
+++ b/tests/test_atomic_write.py
@@ -229,3 +229,35 @@ def test_atomic_write_json_sync_unlink_error(mock_unlink, mock_replace, tmp_path
 
     # Verify target file wasn't created
     assert not target_file.exists()
+
+
+@patch("router.main.os.close")
+@patch("router.main.os.fdopen")
+def test_atomic_write_json_sync_fdopen_non_os_error(mock_fdopen, mock_close, tmp_path):
+    """Test that non-OSError in fdopen closes fd and unlinks temp file without leaking."""
+    target_file = tmp_path / "data.json"
+    data = {"key": "value"}
+
+    mock_fdopen.side_effect = RuntimeError("Mocked fdopen runtime error")
+
+    with pytest.raises(RuntimeError, match="Mocked fdopen runtime error"):
+        _atomic_write_json_sync(str(target_file), data)
+
+    assert mock_close.called
+    assert not target_file.exists()
+    assert list(tmp_path.iterdir()) == []
+
+
+@patch("router.main.json.dump")
+def test_atomic_write_json_sync_unexpected_exception_cleanup(mock_dump, tmp_path):
+    """Test cleanup on unexpected exception types (e.g. RecursionError)."""
+    target_file = tmp_path / "data.json"
+    data = {"key": "value"}
+
+    mock_dump.side_effect = RecursionError("Mocked recursion limit")
+
+    with pytest.raises(RecursionError, match="Mocked recursion limit"):
+        _atomic_write_json_sync(str(target_file), data)
+
+    assert not target_file.exists()
+    assert list(tmp_path.iterdir()) == []