diff --git a/swh/scanner/model.py b/swh/scanner/model.py
--- a/swh/scanner/model.py
+++ b/swh/scanner/model.py
@@ -5,6 +5,7 @@
 
 from __future__ import annotations
 import sys
+import json
 from pathlib import PosixPath
 from typing import Any, Dict
 from enum import Enum
@@ -102,4 +103,4 @@
                 if next_tree:
                     child_tree[rel_path] = next_tree
 
-        return child_tree
+        return json.dumps(child_tree)