diff --git a/backend/chat.py b/backend/chat.py
index 31e9cd0..d222020 100644
--- a/backend/chat.py
+++ b/backend/chat.py
@@ -100,5 +100,6 @@ def _collect(nodes: list, parts: list):
         parts = []
         _collect(tree if isinstance(tree, list) else [tree], parts)
         return "\n\n".join(parts)
-    except Exception:
+    except Exception as e:
+        logger.warning(f"Failed to extract summaries from structure JSON: {e}")
         return ""
diff --git a/tests/test_api.py b/tests/test_api.py
index 899d446..4db0edf 100644
--- a/tests/test_api.py
+++ b/tests/test_api.py
@@ -135,6 +135,44 @@ def test_answer_does_not_contain_suggestion_marker(self, client_with_docs):
         assert "💡 SUGERENCIA:" not in data["answer"]
 
 
+# ---------------------------------------------------------------------------
+# _extract_summaries (unit)
+# ---------------------------------------------------------------------------
+
+class TestExtractSummaries:
+    def test_returns_empty_string_on_invalid_json(self, caplog):
+        import logging
+        from backend.chat import _extract_summaries
+
+        with caplog.at_level(logging.WARNING, logger="backend.chat"):
+            result = _extract_summaries("not valid json{{{")
+
+        assert result == ""
+        assert "Failed to extract summaries" in caplog.text
+
+    def test_extracts_nested_summaries(self):
+        from backend.chat import _extract_summaries
+        import json
+
+        structure = json.dumps([{
+            "title": "Root",
+            "summary": "Root summary",
+            "nodes": [
+                {"title": "Child", "summary": "Child summary", "nodes": []}
+            ]
+        }])
+        result = _extract_summaries(structure)
+        assert "Root summary" in result
+        assert "Child summary" in result
+
+    def test_returns_empty_string_on_empty_nodes(self):
+        from backend.chat import _extract_summaries
+        import json
+
+        result = _extract_summaries(json.dumps([{"title": "T", "nodes": []}]))
+        assert result == ""
+
+
 # ---------------------------------------------------------------------------
 # POST /upload
 # ---------------------------------------------------------------------------