fix: ignore non-list QueryExpander replies (#11630)

Co-authored-by: Stefano Fiorucci <stefanofiorucci@gmail.com>

Author: oyinkanchekwas

haystack/components/query/query_expander.py

@@ -265,6 +265,13 @@ def _parse_expanded_queries(generator_response: str) -> list[str]:
         if parsed is None:
             return []
 
+        if not isinstance(parsed["queries"], list):
+            logger.warning(
+                "Expected 'queries' to be a list but got {type}. Returning no expanded queries.",
+                type=type(parsed["queries"]).__name__,
+            )
+            return []
+
         queries = []
         for item in parsed["queries"]:
             if isinstance(item, str) and item.strip():

releasenotes/notes/query-expander-reject-non-list-queries-6f3b1c2d8e9a4b70.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    ``QueryExpander`` now ignores malformed responses where ``queries`` is not a list, so a string response is no longer
+    split into single-character queries.

test/components/query/test_query_expander.py

@@ -193,6 +193,17 @@ def test_parse_expanded_queries_non_list_json(self, monkeypatch):
         queries = expander._parse_expanded_queries('{"not": "a list"}')
         assert queries == []
 
+    @pytest.mark.parametrize("queries_value", ['"single query"', '{"query": "value"}', "null"])
+    def test_parse_expanded_queries_rejects_non_list_queries_value(self, monkeypatch, caplog, queries_value):
+        monkeypatch.setenv("OPENAI_API_KEY", "test-key-12345")
+        expander = QueryExpander()
+
+        with caplog.at_level(logging.WARNING):
+            queries = expander._parse_expanded_queries(f'{{"queries": {queries_value}}}')
+
+        assert queries == []
+        assert "Expected 'queries' to be a list" in caplog.text
+
     def test_parse_expanded_queries_mixed_types(self, monkeypatch):
         monkeypatch.setenv("OPENAI_API_KEY", "test-key-12345")
         expander = QueryExpander()