diff --git a/strix/agents/factory.py b/strix/agents/factory.py
index e40e60e8..9d599a54 100644
--- a/strix/agents/factory.py
+++ b/strix/agents/factory.py
@@ -160,7 +160,9 @@ def _schema_types(spec: dict[str, Any]) -> set[str]:
 def _decode_structured(value: str, types: set[str]) -> Any:
     stripped = value.strip()
     if not stripped:
-        return value
+        # An empty string is the model's "no value" for a list/dict param; give it
+        # the empty container so it validates instead of failing the type check.
+        return [] if "array" in types else {}
     try:
         decoded = json.loads(stripped)
     except json.JSONDecodeError:
diff --git a/tests/test_agent_factory_tool_arguments.py b/tests/test_agent_factory_tool_arguments.py
index 49dabf5d..70908f26 100644
--- a/tests/test_agent_factory_tool_arguments.py
+++ b/tests/test_agent_factory_tool_arguments.py
@@ -70,7 +70,6 @@ async def test_encoded_list_is_decoded_for_an_array_parameter(schema: dict[str,
         "auth",
         "Endpoint /admin leaks user data, and session tokens never expire",
         '"auth"',
-        "",
     ],
 )
 async def test_free_form_strings_are_never_split_into_an_array(value: str) -> None:
@@ -79,6 +78,29 @@ async def test_free_form_strings_are_never_split_into_an_array(value: str) -> No
     assert parsed["tags"] == value
 
 
+@pytest.mark.asyncio
+@pytest.mark.parametrize("schema", [_ARRAY, _NULLABLE_ARRAY])
+@pytest.mark.parametrize("value", ["", "   "])
+async def test_empty_string_becomes_an_empty_array(schema: dict[str, Any], value: str) -> None:
+    parsed = await _roundtrip(schema, {"tags": value})
+
+    assert parsed["tags"] == []
+
+
+@pytest.mark.asyncio
+async def test_empty_string_becomes_an_empty_object() -> None:
+    parsed = await _roundtrip(_OBJECT, {"modifications": ""})
+
+    assert parsed["modifications"] == {}
+
+
+@pytest.mark.asyncio
+async def test_empty_string_for_a_string_parameter_is_untouched() -> None:
+    parsed = await _roundtrip(_STRING, {"todos": ""})
+
+    assert parsed["todos"] == ""
+
+
 @pytest.mark.asyncio
 async def test_encoded_mapping_is_decoded_for_an_object_parameter() -> None:
     parsed = await _roundtrip(_OBJECT, {"modifications": '{"method": "POST"}'})