diff --git a/engine/agents/openai_event_mapper.py b/engine/agents/openai_event_mapper.py
index e69008f..b0e0ba1 100644
--- a/engine/agents/openai_event_mapper.py
+++ b/engine/agents/openai_event_mapper.py
@@ -93,7 +93,13 @@ def _map_assistant_message(
         raw_item = item.raw_item
         item_id = raw_item.id
         parts = raw_item.content
-        text = "".join(part.text for part in parts if isinstance(part, ResponseOutputText))
+        # ``part.text`` can be ``None`` on leniently-parsed provider payloads:
+        # Gemini (via the LiteLLM Responses translation) emits ``output_text``
+        # parts with no ``text`` field when a turn is tool-calls-only, and the
+        # SDK materializes those without validation. Joining ``None`` raises
+        # ``TypeError: sequence item 0: expected str instance, NoneType found``
+        # and kills the run (INF: analysis-smoke gemini-3.6-flash failures).
+        text = "".join(part.text or "" for part in parts if isinstance(part, ResponseOutputText))
         refusal_text = _extract_refusal_text(parts=parts, text=text)
         if refusal_text is not None:
             return MappedEvent(refusal_text=refusal_text)
diff --git a/tests/unit/agents/test_openai_event_mapper.py b/tests/unit/agents/test_openai_event_mapper.py
index 5e71082..59f0094 100644
--- a/tests/unit/agents/test_openai_event_mapper.py
+++ b/tests/unit/agents/test_openai_event_mapper.py
@@ -196,3 +196,38 @@ def test_tool_call_and_output_have_distinct_item_ids() -> None:
     assert output_mapped.context_item is not None
     assert call_mapped.context_item.item_id == "tool-call-call_xyz"
     assert output_mapped.context_item.item_id == "tool-result-call_xyz"
+
+
+def test_assistant_text_part_with_none_text_maps_as_empty() -> None:
+    """Gemini via the LiteLLM Responses translation emits ``output_text`` parts
+    with no ``text`` field on tool-calls-only turns; the SDK materializes them
+    leniently with ``text=None``. The mapper must treat those as empty instead
+    of raising ``TypeError: sequence item 0: expected str instance, NoneType
+    found`` (which failed whole HALO runs on gemini-3.6-flash).
+    """
+    from agents.items import MessageOutputItem
+    from agents.stream_events import RunItemStreamEvent
+    from openai.types.responses import ResponseOutputMessage, ResponseOutputText
+
+    from tests._sdk_events import SHARED_AGENT
+
+    raw = ResponseOutputMessage.model_construct(
+        id="msg_none_text",
+        type="message",
+        role="assistant",
+        status="completed",
+        content=[ResponseOutputText.model_construct(type="output_text", text=None, annotations=[])],
+    )
+    event = RunItemStreamEvent(
+        name="message_output_created",
+        item=MessageOutputItem(agent=SHARED_AGENT, raw_item=raw),
+    )
+
+    mapper = OpenAiEventMapper()
+    mapped = mapper.to_mapped_event(event, execution=_exec(), is_root=True)
+    assert mapped.refusal_text is None
+    assert mapped.context_item is not None
+    assert mapped.context_item.content is None
+    assert mapped.output_item is not None
+    assert mapped.output_item.final is False
+    assert mapped.output_item.item.content is None