fix(responses-adapter): skip null reasoning summary text and keep thinking-only assistant turns

litellm/completion_extras/litellm_responses_transformation/transformation.py

@@ -428,12 +428,14 @@ class LiteLLMResponsesTransformationHandler(CompletionTransformationBridge):
                 if role == "assistant":
                     input_items.extend(_reasoning_input_items(msg))
                 input_items.append(
-                    {
+                    {  # mutable-ok: API message payload
                         "type": "message",
                         "role": role,
                         "content": self._convert_content_to_responses_format(content, cast(str, role)),
                     }
                 )
+            elif role == "assistant":
+                input_items.extend(_reasoning_input_items(msg))
 
         return input_items, instructions
 

litellm/llms/anthropic/experimental_pass_through/responses_adapters/transformation.py

@@ -110,8 +110,8 @@ class LiteLLMAnthropicToResponsesAPIAdapter:
     def _summary_part_text(part: object) -> str:
         if isinstance(part, Mapping):
             mapping: Final = cast(Mapping[str, Any], part)  # cast-ok: summary parts are untyped provider json
-            return str(mapping.get("text", ""))
-        return str(getattr(part, "text", ""))
+            return str(mapping.get("text") or "")
+        return str(getattr(part, "text", "") or "")
 
     @classmethod
     def _thinking_blocks_from_reasoning_item(

tests/test_litellm/completion_extras/litellm_responses_transformation/test_completion_extras_litellm_responses_transformation_transformation.py

@@ -3824,6 +3824,28 @@ def test_assistant_thinking_blocks_become_a_reasoning_input_item():
     assert "id" not in reasoning_item
 
 
+def test_thinking_only_assistant_turn_still_sends_its_reasoning():
+    """An assistant turn can be pure reasoning, with no visible text and no tool call."""
+    handler = LiteLLMResponsesTransformationHandler()
+    messages = [
+        {"role": "user", "content": "What is the weather in Denver?"},
+        {
+            "role": "assistant",
+            "content": None,
+            "thinking_blocks": [
+                {"type": "thinking", "thinking": "August in Denver is dry.", "signature": "sig1"}
+            ],
+        },
+        {"role": "user", "content": "Why?"},
+    ]
+
+    input_items, _ = handler.convert_chat_completion_messages_to_responses_api(messages)
+
+    reasoning_items = [item for item in input_items if item.get("type") == "reasoning"]
+    assert len(reasoning_items) == 1
+    assert reasoning_items[0]["summary"] == [{"type": "summary_text", "text": "August in Denver is dry."}]
+
+
 def test_stored_reasoning_items_win_over_thinking_blocks():
     """A minted reasoning id beats a re-derived one, so the two must not both be sent."""
     handler = LiteLLMResponsesTransformationHandler()

tests/test_litellm/llms/anthropic/experimental_pass_through/responses_adapters/test_responses_adapters_transformation.py

@@ -1249,6 +1249,20 @@ class TestTranslateResponse:
         result: Any = _ADAPTER.translate_response(response)
         assert result["content"] == []
 
+    def test_null_summary_text_skipped_rather_than_stringified(self):
+        """A summary part whose text is null must not reach the client as the word "None"."""
+        response = _make_mock_response(
+            output=[
+                {
+                    "type": "reasoning",
+                    "id": "rs_null_1",
+                    "summary": [{"type": "summary_text", "text": None}],
+                }
+            ]
+        )
+        result: Any = _ADAPTER.translate_response(response)
+        assert result["content"] == []
+
     def test_reasoning_item_id_never_becomes_a_thinking_signature(self):
         """Only Anthropic can sign a thinking block, so a stand-in signature is never invented."""
         reasoning = _make_reasoning_item(["Part one.", "Part two."], item_id="rs_abc123")