fix(responses): keep bare strings in a system content part list

Filtering a content list down to dicts dropped plain strings, which
upstream normalization leaves in place, so part of the system prompt
went missing from the merged message.

litellm/responses/litellm_completion_transformation/transformation.py

@@ -468,13 +468,24 @@ class LiteLLMCompletionResponsesConfig:
         the backends that reject a trailing system message are the same ones that expect one.
         """
         texts: Final = tuple(
-            content
-            if isinstance(content, str)
-            else "\n\n".join(part.get("text", "") for part in content if isinstance(part, dict))
-            for content in contents
+            text for content in contents for text in LiteLLMCompletionResponsesConfig._system_content_texts(content)
         )
         return ChatCompletionSystemMessage(role="system", content="\n\n".join(text for text in texts if text))
 
+    @staticmethod
+    def _system_content_texts(content: object) -> tuple[str, ...]:
+        """Every text a system content field carries, as a plain string or as a part list
+        that may mix bare strings with text parts."""
+        if isinstance(content, str):
+            return (content,)
+        if not isinstance(content, (list, tuple)):
+            return ()
+        return tuple(
+            part if isinstance(part, str) else str(part.get("text", ""))
+            for part in content
+            if isinstance(part, (str, dict))
+        )
+
     @staticmethod
     async def async_responses_api_session_handler(
         previous_response_id: str,

tests/test_litellm/responses/litellm_completion_transformation/test_system_message_hoisting.py

@@ -73,3 +73,17 @@ def test_a_conversation_without_a_system_message_is_unchanged():
     roles, _ = _roles([{"role": "user", "content": "q"}], {})
 
     assert roles == ["user"]
+
+
+def test_bare_strings_in_a_part_list_survive_the_merge():
+    """Upstream normalization leaves plain strings in a content list, so filtering the
+    list down to dicts silently dropped part of the prompt."""
+    _, messages = _roles(
+        [
+            {"role": "user", "content": "q"},
+            {"role": "system", "content": ["B", {"type": "text", "text": "C"}]},
+        ],
+        {"instructions": "A"},
+    )
+
+    assert messages[0]["content"] == "A\n\nB\n\nC"