fix(responses): keep prompt cache breakpoint on stringified content blocks

litellm/completion_extras/litellm_responses_transformation/transformation.py

@@ -1057,7 +1057,10 @@ class LiteLLMResponsesTransformationHandler(CompletionTransformationBridge):
                             verbose_logger.debug("Chat provider:   passthrough -> %s", item)
                         else:
                             # Default to input_text for unknown types
-                            converted = self._convert_content_str_to_input_text(str(item.get("text", item)), role)
+                            converted = with_prompt_cache_breakpoint(
+                                self._convert_content_str_to_input_text(str(item.get("text", item)), role),
+                                item.get("prompt_cache_breakpoint"),
+                            )
                             result.append(converted)
                             verbose_logger.debug("Chat provider:   unknown(%s) -> %s", original_type, converted)
             verbose_logger.debug("Chat provider: Final converted content: %s", result)

tests/test_litellm/completion_extras/litellm_responses_transformation/test_completion_extras_litellm_responses_transformation_transformation.py

@@ -4015,3 +4015,38 @@ def test_convert_chat_completion_messages_to_responses_api_omits_absent_prompt_c
     response, _ = handler.convert_chat_completion_messages_to_responses_api(messages)
 
     assert all("prompt_cache_breakpoint" not in block for block in response[0]["content"])
+
+
+def test_convert_chat_completion_messages_to_responses_api_keeps_prompt_cache_breakpoint_on_unknown_block():
+    """A block type the bridge cannot map still has to keep the marker.
+
+    The hook marks the last block of the message it targets, so a message ending in a block this
+    bridge does not know falls through to the stringify path. Losing the marker there is the same
+    failure as losing it on a text block: the request goes out in explicit mode with nothing marked.
+    """
+    from litellm.completion_extras.litellm_responses_transformation.transformation import (
+        LiteLLMResponsesTransformationHandler,
+    )
+
+    handler = LiteLLMResponsesTransformationHandler()
+    breakpoint_marker = {"mode": "explicit"}
+
+    messages = [
+        {
+            "role": "user",
+            "content": [
+                {"type": "text", "text": "describe this"},
+                {
+                    "type": "input_audio",
+                    "input_audio": {"data": "Zm9v", "format": "wav"},
+                    "prompt_cache_breakpoint": breakpoint_marker,
+                },
+            ],
+        },
+    ]
+
+    response, _ = handler.convert_chat_completion_messages_to_responses_api(messages)
+
+    content = response[0]["content"]
+    assert [block["type"] for block in content] == ["input_text", "input_text"]
+    assert content[1]["prompt_cache_breakpoint"] == breakpoint_marker

tests/test_litellm/responses/test_responses_prompt_management.py

@@ -428,7 +428,6 @@ class TestResponsesAPIPromptManagement:
         sent_input = mock_handler.call_args.kwargs["input"]
         assert "cache_control" not in sent_input[0]
 
-
     def test_all_non_message_input_items_remain_unchanged(self):
         reasoning_item = {
             "type": "reasoning",