fix(factory): handle list content in map_system_message_pt

Fixes #23757

When the Anthropic pass-through endpoint converts messages to OpenAI
format, content fields can be lists of content blocks rather than
plain strings. map_system_message_pt assumed string content and
crashed with TypeError on concatenation.

Add _get_content_as_str() helper that normalizes both str and list
content to a string before merging, using the existing
convert_content_list_to_str utility.

Tests: 3 new test cases covering list content, mixed str/list, and
list content as last message.

litellm/litellm_core_utils/prompt_templates/factory.py

@@ -86,6 +86,15 @@ DEFAULT_ASSISTANT_CONTINUE_MESSAGE = ChatCompletionAssistantMessage(
 )  # similar to autogen. Only used if `litellm.modify_params=True`.
 
 
+def _get_content_as_str(content: Union[str, list]) -> str:
+    """Extract text from content that may be a string or a list of content blocks."""
+    if isinstance(content, str):
+        return content
+    if isinstance(content, list):
+        return convert_content_list_to_str({"content": content})
+    return str(content)
+
+
 def map_system_message_pt(messages: list) -> list:
     """
     Convert 'system' message to 'user' message if provider doesn't support 'system' role.
@@ -100,6 +109,7 @@ def map_system_message_pt(messages: list) -> list:
     new_messages = []
     for i, m in enumerate(messages):
         if m["role"] == "system":
+            system_text = _get_content_as_str(m["content"])
             if i < len(messages) - 1:  # Not the last message
                 next_m = messages[i + 1]
                 next_role = next_m["role"]
@@ -107,13 +117,14 @@ def map_system_message_pt(messages: list) -> list:
                     next_role == "user" or next_role == "assistant"
                 ):  # Next message is a user or assistant message
                     # Merge system prompt into the next message
-                    next_m["content"] = m["content"] + " " + next_m["content"]
+                    next_text = _get_content_as_str(next_m["content"])
+                    next_m["content"] = system_text + " " + next_text
                 elif next_role == "system":  # Next message is a system message
                     # Append a user message instead of the system message
-                    new_message = {"role": "user", "content": m["content"]}
+                    new_message = {"role": "user", "content": system_text}
                     new_messages.append(new_message)
             else:  # Last message
-                new_message = {"role": "user", "content": m["content"]}
+                new_message = {"role": "user", "content": system_text}
                 new_messages.append(new_message)
         else:  # Not a system message
             new_messages.append(m)

tests/llm_translation/test_optional_params.py

@@ -55,6 +55,61 @@ def test_supports_system_message():
     assert isinstance(response, litellm.ModelResponse)
 
 
+def test_supports_system_message_list_content():
+    """
+    Test map_system_message_pt when content is a list of content blocks
+    (e.g. from Anthropic pass-through endpoint).
+
+    Fixes: https://github.com/BerriAI/litellm/issues/23757
+    """
+    # System message with list content (Anthropic format)
+    messages = [
+        {"role": "system", "content": [{"type": "text", "text": "You are helpful."}]},
+        {"role": "user", "content": [{"type": "text", "text": "Hello!"}]},
+    ]
+
+    new_messages = map_system_message_pt(messages=messages)
+
+    assert len(new_messages) == 1
+    assert new_messages[0]["role"] == "user"
+    assert isinstance(new_messages[0]["content"], str)
+    assert "You are helpful." in new_messages[0]["content"]
+    assert "Hello!" in new_messages[0]["content"]
+
+
+def test_supports_system_message_mixed_content():
+    """
+    Test map_system_message_pt with mixed str and list content types.
+    """
+    messages = [
+        {"role": "system", "content": "System prompt"},
+        {"role": "user", "content": [{"type": "text", "text": "User message"}]},
+    ]
+
+    new_messages = map_system_message_pt(messages=messages)
+
+    assert len(new_messages) == 1
+    assert new_messages[0]["role"] == "user"
+    assert isinstance(new_messages[0]["content"], str)
+    assert "System prompt" in new_messages[0]["content"]
+    assert "User message" in new_messages[0]["content"]
+
+
+def test_supports_system_message_list_content_last_message():
+    """
+    Test map_system_message_pt when system message with list content is the last message.
+    """
+    messages = [
+        {"role": "system", "content": [{"type": "text", "text": "Only system"}]},
+    ]
+
+    new_messages = map_system_message_pt(messages=messages)
+
+    assert len(new_messages) == 1
+    assert new_messages[0]["role"] == "user"
+    assert new_messages[0]["content"] == "Only system"
+
+
 @pytest.mark.parametrize(
     "stop_sequence, expected_count", [("\n", 0), (["\n"], 0), (["finish_reason"], 1)]
 )