fix(factory): merge system message into content-block lists without string concat

litellm/litellm_core_utils/prompt_templates/factory.py

@@ -87,6 +87,18 @@ DEFAULT_ASSISTANT_CONTINUE_MESSAGE = ChatCompletionAssistantMessage(
 )  # similar to autogen. Only used if `litellm.modify_params=True`.
 
 
+def _merge_message_content(system_content: Union[str, list], next_content: Union[str, list]) -> Union[str, list]:
+    if isinstance(system_content, str) and isinstance(next_content, str):
+        return system_content + " " + next_content
+
+    def as_blocks(content: Union[str, list]) -> list:
+        if isinstance(content, str):
+            return [{"type": "text", "text": content}]
+        return list(content)
+
+    return as_blocks(system_content) + as_blocks(next_content)
+
+
 def map_system_message_pt(messages: list) -> list:
     """
     Convert 'system' message to 'user' message if provider doesn't support 'system' role.
@@ -106,7 +118,7 @@ def map_system_message_pt(messages: list) -> list:
                 next_role = next_m["role"]
                 if 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_m["content"] = _merge_message_content(m["content"], next_m["content"])
                 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"]}

tests/llm_translation/test_optional_params.py

@@ -55,6 +55,69 @@ def test_supports_system_message():
     assert isinstance(response, litellm.ModelResponse)
 
 
+def test_map_system_message_pt_content_block_lists():
+    """
+    Regression for https://github.com/BerriAI/litellm/issues/32904
+
+    When system and/or user content is a list of content blocks (Anthropic-style),
+    merging must not string-concatenate a list. Merge as a block list instead.
+    """
+    messages = [
+        {"role": "system", "content": [{"type": "text", "text": "Be brief."}]},
+        {"role": "user", "content": [{"type": "text", "text": "Hi"}]},
+    ]
+
+    new_messages = map_system_message_pt(messages=messages)
+
+    assert len(new_messages) == 1
+    assert new_messages[0]["role"] == "user"
+    assert new_messages[0]["content"] == [
+        {"type": "text", "text": "Be brief."},
+        {"type": "text", "text": "Hi"},
+    ]
+
+
+def test_map_system_message_pt_mixed_string_and_content_blocks():
+    """
+    Mixed forms: a plain string on one side and a content-block list on the other
+    should merge into a block list rather than raising.
+    """
+    system_list_user_str = map_system_message_pt(
+        messages=[
+            {"role": "system", "content": [{"type": "text", "text": "Be brief."}]},
+            {"role": "user", "content": "Hi"},
+        ]
+    )
+    assert system_list_user_str[0]["content"] == [
+        {"type": "text", "text": "Be brief."},
+        {"type": "text", "text": "Hi"},
+    ]
+
+    system_str_user_list = map_system_message_pt(
+        messages=[
+            {"role": "system", "content": "Be brief."},
+            {"role": "user", "content": [{"type": "text", "text": "Hi"}]},
+        ]
+    )
+    assert system_str_user_list[0]["content"] == [
+        {"type": "text", "text": "Be brief."},
+        {"type": "text", "text": "Hi"},
+    ]
+
+
+def test_map_system_message_pt_string_content_unchanged():
+    """
+    Plain string content on both sides keeps the original space-joined behavior.
+    """
+    new_messages = map_system_message_pt(
+        messages=[
+            {"role": "system", "content": "Be brief."},
+            {"role": "user", "content": "Hi"},
+        ]
+    )
+    assert new_messages[0]["content"] == "Be brief. Hi"
+
+
 @pytest.mark.parametrize(
     "stop_sequence, expected_count", [("\n", 0), (["\n"], 0), (["finish_reason"], 1)]
 )