Fix map_system_message_pt for content-block list messages

map_system_message_pt (used by supports_system_message=False) merged the
system prompt into the next message with string concatenation, raising
'TypeError: can only concatenate list (not "str") to list' whenever the
system or user content was a list of content blocks - the format sent by
Anthropic-style clients (e.g. Claude Code / Agent SDK) through the
/v1/messages endpoint.

Merge as a list of content blocks when either side is a list; keep the
existing string behavior otherwise. Adds regression tests.

litellm/litellm_core_utils/prompt_templates/factory.py

@@ -87,6 +87,13 @@ DEFAULT_ASSISTANT_CONTINUE_MESSAGE = ChatCompletionAssistantMessage(
 )  # similar to autogen. Only used if `litellm.modify_params=True`.
 
 
+def _content_as_block_list(content) -> list:
+    """Normalize a message content field to a list of content blocks."""
+    if isinstance(content, list):
+        return content
+    return [{"type": "text", "text": content}]
+
+
 def map_system_message_pt(messages: list) -> list:
     """
     Convert 'system' message to 'user' message if provider doesn't support 'system' role.
@@ -105,8 +112,17 @@ def map_system_message_pt(messages: list) -> list:
                 next_m = messages[i + 1]
                 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"]
+                    # Merge system prompt into the next message. Contents may be
+                    # strings or lists of content blocks (e.g. Anthropic-style
+                    # requests); merge as block lists when either side is a list.
+                    if isinstance(m["content"], list) or isinstance(
+                        next_m["content"], list
+                    ):
+                        next_m["content"] = _content_as_block_list(
+                            m["content"]
+                        ) + _content_as_block_list(next_m["content"])
+                    else:
+                        next_m["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/test_litellm/litellm_core_utils/prompt_templates/test_litellm_core_utils_prompt_templates_factory.py

@@ -3166,3 +3166,58 @@ async def test_bedrock_converse_message_level_cache_point_preserves_ttl_async():
     )
 
     assert _collect_cache_points(result) == [{"type": "default", "ttl": "1h"}]
+
+
+def test_map_system_message_pt_string_contents():
+    """Baseline: string contents keep the existing merge behavior."""
+    from litellm.litellm_core_utils.prompt_templates.factory import (
+        map_system_message_pt,
+    )
+
+    messages = [
+        {"role": "system", "content": "You are helpful."},
+        {"role": "user", "content": "Hi"},
+    ]
+    result = map_system_message_pt(messages=messages)
+    assert result == [{"role": "user", "content": "You are helpful. Hi"}]
+
+
+def test_map_system_message_pt_content_block_lists():
+    """Contents sent as content-block lists (e.g. Anthropic-style requests)
+    must not raise 'can only concatenate list (not "str") to list'.
+    Regression test for supports_system_message=False with block content."""
+    from litellm.litellm_core_utils.prompt_templates.factory import (
+        map_system_message_pt,
+    )
+
+    messages = [
+        {
+            "role": "system",
+            "content": [{"type": "text", "text": "You are helpful."}],
+        },
+        {"role": "user", "content": [{"type": "text", "text": "Hi"}]},
+    ]
+    result = map_system_message_pt(messages=messages)
+    assert len(result) == 1
+    assert result[0]["role"] == "user"
+    assert result[0]["content"] == [
+        {"type": "text", "text": "You are helpful."},
+        {"type": "text", "text": "Hi"},
+    ]
+
+
+def test_map_system_message_pt_mixed_contents():
+    """String system content merged into a block-list user message."""
+    from litellm.litellm_core_utils.prompt_templates.factory import (
+        map_system_message_pt,
+    )
+
+    messages = [
+        {"role": "system", "content": "You are helpful."},
+        {"role": "user", "content": [{"type": "text", "text": "Hi"}]},
+    ]
+    result = map_system_message_pt(messages=messages)
+    assert result[0]["content"] == [
+        {"type": "text", "text": "You are helpful."},
+        {"type": "text", "text": "Hi"},
+    ]