fix(zai): flatten list-format content in tool/assistant messages for GLM

GLM's Jinja chat template checks `m.content is string` — when it
receives list-format content parts (as sent by Go clients like
openai-go), the content is silently replaced with an empty artifact.

Flatten list-format content to strings for tool/assistant messages
using the existing convert_content_list_to_str utility. User messages
are left intact so the parent's image_url processing still works.

Fixes #25868

Signed-off-by: Jay <moonandstar99@yahoo.com>

litellm/llms/zai/chat/transformation.py

@@ -1,5 +1,8 @@
-from typing import List, Optional, Tuple
+from typing import Any, Coroutine, List, Literal, Optional, Tuple, Union, overload
 
+from litellm.litellm_core_utils.prompt_templates.common_utils import (
+    convert_content_list_to_str,
+)
 from litellm.secret_managers.main import get_secret_str
 from litellm.types.llms.openai import AllMessageValues, ChatCompletionToolParam
 
@@ -20,6 +23,50 @@ class ZAIChatConfig(OpenAIGPTConfig):
         dynamic_api_key = api_key or get_secret_str("ZAI_API_KEY")
         return api_base, dynamic_api_key
 
+    @overload
+    def _transform_messages(
+        self, messages: List[AllMessageValues], model: str, is_async: Literal[True]
+    ) -> Coroutine[Any, Any, List[AllMessageValues]]: ...
+
+    @overload
+    def _transform_messages(
+        self,
+        messages: List[AllMessageValues],
+        model: str,
+        is_async: Literal[False] = False,
+    ) -> List[AllMessageValues]: ...
+
+    def _transform_messages(
+        self, messages: List[AllMessageValues], model: str, is_async: bool = False
+    ) -> Union[List[AllMessageValues], Coroutine[Any, Any, List[AllMessageValues]]]:
+        """Flatten list-format content in tool/assistant messages for GLM.
+
+        GLM's Jinja template checks ``m.content is string`` — list-format
+        content parts (used by Go clients like openai-go) are silently
+        dropped.  Flatten them to strings before forwarding.
+
+        Only tool/assistant roles are flattened — user messages are left
+        intact so the parent's image_url processing can handle them.
+
+        See: https://github.com/BerriAI/litellm/issues/25868
+        """
+        for message in messages:
+            role = message.get("role")
+            if role in ("tool", "assistant"):
+                content = message.get("content")
+                if content is not None and not isinstance(content, str):
+                    text = convert_content_list_to_str(message)
+                    message["content"] = text if text else ""
+
+        if is_async:
+            return super()._transform_messages(
+                messages=messages, model=model, is_async=True
+            )
+        else:
+            return super()._transform_messages(
+                messages=messages, model=model, is_async=False
+            )
+
     def remove_cache_control_flag_from_messages_and_tools(
         self,
         model: str,

tests/litellm/llms/zai/chat/test_zai_chat_transformation.py

@@ -0,0 +1,157 @@
+"""
+Unit tests for ZAI/GLM chat transformation.
+
+Tests that list-format content in tool/assistant messages is flattened
+to strings before sending to GLM, which requires string-type content.
+
+See: https://github.com/BerriAI/litellm/issues/25868
+"""
+
+import pytest
+
+from litellm.llms.zai.chat.transformation import ZAIChatConfig
+
+
+class TestZAITransformMessages:
+    """Test that ZAIChatConfig._transform_messages flattens tool/assistant content."""
+
+    def setup_method(self):
+        self.config = ZAIChatConfig()
+
+    def test_tool_message_list_content_flattened(self):
+        """Tool message with list content is flattened to string."""
+        messages = [
+            {"role": "user", "content": "What is 1+1?"},
+            {
+                "role": "assistant",
+                "content": None,
+                "tool_calls": [
+                    {
+                        "id": "call_1",
+                        "type": "function",
+                        "function": {"name": "calc", "arguments": '{"x": 1}'},
+                    }
+                ],
+            },
+            {
+                "role": "tool",
+                "tool_call_id": "call_1",
+                "content": [{"type": "text", "text": "2"}],
+            },
+        ]
+
+        result = self.config._transform_messages(messages, model="glm-4.6")
+
+        tool_msg = [m for m in result if m.get("role") == "tool"][0]
+        assert isinstance(tool_msg["content"], str)
+        assert tool_msg["content"] == "2"
+
+    def test_tool_message_string_content_unchanged(self):
+        """Tool message with string content passes through."""
+        messages = [
+            {"role": "tool", "tool_call_id": "call_1", "content": "result text"},
+        ]
+
+        result = self.config._transform_messages(messages, model="glm-4.6")
+
+        assert result[0]["content"] == "result text"
+
+    def test_assistant_message_list_content_flattened(self):
+        """Assistant message with list content is flattened."""
+        messages = [
+            {
+                "role": "assistant",
+                "content": [{"type": "text", "text": "Hello there"}],
+            },
+        ]
+
+        result = self.config._transform_messages(messages, model="glm-4.6")
+
+        assert isinstance(result[0]["content"], str)
+        assert result[0]["content"] == "Hello there"
+
+    def test_user_message_not_modified(self):
+        """User messages are not modified by the ZAI transform.
+
+        User messages may contain image_url content parts that the parent
+        class processes — we must not flatten those.
+        """
+        messages = [
+            {
+                "role": "user",
+                "content": [{"type": "text", "text": "hello"}],
+            },
+        ]
+
+        result = self.config._transform_messages(messages, model="glm-4.6")
+
+        assert isinstance(result[0]["content"], list)
+
+    def test_system_message_not_modified(self):
+        """System messages are not modified."""
+        messages = [
+            {"role": "system", "content": "You are helpful."},
+        ]
+
+        result = self.config._transform_messages(messages, model="glm-4.6")
+
+        assert result[0]["content"] == "You are helpful."
+
+    def test_tool_message_none_content_unchanged(self):
+        """Tool message with None content stays None."""
+        messages = [
+            {"role": "tool", "tool_call_id": "call_1", "content": None},
+        ]
+
+        result = self.config._transform_messages(messages, model="glm-4.6")
+
+        assert result[0]["content"] is None
+
+    def test_multiple_tool_messages_all_flattened(self):
+        """Multiple tool messages with list content are all flattened."""
+        messages = [
+            {
+                "role": "tool",
+                "tool_call_id": "call_1",
+                "content": [{"type": "text", "text": "result 1"}],
+            },
+            {
+                "role": "tool",
+                "tool_call_id": "call_2",
+                "content": [{"type": "text", "text": "result 2"}],
+            },
+        ]
+
+        result = self.config._transform_messages(messages, model="glm-4.6")
+
+        assert all(isinstance(m["content"], str) for m in result)
+        assert result[0]["content"] == "result 1"
+        assert result[1]["content"] == "result 2"
+
+    def test_tool_message_empty_list_becomes_empty_string(self):
+        """Tool message with empty list content becomes empty string."""
+        messages = [
+            {"role": "tool", "tool_call_id": "call_1", "content": []},
+        ]
+
+        result = self.config._transform_messages(messages, model="glm-4.6")
+
+        assert result[0]["content"] == ""
+
+    def test_non_text_content_parts_dropped(self):
+        """Non-text content parts (e.g., image_url) in tool messages are dropped."""
+        messages = [
+            {
+                "role": "tool",
+                "tool_call_id": "call_1",
+                "content": [
+                    {"type": "image_url", "image_url": {"url": "https://example.com/img.png"}},
+                    {"type": "text", "text": "caption"},
+                ],
+            },
+        ]
+
+        result = self.config._transform_messages(messages, model="glm-4.6")
+
+        assert isinstance(result[0]["content"], str)
+        assert "caption" in result[0]["content"]