fix(logging): keep system prompt only in system_prompt field

Stop prepending string system prompts into messages when building
StandardLoggingPayload. With the dedicated system_prompt field, duplicating
string prompts in messages was inconsistent with list/block prompts.

Removes append_system_prompt_messages and updates tests to assert both
string and block system prompts stay separate from messages.

litellm/litellm_core_utils/litellm_logging.py

@@ -4724,31 +4724,6 @@ class StandardLoggingPayloadSetup:
                 return value
         return None
 
-    @staticmethod
-    def append_system_prompt_messages(kwargs: dict | None = None, messages: Any | None = None):
-        """
-        Append system prompt messages to the messages
-        """
-        if kwargs is not None:
-            if kwargs.get("system") is not None and isinstance(kwargs.get("system"), str):
-                if messages is None:
-                    return [{"role": "system", "content": kwargs.get("system")}]
-                elif isinstance(messages, list):
-                    if len(messages) == 0:
-                        return [{"role": "system", "content": kwargs.get("system")}]
-                    # check for duplicates
-                    if messages[0].get("role") == "system" and messages[0].get("content") == kwargs.get("system"):
-                        return messages
-                    messages = [{"role": "system", "content": kwargs.get("system")}] + messages
-                elif isinstance(messages, str):
-                    messages = [
-                        {"role": "system", "content": kwargs.get("system")},
-                        {"role": "user", "content": messages},
-                    ]
-                return messages
-
-        return messages
-
     @staticmethod
     def merge_litellm_metadata(litellm_params: dict) -> dict:
         """
@@ -5644,11 +5619,7 @@ def get_standard_logging_object_payload(
             model_id=_model_id,
             requester_ip_address=clean_metadata.get("requester_ip_address", None),
             user_agent=clean_metadata.get("user_agent", None),
-            messages=truncate_base64_in_messages(
-                StandardLoggingPayloadSetup.append_system_prompt_messages(
-                    kwargs=kwargs, messages=kwargs.get("messages")
-                )
-            ),
+            messages=truncate_base64_in_messages(kwargs.get("messages")),
             system_prompt=StandardLoggingPayloadSetup.get_system_prompt_from_kwargs(kwargs=kwargs),
             response=final_response_obj,
             model_parameters=ModelParamHelper.get_standard_logging_model_parameters(

tests/test_litellm/litellm_core_utils/test_litellm_logging.py

@@ -2182,65 +2182,6 @@ def test_get_usage_as_dict():
     assert result == {"prompt_tokens": 0, "completion_tokens": 0, "total_tokens": 0}
 
 
-def test_append_system_prompt_messages():
-    """
-    Test append_system_prompt_messages prepends system message from kwargs to messages list.
-    """
-    from litellm.litellm_core_utils.litellm_logging import StandardLoggingPayloadSetup
-
-    # Test case 1: system in kwargs with existing messages
-    kwargs = {"system": "You are a helpful assistant"}
-    messages = [{"role": "user", "content": "Hello"}]
-    result = StandardLoggingPayloadSetup.append_system_prompt_messages(
-        kwargs=kwargs, messages=messages
-    )
-    assert len(result) == 2
-    assert result[0] == {"role": "system", "content": "You are a helpful assistant"}
-    assert result[1] == {"role": "user", "content": "Hello"}
-
-    # Test case 2: system in kwargs with None messages
-    kwargs = {"system": "You are a helpful assistant"}
-    result = StandardLoggingPayloadSetup.append_system_prompt_messages(
-        kwargs=kwargs, messages=None
-    )
-    assert len(result) == 1
-    assert result[0] == {"role": "system", "content": "You are a helpful assistant"}
-
-    # Test case 3: system in kwargs with empty messages list
-    kwargs = {"system": "You are a helpful assistant"}
-    result = StandardLoggingPayloadSetup.append_system_prompt_messages(
-        kwargs=kwargs, messages=[]
-    )
-    assert len(result) == 1
-    assert result[0] == {"role": "system", "content": "You are a helpful assistant"}
-
-    # Test case 4: duplicate system message should not be added
-    kwargs = {"system": "You are a helpful assistant"}
-    messages = [
-        {"role": "system", "content": "You are a helpful assistant"},
-        {"role": "user", "content": "Hello"},
-    ]
-    result = StandardLoggingPayloadSetup.append_system_prompt_messages(
-        kwargs=kwargs, messages=messages
-    )
-    assert len(result) == 2
-    assert result[0] == {"role": "system", "content": "You are a helpful assistant"}
-
-    # Test case 5: no system in kwargs returns messages unchanged
-    kwargs = {}
-    messages = [{"role": "user", "content": "Hello"}]
-    result = StandardLoggingPayloadSetup.append_system_prompt_messages(
-        kwargs=kwargs, messages=messages
-    )
-    assert result == messages
-
-    # Test case 6: None kwargs returns messages unchanged
-    result = StandardLoggingPayloadSetup.append_system_prompt_messages(
-        kwargs=None, messages=messages
-    )
-    assert result == messages
-
-
 def test_get_system_prompt_from_kwargs():
     from litellm.litellm_core_utils.litellm_logging import StandardLoggingPayloadSetup
 
@@ -2282,21 +2223,12 @@ def test_get_system_prompt_from_kwargs():
     assert StandardLoggingPayloadSetup.get_system_prompt_from_kwargs(kwargs=None) is None
 
 
-def test_get_standard_logging_object_payload_includes_system_prompt_for_list_system(logging_obj):
+def test_get_standard_logging_object_payload_keeps_system_prompt_separate_from_messages(logging_obj):
     import datetime
 
     from litellm.litellm_core_utils.litellm_logging import get_standard_logging_object_payload
 
-    system_blocks = [
-        {"type": "text", "text": "SHAPE-SECRET", "cache_control": {"type": "ephemeral"}},
-    ]
     user_messages = [{"role": "user", "content": "hello"}]
-    kwargs = {
-        "model": "anthropic/claude-sonnet-4-5",
-        "system": system_blocks,
-        "messages": user_messages,
-        "litellm_params": {},
-    }
     mock_response = {
         "id": "msg_123",
         "type": "message",
@@ -2307,8 +2239,17 @@ def test_get_standard_logging_object_payload_includes_system_prompt_for_list_sys
     }
     now = datetime.datetime.now()
 
-    payload = get_standard_logging_object_payload(
-        kwargs=kwargs,
+    system_blocks = [
+        {"type": "text", "text": "SHAPE-SECRET", "cache_control": {"type": "ephemeral"}},
+    ]
+    list_kwargs = {
+        "model": "anthropic/claude-sonnet-4-5",
+        "system": system_blocks,
+        "messages": user_messages,
+        "litellm_params": {},
+    }
+    list_payload = get_standard_logging_object_payload(
+        kwargs=list_kwargs,
         init_response_obj=mock_response,
         start_time=now,
         end_time=now,
@@ -2316,9 +2257,28 @@ def test_get_standard_logging_object_payload_includes_system_prompt_for_list_sys
         status="success",
     )
 
-    assert payload is not None
-    assert payload["system_prompt"] == system_blocks
-    assert payload["messages"] == user_messages
+    assert list_payload is not None
+    assert list_payload["system_prompt"] == system_blocks
+    assert list_payload["messages"] == user_messages
+
+    string_kwargs = {
+        "model": "anthropic/claude-sonnet-4-5",
+        "system": "Be helpful",
+        "messages": user_messages,
+        "litellm_params": {},
+    }
+    string_payload = get_standard_logging_object_payload(
+        kwargs=string_kwargs,
+        init_response_obj=mock_response,
+        start_time=now,
+        end_time=now,
+        logging_obj=logging_obj,
+        status="success",
+    )
+
+    assert string_payload is not None
+    assert string_payload["system_prompt"] == "Be helpful"
+    assert string_payload["messages"] == user_messages
 
 
 @pytest.mark.asyncio