fix(anthropic): remove empty system messages from message list

Empty system messages were skipped for Anthropic's system param but
not removed from the messages list, causing BadRequestError when
anthropic_messages_pt encountered the unsupported "system" role.

Fixes #21622

litellm/llms/anthropic/chat/transformation.py

@@ -1048,7 +1048,7 @@ class AnthropicConfig(AnthropicModelInfo, BaseConfig):
         anthropic_system_message_list: List[AnthropicSystemMessageContent] = []
         for idx, message in enumerate(messages):
             if message["role"] == "system":
-                valid_content: bool = False
+                system_prompt_indices.append(idx)
                 system_message_block = ChatCompletionSystemMessage(**message)
                 if isinstance(system_message_block["content"], str):
                     # Skip empty text blocks - Anthropic API raises errors for empty text
@@ -1068,7 +1068,6 @@ class AnthropicConfig(AnthropicModelInfo, BaseConfig):
                     anthropic_system_message_list.append(
                         anthropic_system_message_content
                     )
-                    valid_content = True
                 elif isinstance(message["content"], list):
                     for _content in message["content"]:
                         # Skip empty text blocks - Anthropic API raises errors for empty text
@@ -1092,10 +1091,7 @@ class AnthropicConfig(AnthropicModelInfo, BaseConfig):
                         anthropic_system_message_list.append(
                             anthropic_system_message_content
                         )
-                    valid_content = True
 
-                if valid_content:
-                    system_prompt_indices.append(idx)
         if len(system_prompt_indices) > 0:
             for idx in reversed(system_prompt_indices):
                 messages.pop(idx)

tests/test_litellm/llms/anthropic/chat/test_anthropic_chat_transformation.py

@@ -1739,6 +1739,8 @@ def test_translate_system_message_skips_empty_string_content():
 
     # Empty system message should produce no anthropic content blocks
     assert len(result) == 0
+    # System message must be removed from messages so it doesn't reach anthropic_messages_pt
+    assert all(m["role"] != "system" for m in messages)
 
 
 def test_translate_system_message_skips_empty_list_content():