Fix: Skip empty text blocks in Anthropic system messages (#17442)

When translating system messages for the Anthropic API, empty text
content blocks cause the error "messages: text content blocks must be
non-empty". This fix skips empty string content and empty text blocks
in list content to prevent this error.

Fixes issue with Vertex AI Anthropic API calls.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

litellm/llms/anthropic/chat/transformation.py

@@ -786,6 +786,9 @@ class AnthropicConfig(AnthropicModelInfo, BaseConfig):
                 valid_content: bool = False
                 system_message_block = ChatCompletionSystemMessage(**message)
                 if isinstance(system_message_block["content"], str):
+                    # Skip empty text blocks - Anthropic API raises errors for empty text
+                    if not system_message_block["content"]:
+                        continue
                     anthropic_system_message_content = AnthropicSystemMessageContent(
                         type="text",
                         text=system_message_block["content"],
@@ -800,10 +803,14 @@ class AnthropicConfig(AnthropicModelInfo, BaseConfig):
                     valid_content = True
                 elif isinstance(message["content"], list):
                     for _content in message["content"]:
+                        # Skip empty text blocks - Anthropic API raises errors for empty text
+                        text_value = _content.get("text")
+                        if _content.get("type") == "text" and not text_value:
+                            continue
                         anthropic_system_message_content = (
                             AnthropicSystemMessageContent(
                                 type=_content.get("type"),
-                                text=_content.get("text"),
+                                text=text_value,
                             )
                         )
                         if "cache_control" in _content:

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

@@ -1290,10 +1290,95 @@ def test_effort_with_other_features():
         litellm_params={},
         headers={}
     )
-    
+
     # Verify all features are present
     assert "output_config" in result
     assert result["output_config"]["effort"] == "low"
     assert "tools" in result
     assert len(result["tools"]) > 0
     assert "thinking" in result
+
+
+def test_translate_system_message_skips_empty_string_content():
+    """
+    Test that translate_system_message skips system messages with empty string content.
+
+    Fixes: Vertex AI Anthropic API error "messages: text content blocks must be non-empty"
+    """
+    config = AnthropicConfig()
+
+    # Test empty string content - should not produce any anthropic system message content
+    messages = [
+        {"role": "system", "content": ""},
+        {"role": "user", "content": "Hello"},
+    ]
+
+    result = config.translate_system_message(messages)
+
+    # Empty system message should produce no anthropic content blocks
+    assert len(result) == 0
+
+
+def test_translate_system_message_skips_empty_list_content():
+    """
+    Test that translate_system_message skips empty text blocks in list content.
+
+    Fixes: Vertex AI Anthropic API error "messages: text content blocks must be non-empty"
+    """
+    config = AnthropicConfig()
+
+    # Test list content with empty text block
+    messages = [
+        {"role": "system", "content": [
+            {"type": "text", "text": ""},
+            {"type": "text", "text": "Valid content"},
+            {"type": "text", "text": ""},
+        ]},
+        {"role": "user", "content": "Hello"},
+    ]
+
+    result = config.translate_system_message(messages)
+
+    # Only non-empty text blocks should be included
+    assert len(result) == 1
+    assert result[0]["text"] == "Valid content"
+
+
+def test_translate_system_message_preserves_valid_content():
+    """
+    Test that translate_system_message preserves valid system message content.
+    """
+    config = AnthropicConfig()
+
+    # Test valid string content
+    messages = [
+        {"role": "system", "content": "You are a helpful assistant."},
+        {"role": "user", "content": "Hello"},
+    ]
+
+    result = config.translate_system_message(messages)
+
+    assert len(result) == 1
+    assert result[0]["type"] == "text"
+    assert result[0]["text"] == "You are a helpful assistant."
+
+
+def test_translate_system_message_preserves_cache_control():
+    """
+    Test that translate_system_message preserves cache_control on valid content.
+    """
+    config = AnthropicConfig()
+
+    # Test list content with cache_control
+    messages = [
+        {"role": "system", "content": [
+            {"type": "text", "text": "Cached content", "cache_control": {"type": "ephemeral"}},
+        ]},
+        {"role": "user", "content": "Hello"},
+    ]
+
+    result = config.translate_system_message(messages)
+
+    assert len(result) == 1
+    assert result[0]["text"] == "Cached content"
+    assert result[0]["cache_control"] == {"type": "ephemeral"}