Fix empty assistant message handling in AWS Bedrock Converse API to prevent 400 Bad Request errors

litellm/litellm_core_utils/prompt_templates/factory.py

@@ -3803,7 +3803,9 @@ def _bedrock_converse_messages_pt(  # noqa: PLR0915
                                 assistant_parts=assistants_parts,
                             )
                         elif element["type"] == "text":
-                            assistants_part = BedrockContentBlock(text=element["text"])
+                            # AWS Bedrock doesn't allow empty or whitespace-only text content, so use placeholder for empty strings
+                            text_content = element["text"] if element["text"].strip() else "."
+                            assistants_part = BedrockContentBlock(text=text_content)
                             assistants_parts.append(assistants_part)
                         elif element["type"] == "image_url":
                             if isinstance(element["image_url"], dict):
@@ -3827,7 +3829,9 @@ def _bedrock_converse_messages_pt(  # noqa: PLR0915
                             assistants_parts.append(_cache_point_block)
                 assistant_content.extend(assistants_parts)
             elif _assistant_content is not None and isinstance(_assistant_content, str):
-                assistant_content.append(BedrockContentBlock(text=_assistant_content))
+                # AWS Bedrock doesn't allow empty or whitespace-only text content, so use placeholder for empty strings
+                text_content = _assistant_content if _assistant_content.strip() else "."
+                assistant_content.append(BedrockContentBlock(text=text_content))
                 # Add cache point block for assistant string content
                 _cache_point_block = (
                     litellm.AmazonConverseConfig()._get_cache_point_block(

tests/test_litellm/llms/bedrock/chat/test_converse_transformation.py

@@ -2584,3 +2584,95 @@ def test_request_metadata_not_provided():
 
     # requestMetadata should not be in the request
     assert "requestMetadata" not in request_data
+
+
+def test_empty_assistant_message_handling():
+    """
+    Test that empty assistant messages are handled correctly by replacing
+    empty or whitespace-only content with a placeholder to prevent AWS Bedrock
+    Converse API 400 Bad Request errors.
+    """
+    from litellm.litellm_core_utils.prompt_templates.factory import _bedrock_converse_messages_pt
+
+    # Test case 1: Empty string content - test with modify_params=True to prevent merging
+    messages = [
+        {"role": "user", "content": "Hello"},
+        {"role": "assistant", "content": ""},  # Empty content
+        {"role": "user", "content": "How are you?"}
+    ]
+
+    # Enable modify_params to prevent consecutive user message merging
+    original_modify_params = litellm.modify_params
+    litellm.modify_params = True
+
+    try:
+        result = _bedrock_converse_messages_pt(
+            messages=messages,
+            model="anthropic.claude-3-5-sonnet-20240620-v1:0",
+            llm_provider="bedrock_converse"
+        )
+
+        # Should have 3 messages: user, assistant (with placeholder), user
+        assert len(result) == 3
+        assert result[0]["role"] == "user"
+        assert result[1]["role"] == "assistant"
+        assert result[2]["role"] == "user"
+
+        # Assistant message should have placeholder text instead of empty content
+        assert len(result[1]["content"]) == 1
+        assert result[1]["content"][0]["text"] == "Please continue."
+
+        # Test case 2: Whitespace-only content
+        messages = [
+            {"role": "user", "content": "Hello"},
+            {"role": "assistant", "content": "   "},  # Whitespace-only content
+            {"role": "user", "content": "How are you?"}
+        ]
+
+        result = _bedrock_converse_messages_pt(
+            messages=messages,
+            model="anthropic.claude-3-5-sonnet-20240620-v1:0",
+            llm_provider="bedrock_converse"
+        )
+
+        # Assistant message should have placeholder text instead of whitespace
+        assert len(result[1]["content"]) == 1
+        assert result[1]["content"][0]["text"] == "Please continue."
+
+        # Test case 3: Empty list content
+        messages = [
+            {"role": "user", "content": "Hello"},
+            {"role": "assistant", "content": [{"type": "text", "text": ""}]},  # Empty text in list
+            {"role": "user", "content": "How are you?"}
+        ]
+
+        result = _bedrock_converse_messages_pt(
+            messages=messages,
+            model="anthropic.claude-3-5-sonnet-20240620-v1:0",
+            llm_provider="bedrock_converse"
+        )
+
+        # Assistant message should have placeholder text instead of empty text
+        assert len(result[1]["content"]) == 1
+        assert result[1]["content"][0]["text"] == "Please continue."
+
+        # Test case 4: Normal content should not be affected
+        messages = [
+            {"role": "user", "content": "Hello"},
+            {"role": "assistant", "content": "I'm doing well, thank you!"},  # Normal content
+            {"role": "user", "content": "How are you?"}
+        ]
+
+        result = _bedrock_converse_messages_pt(
+            messages=messages,
+            model="anthropic.claude-3-5-sonnet-20240620-v1:0",
+            llm_provider="bedrock_converse"
+        )
+
+        # Assistant message should keep original content
+        assert len(result[1]["content"]) == 1
+        assert result[1]["content"][0]["text"] == "I'm doing well, thank you!"
+
+    finally:
+        # Restore original modify_params setting
+        litellm.modify_params = original_modify_params