fix(bedrock): extract system messages from messages list for Claude messages API

litellm/llms/bedrock/messages/invoke_transformations/anthropic_claude3_transformation.py

@@ -564,6 +564,53 @@ class AmazonAnthropicClaudeMessagesConfig(
         normalize_bedrock_opus_output_config_effort(model=model, output_config=clamped)
         optional_params["reasoning_effort"] = clamped["effort"]
 
+    def _extract_system_messages_from_messages(
+        self, anthropic_messages_request: Dict
+    ) -> None:
+        """
+        Extract system messages from the messages array and merge them into the system parameter.
+        """
+        system_messages_to_add = []
+        messages_list = anthropic_messages_request.get("messages", [])
+        new_messages = []
+        system_messages_found = False
+        for msg in messages_list:
+            if isinstance(msg, dict) and msg.get("role") == "system":
+                system_messages_found = True
+                content = msg.get("content")
+                if isinstance(content, str):
+                    if content:
+                        block = {"type": "text", "text": content}
+                        if "cache_control" in msg:
+                            block["cache_control"] = msg["cache_control"]
+                        system_messages_to_add.append(block)
+                elif isinstance(content, list):
+                    for item in content:
+                        if isinstance(item, dict):
+                            if item.get("type") == "text":
+                                text_val = item.get("text")
+                                if text_val:
+                                    block = {"type": "text", "text": text_val}
+                                    if "cache_control" in item:
+                                        block["cache_control"] = item["cache_control"]
+                                    system_messages_to_add.append(block)
+            else:
+                new_messages.append(msg)
+
+        if system_messages_found:
+            anthropic_messages_request["messages"] = new_messages
+            if system_messages_to_add:
+                existing_system = anthropic_messages_request.get("system")
+                if existing_system is None:
+                    anthropic_messages_request["system"] = system_messages_to_add
+                elif isinstance(existing_system, str):
+                    if existing_system:
+                        anthropic_messages_request["system"] = [{"type": "text", "text": existing_system}] + system_messages_to_add
+                    else:
+                        anthropic_messages_request["system"] = system_messages_to_add
+                elif isinstance(existing_system, list):
+                    anthropic_messages_request["system"] = list(existing_system) + system_messages_to_add
+
     def transform_anthropic_messages_request(
         self,
         model: str,
@@ -584,6 +631,9 @@ class AmazonAnthropicClaudeMessagesConfig(
             litellm_params=litellm_params,
             headers=headers,
         )
+
+        # Extract system messages from messages list (Bedrock does not support system role in messages)
+        self._extract_system_messages_from_messages(anthropic_messages_request)
         #########################################################
         ############## BEDROCK Invoke SPECIFIC TRANSFORMATION ###
         #########################################################

tests/pass_through_unit_tests/test_bedrock_anthropic_messages_test.py

@@ -136,3 +136,84 @@ async def test_should_not_fail_with_forwarded_headers_bedrock_invoke_messages():
     print("INVOKE API RESPONSE: ", response)
 
     INSTANCE_BASE_ANTHROPIC_MESSAGES_TEST._validate_response(response)
+
+
+def test_bedrock_anthropic_messages_system_role_transformation():
+    """
+    Test that system messages in the messages array are correctly extracted and moved
+    to the top-level system parameter for Bedrock.
+    """
+    config = litellm.AmazonAnthropicClaudeMessagesConfig()
+    
+    # 1. Test case: System message in messages array, no existing system param
+    req = config.transform_anthropic_messages_request(
+        model="bedrock/us.anthropic.claude-sonnet-4-5-20250929-v1:0",
+        messages=[
+            {"role": "system", "content": "You are a helpful assistant"},
+            {"role": "user", "content": "Hello"}
+        ],
+        anthropic_messages_optional_request_params={"max_tokens": 100},
+        litellm_params={},
+        headers={}
+    )
+    
+    assert req["messages"] == [{"role": "user", "content": "Hello"}]
+    assert req["system"] == [{"type": "text", "text": "You are a helpful assistant"}]
+    
+    # 2. Test case: System message in messages array, and existing system param
+    req = config.transform_anthropic_messages_request(
+        model="bedrock/us.anthropic.claude-sonnet-4-5-20250929-v1:0",
+        messages=[
+            {"role": "system", "content": "System message in messages array"},
+            {"role": "user", "content": "Hello"}
+        ],
+        anthropic_messages_optional_request_params={
+            "max_tokens": 100,
+            "system": "Existing top-level system message"
+        },
+        litellm_params={},
+        headers={}
+    )
+    
+    assert req["messages"] == [{"role": "user", "content": "Hello"}]
+    assert req["system"] == [
+        {"type": "text", "text": "Existing top-level system message"},
+        {"type": "text", "text": "System message in messages array"}
+    ]
+    
+    # 3. Test case: Empty system message content (should remove system message from messages, system param unaffected)
+    req = config.transform_anthropic_messages_request(
+        model="bedrock/us.anthropic.claude-sonnet-4-5-20250929-v1:0",
+        messages=[
+            {"role": "system", "content": ""},
+            {"role": "user", "content": "Hello"}
+        ],
+        anthropic_messages_optional_request_params={"max_tokens": 100},
+        litellm_params={},
+        headers={}
+    )
+    
+    assert req["messages"] == [{"role": "user", "content": "Hello"}]
+    assert "system" not in req
+    
+    # 4. Test case: Non-text items in list content (should only extract text items, ignore non-text)
+    req = config.transform_anthropic_messages_request(
+        model="bedrock/us.anthropic.claude-sonnet-4-5-20250929-v1:0",
+        messages=[
+            {
+                "role": "system",
+                "content": [
+                    {"type": "text", "text": "This is text"},
+                    {"type": "image", "source": {"type": "base64", "media_type": "image/jpeg", "data": "abc"}}
+                ]
+            },
+            {"role": "user", "content": "Hello"}
+        ],
+        anthropic_messages_optional_request_params={"max_tokens": 100},
+        litellm_params={},
+        headers={}
+    )
+    
+    assert req["messages"] == [{"role": "user", "content": "Hello"}]
+    assert req["system"] == [{"type": "text", "text": "This is text"}]
+