Support multiple system messages for Anthropic

libs/aws/langchain_aws/chat_models/bedrock.py

@@ -362,7 +362,10 @@ def _merge_messages(
                     ]
                 )
         last = merged[-1] if merged else None
-        if isinstance(last, HumanMessage) and isinstance(curr, HumanMessage):
+        if any(
+            all(isinstance(m, c) for m in (curr, last))
+            for c in (SystemMessage, HumanMessage)
+        ):
             if isinstance(last.content, str):
                 new_content: List = [{"type": "text", "text": last.content}]
             else:
@@ -387,9 +390,11 @@ def _format_anthropic_messages(
     merged_messages = _merge_messages(messages)
     for i, message in enumerate(merged_messages):
         if message.type == "system":
-            if i != 0:
-                raise ValueError("System message must be at beginning of message list.")
-            if isinstance(message.content, str):
+            if system is not None:
+                raise ValueError(
+                    "Received multiple non-consecutive system messages."
+                )
+            elif isinstance(message.content, str):
                 system = message.content
             elif isinstance(message.content, list):
                 system_blocks = []

libs/aws/langchain_aws/chat_models/bedrock_converse.py

@@ -1037,8 +1037,8 @@ def _messages_to_bedrock(
     """Handle Bedrock converse and Anthropic style content blocks"""
     bedrock_messages: List[Dict[str, Any]] = []
     bedrock_system: List[Dict[str, Any]] = []
-    # Merge system, human, ai message runs because Anthropic expects (at most) 1
-    # system message then alternating human/ai messages.
+    # Merge system, human, ai message runs because Anthropic expects
+    # (optional) system messages first, then alternating human/ai messages.
     messages = merge_message_runs(messages)
     for msg in messages:
         content = _lc_content_to_bedrock(msg.content)

libs/aws/tests/unit_tests/chat_models/test_bedrock.py

@@ -26,6 +26,7 @@
 def test__merge_messages() -> None:
     messages = [
         SystemMessage("foo"),  # type: ignore[misc]
+        SystemMessage("barfoo"),  # type: ignore[misc]
         HumanMessage("bar"),  # type: ignore[misc]
         AIMessage(  # type: ignore[misc]
             [
@@ -52,7 +53,12 @@ def test__merge_messages() -> None:
         HumanMessage("next thing"),  # type: ignore[misc]
     ]
     expected = [
-        SystemMessage("foo"),  # type: ignore[misc]
+        SystemMessage(
+            [
+                {'type': 'text', 'text': 'foo'},
+                {'type': 'text', 'text': 'barfoo'}
+            ]
+        ),  # type: ignore[misc]
         HumanMessage("bar"),  # type: ignore[misc]
         AIMessage(  # type: ignore[misc]
             [
@@ -345,6 +351,34 @@ def test__format_anthropic_messages_system_message_list_content() -> None:
     actual = _format_anthropic_messages(messages)
     assert expected == actual
 
+def test__format_anthropic_multiple_system_messages() -> None:
+    """Test that multiple system messages can be passed, and that none of them are required to be at position 0."""
+    system1 = SystemMessage("foo")  # type: ignore[misc]
+    system2 = SystemMessage("bar")  # type: ignore[misc]
+    human = HumanMessage("Hello!")
+    messages = [human, system1, system2]
+    expected_system = [
+        {'text': 'foo', 'type': 'text'},
+        {'text': 'bar', 'type': 'text'}
+    ]
+    expected_messages = [
+        {"role": "user", "content": "Hello!"}
+    ]
+
+    actual_system, actual_messages = _format_anthropic_messages(messages)
+    assert expected_system == actual_system
+    assert expected_messages == actual_messages
+
+def test__format_anthropic_nonconsecutive_system_messages() -> None:
+    """Test that we fail when non-consecutive system messages are passed."""
+    system1 = SystemMessage("foo")  # type: ignore[misc]
+    system2 = SystemMessage("bar")  # type: ignore[misc]
+    human = HumanMessage("Hello!")
+    messages = [system1, human, system2]
+
+    with pytest.raises(ValueError, match="Received multiple non-consecutive system messages."):
+        _format_anthropic_messages(messages)
+
 
 @pytest.fixture()
 def pydantic() -> Type[BaseModel]: