fix(vertexai): allow system messages at any position in ChatAnthropicVertex (#1610)

Author: zamal-db

libs/vertexai/langchain_google_vertexai/_anthropic_utils.py

@@ -267,10 +267,10 @@ def _format_messages_anthropic(
     formatted_messages: list[dict] = []
 
     merged_messages = _merge_messages(messages)
-    for i, message in enumerate(merged_messages):
+    for message in merged_messages:
         if message.type == "system":
-            if i != 0:
-                msg = "System message must be at beginning of message list."
+            if system_messages is not None:
+                msg = "Received multiple non-consecutive system messages."
                 raise ValueError(msg)
             fm = _format_message_anthropic(message, project)
             if fm:
@@ -458,16 +458,21 @@ def _merge_messages(
                     curr = curr.model_copy(deep=True)
                     curr.content = cleaned_content
         last = merged[-1] if merged else None
-        if isinstance(last, HumanMessage) and isinstance(curr, HumanMessage):
-            if isinstance(last.content, str):
-                new_content: list = [{"type": "text", "text": last.content}]
+        if any(
+            all(isinstance(m, c) for m in (curr, last))
+            for c in (SystemMessage, HumanMessage)
+        ):
+            if isinstance(cast("BaseMessage", last).content, str):
+                new_content: list = [
+                    {"type": "text", "text": cast("BaseMessage", last).content}
+                ]
             else:
-                new_content = last.content
+                new_content = cast("list", cast("BaseMessage", last).content)
             if isinstance(curr.content, str):
                 new_content.append({"type": "text", "text": curr.content})
             else:
                 new_content.extend(curr.content)
-            last.content = new_content
+            merged[-1] = curr.model_copy(update={"content": new_content})
         else:
             merged.append(curr)
     return merged

libs/vertexai/tests/unit_tests/test_anthropic_utils.py

@@ -1497,3 +1497,58 @@ def test_format_image(image_url: str, expected_media_type: str) -> None:
         }
 
         mock_loader_instance.load_bytes.assert_called_once_with(image_url)
+
+
+def test_format_messages_anthropic_system_not_first() -> None:
+    """Test that system messages are accepted when not at position 0.
+
+    Regression test for https://github.com/langchain-ai/langchain-google/issues/1022
+    """
+    messages = [
+        HumanMessage(content="Hello"),
+        AIMessage(content="Hi there!"),
+        SystemMessage(content="You are a helpful assistant."),
+        HumanMessage(content="What is 2+2?"),
+    ]
+    system_messages, formatted_messages = _format_messages_anthropic(
+        messages, project="test-project"
+    )
+
+    assert system_messages == [{"type": "text", "text": "You are a helpful assistant."}]
+    # System message should be extracted; remaining messages should be formatted
+    assert len(formatted_messages) == 3
+    assert formatted_messages[0]["role"] == "user"
+    assert formatted_messages[1]["role"] == "assistant"
+    assert formatted_messages[2]["role"] == "user"
+
+
+def test_format_messages_anthropic_consecutive_system_merged() -> None:
+    """Test that consecutive system messages are merged into one."""
+    messages = [
+        SystemMessage(content="Rule 1."),
+        SystemMessage(content="Rule 2."),
+        HumanMessage(content="Hello"),
+    ]
+    system_messages, formatted_messages = _format_messages_anthropic(
+        messages, project="test-project"
+    )
+
+    # Consecutive system messages should be merged
+    assert system_messages == [
+        {"type": "text", "text": "Rule 1."},
+        {"type": "text", "text": "Rule 2."},
+    ]
+    assert len(formatted_messages) == 1
+    assert formatted_messages[0]["role"] == "user"
+
+
+def test_format_messages_anthropic_multiple_non_consecutive_system_raises() -> None:
+    """Test that multiple non-consecutive system messages raise an error."""
+    messages = [
+        SystemMessage(content="First system."),
+        HumanMessage(content="Hello"),
+        SystemMessage(content="Second system."),
+        HumanMessage(content="World"),
+    ]
+    with pytest.raises(ValueError, match="multiple non-consecutive system messages"):
+        _format_messages_anthropic(messages, project="test-project")