core: add RemoveMessage (#23636)

This change adds a new message type `RemoveMessage`. This will enable
`langgraph` users to manually modify graph state (or have the graph
nodes modify the state) to remove messages by `id`

Examples:

* allow users to delete messages from state by calling

```python
graph.update_state(config, values=[RemoveMessage(id=state.values[-1].id)])
```

* allow nodes to delete messages

```python
graph.add_node("delete_messages", lambda state: [RemoveMessage(id=state[-1].id)])
```

libs/community/tests/unit_tests/load/test_serializable.py

@@ -95,6 +95,13 @@ def test_serializable_mapping() -> None:
             "structured",
             "StructuredPrompt",
         ),
+        # This is not exported from langchain, only langchain_core
+        ("langchain", "schema", "messages", "RemoveMessage"): (
+            "langchain_core",
+            "messages",
+            "modifier",
+            "RemoveMessage",
+        ),
     }
     serializable_modules = import_all_modules("langchain")
 

libs/core/langchain_core/load/mapping.py

@@ -76,6 +76,12 @@ SERIALIZABLE_MAPPING: Dict[Tuple[str, ...], Tuple[str, ...]] = {
         "tool",
         "ToolMessage",
     ),
+    ("langchain", "schema", "messages", "RemoveMessage"): (
+        "langchain_core",
+        "messages",
+        "modifier",
+        "RemoveMessage",
+    ),
     ("langchain", "schema", "agent", "AgentAction"): (
         "langchain_core",
         "agents",

libs/core/langchain_core/messages/__init__.py

@@ -29,6 +29,7 @@ from langchain_core.messages.base import (
 from langchain_core.messages.chat import ChatMessage, ChatMessageChunk
 from langchain_core.messages.function import FunctionMessage, FunctionMessageChunk
 from langchain_core.messages.human import HumanMessage, HumanMessageChunk
+from langchain_core.messages.modifier import RemoveMessage
 from langchain_core.messages.system import SystemMessage, SystemMessageChunk
 from langchain_core.messages.tool import (
     InvalidToolCall,
@@ -70,6 +71,7 @@ __all__ = [
     "ToolCallChunk",
     "ToolMessage",
     "ToolMessageChunk",
+    "RemoveMessage",
     "_message_from_dict",
     "convert_to_messages",
     "get_buffer_string",

libs/core/langchain_core/messages/modifier.py

@@ -0,0 +1,23 @@
+from typing import Any, List, Literal
+
+from langchain_core.messages.base import BaseMessage
+
+
+class RemoveMessage(BaseMessage):
+    """Message responsible for deleting other messages."""
+
+    type: Literal["remove"] = "remove"
+
+    def __init__(self, id: str, **kwargs: Any) -> None:
+        if kwargs.pop("content", None):
+            raise ValueError("RemoveMessage does not support 'content' field.")
+
+        return super().__init__("", id=id, **kwargs)
+
+    @classmethod
+    def get_lc_namespace(cls) -> List[str]:
+        """Get the namespace of the langchain object."""
+        return ["langchain", "schema", "messages"]
+
+
+RemoveMessage.update_forward_refs()

libs/core/tests/unit_tests/messages/test_imports.py

@@ -21,6 +21,7 @@ EXPECTED_ALL = [
     "ToolCallChunk",
     "ToolMessage",
     "ToolMessageChunk",
+    "RemoveMessage",
     "convert_to_messages",
     "get_buffer_string",
     "merge_content",