core[minor]: Add RemoveMessage class (#6089)

* core[minor]: Add RemoveMessage class

* add comment

langchain-core/src/messages/base.ts

@@ -36,7 +36,8 @@ export type MessageType =
   | "generic"
   | "system"
   | "function"
-  | "tool";
+  | "tool"
+  | "remove";
 
 export type ImageDetail = "auto" | "low" | "high";
 

langchain-core/src/messages/index.ts

@@ -6,6 +6,7 @@ export * from "./human.js";
 export * from "./system.js";
 export * from "./utils.js";
 export * from "./transformers.js";
+export * from "./modifier.js";
 // TODO: Use a star export when we deprecate the
 // existing "ToolCall" type in "base.js".
 export {

langchain-core/src/messages/modifier.ts

@@ -0,0 +1,31 @@
+import { BaseMessage, BaseMessageFields, MessageType } from "./base.js";
+
+export interface RemoveMessageFields
+  extends Omit<BaseMessageFields, "content"> {
+  /**
+   * The ID of the message to remove.
+   */
+  id: string;
+}
+
+/**
+ * Message responsible for deleting other messages.
+ */
+export class RemoveMessage extends BaseMessage {
+  /**
+   * The ID of the message to remove.
+   */
+  id: string;
+
+  constructor(fields: RemoveMessageFields) {
+    super({
+      ...fields,
+      content: "",
+    });
+    this.id = fields.id;
+  }
+
+  _getType(): MessageType {
+    return "remove";
+  }
+}

langchain-core/src/messages/transformers.ts

@@ -19,6 +19,7 @@ import {
   FunctionMessageFieldsWithName,
 } from "./function.js";
 import { HumanMessage, HumanMessageChunk } from "./human.js";
+import { RemoveMessage } from "./modifier.js";
 import { SystemMessage, SystemMessageChunk } from "./system.js";
 import {
   ToolMessage,
@@ -33,14 +34,16 @@ export type MessageUnion =
   | typeof SystemMessage
   | typeof ChatMessage
   | typeof FunctionMessage
-  | typeof ToolMessage;
+  | typeof ToolMessage
+  | typeof RemoveMessage;
 export type MessageChunkUnion =
   | typeof HumanMessageChunk
   | typeof AIMessageChunk
   | typeof SystemMessageChunk
   | typeof FunctionMessageChunk
   | typeof ToolMessageChunk
-  | typeof ChatMessageChunk;
+  | typeof ChatMessageChunk
+  | typeof RemoveMessage; // RemoveMessage does not have a chunk class.
 export type MessageTypeOrClass = MessageType | MessageUnion | MessageChunkUnion;
 
 const _isMessageType = (msg: BaseMessage, types: MessageTypeOrClass[]) => {
@@ -915,6 +918,10 @@ const _MSG_CHUNK_MAP: Record<
     message: ChatMessage,
     messageChunk: ChatMessageChunk,
   },
+  remove: {
+    message: RemoveMessage,
+    messageChunk: RemoveMessage, // RemoveMessage does not have a chunk class.
+  },
 };
 
 function _switchTypeToMessage(