fix(core): handle message coercion of RemoveMessage (langchain-ai#8941)

Author: dqbd

.changeset/spotty-cherries-carry.md

@@ -0,0 +1,5 @@
+---
+"@langchain/core": patch
+---
+
+Fix deserialization of RemoveMessage if represented as a plain object

langchain-core/src/messages/tests/base_message.test.ts

@@ -5,6 +5,7 @@ import {
   AIMessage,
   ToolMessage,
   ToolMessageChunk,
+  RemoveMessage,
   AIMessageChunk,
   coerceMessageLikeToMessage,
   SystemMessage,
@@ -551,6 +552,11 @@ describe("Message like coercion", () => {
         content: "10.2",
         tool_call_id: "10.2",
       },
+      {
+        role: "remove",
+        id: "1234",
+        content: "",
+      },
     ].map(coerceMessageLikeToMessage);
     expect(messages).toEqual([
       new SystemMessage({
@@ -583,6 +589,9 @@ describe("Message like coercion", () => {
         content: "10.2",
         tool_call_id: "10.2",
       }),
+      new RemoveMessage({
+        id: "1234",
+      }),
     ]);
   });
   it("should convert serialized messages", async () => {

langchain-core/src/messages/utils.ts

@@ -22,6 +22,7 @@ import {
   FunctionMessageChunk,
 } from "./function.js";
 import { HumanMessage, HumanMessageChunk } from "./human.js";
+import { RemoveMessage } from "./modifier.js";
 import { SystemMessage, SystemMessageChunk } from "./system.js";
 import {
   ToolCall,
@@ -132,6 +133,8 @@ function _constructMessageFromParams(
       tool_call_id: rest.tool_call_id as string,
       name: rest.name,
     });
+  } else if (type === "remove" && "id" in rest && typeof rest.id === "string") {
+    return new RemoveMessage({ ...rest, id: rest.id });
   } else {
     const error = addLangChainErrorFields(
       new Error(