AIMessage content erased when doing `updateState`

[Specy]

Checked other resources

• I added a very descriptive title to this issue.
• I searched the LangGraph.js documentation with the integrated search.
• I used the GitHub search to find a similar question and didn't find it.
• I am sure that this is a bug in LangGraph.js rather than my code.
• The bug is not resolved by updating to the latest stable version of LangGraph (or the specific integration package).

Example Code

import { END, MemorySaver, MessagesValue, START, StateGraph, StateSchema, } from '@langchain/langgraph';
import { AIMessage, HumanMessage } from '@langchain/core/messages';

const state = new StateGraph(
    new StateSchema({
        messages: MessagesValue,
    }),
);
const checkpointer = new MemorySaver();

const g = state
    .addNode('answerNode', (state) => {
        console.log(state);
        return {};
    })
    .addEdge(START, 'answerNode')
    .addEdge('answerNode', END)
    .compile({
        checkpointer,
    });

const id = {
    configurable: {
        thread_id: 'thread1',
    },
};
const messages = [
    new HumanMessage({
        contentBlocks: [{ type: 'text', text: 'Hello' }],
    }),
    new AIMessage({
        contentBlocks: [{ type: 'text', text: 'Hi there!' }],
    }),
    new HumanMessage({
        contentBlocks: [{ type: 'text', text: 'How are you' }],
    }),
];
async function run() {
    await g.updateState(id, {
        messages,
    });

    await g.invoke({}, id);
}

run();

Error Message and Stack Trace (if applicable)

{
  messages: [
    HumanMessage {
      "id": "a94629d1-02b3-4c2e-b8c2-863e08e26042",
      "content": [
        {
          "type": "text",
          "text": "Hello"
        }
      ],
      "additional_kwargs": {},
      "response_metadata": {
        "output_version": "v1"
      }
    },
    AIMessage {
      "id": "ddb6123c-5419-43b6-8467-2bd04880c942",
      "content": [], //<----------------
      "additional_kwargs": {},
      "response_metadata": {
        "output_version": "v1"
      },
      "tool_calls": [],
      "invalid_tool_calls": []
    },
    HumanMessage {
      "id": "656f0ebd-edd5-49c3-b763-ddd829c28526",
      "content": [
        {
          "type": "text",
          "text": "How are you"
        }
      ],
      "additional_kwargs": {},
      "response_metadata": {
        "output_version": "v1"
      }
    }
  ]
}

Description

When using langgraph, doing a .updateState while updating messages erases content inside of AIMessage.

System Info

Details

Node version: v24.6.0
Operating system: win32 x64
Package manager: npm
Package manager version: N/A

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+-- @elevenlabs/elevenlabs-js@2.34.0
+-- @googleapis/youtube@31.0.0
+-- @langchain/anthropic@1.3.15
+-- @langchain/core@1.1.19
+-- @langchain/google-vertexai@2.1.15
+-- @langchain/groq@1.0.4
+-- @langchain/langgraph@1.1.4
+-- @langchain/openai@1.2.5
+-- @langchain/textsplitters@1.0.1
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`-- zod@4.3.6