diff --git a/langchain-core/src/messages/base.ts b/langchain-core/src/messages/base.ts
index 264c4ffa52bb..41a7cb2c2f70 100644
--- a/langchain-core/src/messages/base.ts
+++ b/langchain-core/src/messages/base.ts
@@ -305,6 +305,35 @@ export function _mergeLists(left?: any[], right?: any[]) {
   }
 }
 
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+export function _mergeObj<T = any>(
+  left: T | undefined,
+  right: T | undefined
+): T {
+  if (!left && !right) {
+    throw new Error("Cannot merge two undefined objects.");
+  }
+  if (!left || !right) {
+    return left || (right as T);
+  } else if (typeof left !== typeof right) {
+    throw new Error(
+      `Cannot merge objects of different types.\nLeft ${typeof left}\nRight ${typeof right}`
+    );
+  } else if (typeof left === "string" && typeof right === "string") {
+    return (left + right) as T;
+  } else if (Array.isArray(left) && Array.isArray(right)) {
+    return _mergeLists(left, right) as T;
+  } else if (typeof left === "object" && typeof right === "object") {
+    return _mergeDicts(left, right) as T;
+  } else if (left === right) {
+    return left;
+  } else {
+    throw new Error(
+      `Can not merge objects of different types.\nLeft ${left}\nRight ${right}`
+    );
+  }
+}
+
 /**
  * Represents a chunk of a message, which can be concatenated with other
  * message chunks. It includes a method `_merge_kwargs_dict()` for merging
diff --git a/langchain-core/src/messages/tests/base_message.test.ts b/langchain-core/src/messages/tests/base_message.test.ts
index 257d1c4cebad..fb6f3797f31f 100644
--- a/langchain-core/src/messages/tests/base_message.test.ts
+++ b/langchain-core/src/messages/tests/base_message.test.ts
@@ -1,6 +1,11 @@
 import { test } from "@jest/globals";
 import { ChatPromptTemplate } from "../../prompts/chat.js";
-import { HumanMessage, AIMessage, ToolMessage } from "../index.js";
+import {
+  HumanMessage,
+  AIMessage,
+  ToolMessage,
+  ToolMessageChunk,
+} from "../index.js";
 import { load } from "../../load/index.js";
 
 test("Test ChatPromptTemplate can format OpenAI content image messages", async () => {
@@ -127,3 +132,64 @@ test("Deserialisation and serialisation of messages with ID", async () => {
   expect(deserialized).toEqual(message);
   expect(deserialized.id).toBe(messageId);
 });
+
+test("Can concat raw_output (string) of ToolMessageChunk", () => {
+  const rawOutputOne = "Hello";
+  const rawOutputTwo = " world";
+  const chunk1 = new ToolMessageChunk({
+    content: "Hello",
+    tool_call_id: "1",
+    raw_output: rawOutputOne,
+  });
+  const chunk2 = new ToolMessageChunk({
+    content: " world",
+    tool_call_id: "1",
+    raw_output: rawOutputTwo,
+  });
+
+  const concated = chunk1.concat(chunk2);
+  expect(concated.raw_output).toBe(`${rawOutputOne}${rawOutputTwo}`);
+});
+
+test("Can concat raw_output (array) of ToolMessageChunk", () => {
+  const rawOutputOne = ["Hello", " world"];
+  const rawOutputTwo = ["!!"];
+  const chunk1 = new ToolMessageChunk({
+    content: "Hello",
+    tool_call_id: "1",
+    raw_output: rawOutputOne,
+  });
+  const chunk2 = new ToolMessageChunk({
+    content: " world",
+    tool_call_id: "1",
+    raw_output: rawOutputTwo,
+  });
+
+  const concated = chunk1.concat(chunk2);
+  expect(concated.raw_output).toEqual(["Hello", " world", "!!"]);
+});
+
+test("Can concat raw_output (object) of ToolMessageChunk", () => {
+  const rawOutputOne = {
+    foo: "bar",
+  };
+  const rawOutputTwo = {
+    bar: "baz",
+  };
+  const chunk1 = new ToolMessageChunk({
+    content: "Hello",
+    tool_call_id: "1",
+    raw_output: rawOutputOne,
+  });
+  const chunk2 = new ToolMessageChunk({
+    content: " world",
+    tool_call_id: "1",
+    raw_output: rawOutputTwo,
+  });
+
+  const concated = chunk1.concat(chunk2);
+  expect(concated.raw_output).toEqual({
+    foo: "bar",
+    bar: "baz",
+  });
+});
diff --git a/langchain-core/src/messages/tool.ts b/langchain-core/src/messages/tool.ts
index 3375cfd8572d..996e2cdba8cc 100644
--- a/langchain-core/src/messages/tool.ts
+++ b/langchain-core/src/messages/tool.ts
@@ -5,9 +5,18 @@ import {
   mergeContent,
   _mergeDicts,
   type MessageType,
+  _mergeObj,
 } from "./base.js";
 
 export interface ToolMessageFieldsWithToolCallId extends BaseMessageFields {
+  /**
+   * The raw output of the tool.
+   *
+   * **Not part of the payload sent to the model.** Should only be specified if it is
+   * different from the message content, i.e. if only a subset of the full tool output
+   * is being passed as message content.
+   */
+  raw_output?: unknown;
   tool_call_id: string;
 }
 
@@ -26,6 +35,15 @@ export class ToolMessage extends BaseMessage {
 
   tool_call_id: string;
 
+  /**
+   * The raw output of the tool.
+   *
+   * **Not part of the payload sent to the model.** Should only be specified if it is
+   * different from the message content, i.e. if only a subset of the full tool output
+   * is being passed as message content.
+   */
+  raw_output?: unknown;
+
   constructor(fields: ToolMessageFieldsWithToolCallId);
 
   constructor(
@@ -45,6 +63,7 @@ export class ToolMessage extends BaseMessage {
     }
     super(fields);
     this.tool_call_id = fields.tool_call_id;
+    this.raw_output = fields.raw_output;
   }
 
   _getType(): MessageType {
@@ -63,9 +82,19 @@ export class ToolMessage extends BaseMessage {
 export class ToolMessageChunk extends BaseMessageChunk {
   tool_call_id: string;
 
+  /**
+   * The raw output of the tool.
+   *
+   * **Not part of the payload sent to the model.** Should only be specified if it is
+   * different from the message content, i.e. if only a subset of the full tool output
+   * is being passed as message content.
+   */
+  raw_output?: unknown;
+
   constructor(fields: ToolMessageFieldsWithToolCallId) {
     super(fields);
     this.tool_call_id = fields.tool_call_id;
+    this.raw_output = fields.raw_output;
   }
 
   static lc_name() {
@@ -87,6 +116,7 @@ export class ToolMessageChunk extends BaseMessageChunk {
         this.response_metadata,
         chunk.response_metadata
       ),
+      raw_output: _mergeObj(this.raw_output, chunk.raw_output),
       tool_call_id: this.tool_call_id,
       id: this.id ?? chunk.id,
     });