diff --git a/langchain-core/src/callbacks/base.ts b/langchain-core/src/callbacks/base.ts
index 229119697753..802e385ad0a2 100644
--- a/langchain-core/src/callbacks/base.ts
+++ b/langchain-core/src/callbacks/base.ts
@@ -16,6 +16,7 @@ import {
 import type { SerializedFields } from "../load/map_keys.js";
 import type { DocumentInterface } from "../documents/document.js";
 import { getEnvironmentVariable } from "../utils/env.js";
+import { ToolMessage } from "../messages/tool.js";
 
 // eslint-disable-next-line @typescript-eslint/no-explicit-any
 type Error = any;
@@ -197,7 +198,7 @@ abstract class BaseCallbackHandlerMethodsClass {
    * Called at the end of a Tool run, with the tool output and the run ID.
    */
   handleToolEnd?(
-    output: string,
+    output: string | ToolMessage,
     runId: string,
     parentRunId?: string,
     tags?: string[]
diff --git a/langchain-core/src/callbacks/manager.ts b/langchain-core/src/callbacks/manager.ts
index 834ee61d6a38..a04aef323293 100644
--- a/langchain-core/src/callbacks/manager.ts
+++ b/langchain-core/src/callbacks/manager.ts
@@ -20,6 +20,7 @@ import { consumeCallback } from "./promises.js";
 import { Serialized } from "../load/serializable.js";
 import type { DocumentInterface } from "../documents/document.js";
 import { isTracingEnabled } from "../utils/callbacks.js";
+import { ToolMessage } from "../messages/tool.js";
 
 if (
   /* #__PURE__ */ getEnvironmentVariable("LANGCHAIN_TRACING_V2") === "true" &&
@@ -493,7 +494,7 @@ export class CallbackManagerForToolRun
     );
   }
 
-  async handleToolEnd(output: string): Promise<void> {
+  async handleToolEnd(output: string | ToolMessage): Promise<void> {
     await Promise.all(
       this.handlers.map((handler) =>
         consumeCallback(async () => {
diff --git a/langchain-core/src/tools.ts b/langchain-core/src/tools.ts
index 252571ca084a..4e05f8cd8158 100644
--- a/langchain-core/src/tools.ts
+++ b/langchain-core/src/tools.ts
@@ -11,6 +11,7 @@ import {
 } from "./language_models/base.js";
 import { ensureConfig, type RunnableConfig } from "./runnables/config.js";
 import type { RunnableFunc, RunnableInterface } from "./runnables/base.js";
+import type { ToolMessage } from "./messages/tool.js";
 
 // eslint-disable-next-line @typescript-eslint/no-explicit-any
 type ZodAny = z.ZodObject<any, any, any, any>;
@@ -34,10 +35,12 @@ export class ToolInputParsingException extends Error {
   }
 }
 
-export interface StructuredToolInterface<T extends ZodAny = ZodAny>
-  extends RunnableInterface<
+export interface StructuredToolInterface<
+  T extends ZodAny = ZodAny,
+  RunOutput extends string | ToolMessage = string
+> extends RunnableInterface<
     (z.output<T> extends string ? string : never) | z.input<T>,
-    string
+    RunOutput
   > {
   lc_namespace: string[];
 
@@ -59,7 +62,7 @@ export interface StructuredToolInterface<T extends ZodAny = ZodAny>
     configArg?: Callbacks | RunnableConfig,
     /** @deprecated */
     tags?: string[]
-  ): Promise<string>;
+  ): Promise<RunOutput>;
 
   name: string;
 
@@ -72,10 +75,11 @@ export interface StructuredToolInterface<T extends ZodAny = ZodAny>
  * Base class for Tools that accept input of any shape defined by a Zod schema.
  */
 export abstract class StructuredTool<
-  T extends ZodAny = ZodAny
+  T extends ZodAny = ZodAny,
+  RunOutput extends string | ToolMessage = string
 > extends BaseLangChain<
   (z.output<T> extends string ? string : never) | z.input<T>,
-  string
+  RunOutput
 > {
   abstract schema: T | z.ZodEffects<T>;
 
@@ -91,7 +95,7 @@ export abstract class StructuredTool<
     arg: z.output<T>,
     runManager?: CallbackManagerForToolRun,
     config?: RunnableConfig
-  ): Promise<string>;
+  ): Promise<RunOutput>;
 
   /**
    * Invokes the tool with the provided input and configuration.
@@ -102,8 +106,8 @@ export abstract class StructuredTool<
   async invoke(
     input: (z.output<T> extends string ? string : never) | z.input<T>,
     config?: RunnableConfig
-  ): Promise<string> {
-    return this.call(input, ensureConfig(config));
+  ): Promise<RunOutput> {
+    return this.call(input, ensureConfig(config)) as Promise<RunOutput>;
   }
 
   /**
@@ -122,7 +126,7 @@ export abstract class StructuredTool<
     configArg?: Callbacks | RunnableConfig,
     /** @deprecated */
     tags?: string[]
-  ): Promise<string> {
+  ): Promise<RunOutput> {
     let parsed;
     try {
       parsed = await this.schema.parseAsync(arg);
@@ -170,7 +174,10 @@ export abstract class StructuredTool<
   returnDirect = false;
 }
 
-export interface ToolInterface extends StructuredToolInterface {
+export interface ToolInterface<
+  T extends ZodAny = ZodAny,
+  RunOutput extends string | ToolMessage = string
+> extends StructuredToolInterface<T, RunOutput> {
   /**
    * @deprecated Use .invoke() instead. Will be removed in 0.3.0.
    *
@@ -183,13 +190,15 @@ export interface ToolInterface extends StructuredToolInterface {
   call(
     arg: string | undefined | z.input<this["schema"]>,
     callbacks?: Callbacks | RunnableConfig
-  ): Promise<string>;
+  ): Promise<RunOutput>;
 }
 
 /**
  * Base class for Tools that accept input as a string.
  */
-export abstract class Tool extends StructuredTool {
+export abstract class Tool<
+  RunOutput extends string | ToolMessage = string
+> extends StructuredTool<ZodAny, RunOutput> {
   schema = z
     .object({ input: z.string().optional() })
     .transform((obj) => obj.input);
@@ -210,7 +219,7 @@ export abstract class Tool extends StructuredTool {
   call(
     arg: string | undefined | z.input<this["schema"]>,
     callbacks?: Callbacks | RunnableConfig
-  ): Promise<string> {
+  ): Promise<RunOutput> {
     return super.call(
       typeof arg === "string" || !arg ? { input: arg } : arg,
       callbacks
@@ -227,31 +236,37 @@ export interface BaseDynamicToolInput extends ToolParams {
 /**
  * Interface for the input parameters of the DynamicTool class.
  */
-export interface DynamicToolInput extends BaseDynamicToolInput {
+export interface DynamicToolInput<
+  RunOutput extends string | ToolMessage = string
+> extends BaseDynamicToolInput {
   func: (
     input: string,
     runManager?: CallbackManagerForToolRun,
     config?: RunnableConfig
-  ) => Promise<string>;
+  ) => Promise<RunOutput>;
 }
 
 /**
  * Interface for the input parameters of the DynamicStructuredTool class.
  */
-export interface DynamicStructuredToolInput<T extends ZodAny = ZodAny>
-  extends BaseDynamicToolInput {
+export interface DynamicStructuredToolInput<
+  T extends ZodAny = ZodAny,
+  RunOutput extends string | ToolMessage = string
+> extends BaseDynamicToolInput {
   func: (
     input: z.infer<T>,
     runManager?: CallbackManagerForToolRun,
     config?: RunnableConfig
-  ) => Promise<string>;
+  ) => Promise<RunOutput>;
   schema: T;
 }
 
 /**
  * A tool that can be created dynamically from a function, name, and description.
  */
-export class DynamicTool extends Tool {
+export class DynamicTool<
+  RunOutput extends string | ToolMessage = string
+> extends Tool<RunOutput> {
   static lc_name() {
     return "DynamicTool";
   }
@@ -260,9 +275,9 @@ export class DynamicTool extends Tool {
 
   description: string;
 
-  func: DynamicToolInput["func"];
+  func: DynamicToolInput<RunOutput>["func"];
 
-  constructor(fields: DynamicToolInput) {
+  constructor(fields: DynamicToolInput<RunOutput>) {
     super(fields);
     this.name = fields.name;
     this.description = fields.description;
@@ -276,7 +291,7 @@ export class DynamicTool extends Tool {
   async call(
     arg: string | undefined | z.input<this["schema"]>,
     configArg?: RunnableConfig | Callbacks
-  ): Promise<string> {
+  ): Promise<RunOutput> {
     const config = parseCallbackConfigArg(configArg);
     if (config.runName === undefined) {
       config.runName = this.name;
@@ -289,7 +304,7 @@ export class DynamicTool extends Tool {
     input: string,
     runManager?: CallbackManagerForToolRun,
     config?: RunnableConfig
-  ): Promise<string> {
+  ): Promise<RunOutput> {
     return this.func(input, runManager, config);
   }
 }
@@ -301,8 +316,9 @@ export class DynamicTool extends Tool {
  * provided function when the tool is called.
  */
 export class DynamicStructuredTool<
-  T extends ZodAny = ZodAny
-> extends StructuredTool<T> {
+  T extends ZodAny = ZodAny,
+  RunOutput extends string | ToolMessage = string
+> extends StructuredTool<T, RunOutput> {
   static lc_name() {
     return "DynamicStructuredTool";
   }
@@ -311,11 +327,11 @@ export class DynamicStructuredTool<
 
   description: string;
 
-  func: DynamicStructuredToolInput<T>["func"];
+  func: DynamicStructuredToolInput<T, RunOutput>["func"];
 
   schema: T;
 
-  constructor(fields: DynamicStructuredToolInput<T>) {
+  constructor(fields: DynamicStructuredToolInput<T, RunOutput>) {
     super(fields);
     this.name = fields.name;
     this.description = fields.description;
@@ -332,7 +348,7 @@ export class DynamicStructuredTool<
     configArg?: RunnableConfig | Callbacks,
     /** @deprecated */
     tags?: string[]
-  ): Promise<string> {
+  ): Promise<RunOutput> {
     const config = parseCallbackConfigArg(configArg);
     if (config.runName === undefined) {
       config.runName = this.name;
@@ -344,7 +360,7 @@ export class DynamicStructuredTool<
     arg: z.output<T>,
     runManager?: CallbackManagerForToolRun,
     config?: RunnableConfig
-  ): Promise<string> {
+  ): Promise<RunOutput> {
     return this.func(arg, runManager, config);
   }
 }
@@ -365,6 +381,7 @@ export abstract class BaseToolkit {
 /**
  * Parameters for the tool function.
  * @template {ZodAny} RunInput The input schema for the tool.
+ * @template {string | ToolMessage} RunOutput The output type for the tool.
  */
 interface ToolWrapperParams<RunInput extends ZodAny = ZodAny>
   extends ToolParams {
@@ -390,6 +407,7 @@ interface ToolWrapperParams<RunInput extends ZodAny = ZodAny>
  * Creates a new StructuredTool instance with the provided function, name, description, and schema.
  * @function
  * @template {ZodAny} RunInput The input schema for the tool.
+ * @template {string | ToolMessage} RunOutput The output type for the tool.
  *
  * @param {RunnableFunc<RunInput, string>} func - The function to invoke when the tool is called.
  * @param fields - An object containing the following properties:
@@ -399,8 +417,11 @@ interface ToolWrapperParams<RunInput extends ZodAny = ZodAny>
  *
  * @returns {StructuredTool<RunInput, string>} A new StructuredTool instance.
  */
-export function tool<RunInput extends ZodAny = ZodAny>(
-  func: RunnableFunc<z.infer<RunInput>, string>,
+export function tool<
+  RunInput extends ZodAny = ZodAny,
+  RunOutput extends string | ToolMessage = string
+>(
+  func: RunnableFunc<z.infer<RunInput>, RunOutput>,
   fields: ToolWrapperParams<RunInput>
 ) {
   const schema =
@@ -409,7 +430,7 @@ export function tool<RunInput extends ZodAny = ZodAny>(
 
   const description =
     fields.description ?? schema.description ?? `${fields.name} tool`;
-  return new DynamicStructuredTool<RunInput>({
+  return new DynamicStructuredTool<RunInput, RunOutput>({
     name: fields.name,
     description,
     schema: schema as RunInput,