tools

Tools

Tools are a way to encapsulate a function and its input schema in a way that can be passed to a chat model that supports tool calling. This allows the model to request the execution of this function with specific inputs.

You can either define your own tools or use prebuilt integrations that LangChain provides.

Define tools

You create tools using the tool function:

import { createReactAgent } from "@langchain/langgraph/prebuilt";
import { initChatModel } from "langchain/chat_models/universal";
// highlight-next-line
import { tool } from "@langchain/core/tools";
import { z } from "zod";

const multiply = tool(
  async (input: { a: number; b: number }) => {
    return input.a * input.b;
  },
  {
    name: "multiply",
    schema: z.object({
      a: z.number().describe("First operand"),
      b: z.number().describe("Second operand"),
    }),
    description: "Multiply two numbers.",
  }
);

const llm = await initChatModel("anthropic:claude-3-7-sonnet-latest");
const agent = createReactAgent({
  llm,
  tools: [multiply],
});

For additional customization, refer to the custom tools guide.

Hide arguments from the model

Some tools require runtime-only arguments (e.g., user ID or session context) that should not be controllable by the model.

You can put these arguments in the state or config of the agent, and access this information inside the tool:

import { z } from "zod";
import { tool, type ToolRuntime } from "@langchain/core/tools";
// highlight-next-line
import { MessagesAnnotation } from "@langchain/langgraph";

const myTool = tool(
  async (
    input: {
      // This will be populated by an LLM
      toolArg: string;
    },
    // access runtime values from the second argument
    // highlight-next-line
    runtime: ToolRuntime<typeof MessagesAnnotation.State>
  ) => {
    // Fetch the current agent state
    // highlight-next-line
    const state = runtime.state;
    doSomethingWithState(state.messages);
    doSomethingWithConfig(runtime.config);
    // ...
  },
  {
    name: "myTool",
    schema: z.object({
      myToolArg: z.number().describe("Tool arg"),
    }),
    description: "My tool.",
  }
);