Frontend Tools

"""LangGraph agent backing the Frontend Tools demo.This cell demonstrates `useFrontendTool` with a synchronous handler.The backend graph registers no tools of its own — CopilotKit forwardsthe frontend tool schema(s) to the agent at runtime, and the handlerexecutes in the browser. CopilotKitMiddleware is attached so frontendtools, shared state, and agent context flow into every turn.Like the sibling `frontend_tools_async` cell, the agent has no custombehavior beyond a permissive system prompt — the demo's value is inshowing the wiring contract, not the agent logic."""# region: middlewarefrom langchain.agents import create_agentfrom langchain_openai import ChatOpenAIfrom copilotkit import CopilotKitMiddlewaregraph = create_agent(    model=ChatOpenAI(model="gpt-5.4"),    tools=[],    middleware=[CopilotKitMiddleware()],    system_prompt="You are a helpful, concise assistant.",)# endregion

What is this?#

Frontend tools let your agent define and invoke client-side functions that run entirely in the user's browser. Because the handler executes on the frontend, it has direct access to component state, browser APIs, and any third-party UI library the page already uses. That's how an agent can "reach into" the app: update React state, trigger animations, read localStorage, pop a toast, or steer the user's view.

This page covers the "agent drives the UI" shape of frontend tools. The same primitive also powers Generative UI and Human-in-the-loop; see those pages for interaction patterns.

When should I use this?#

Use frontend tools when your agent needs to:

• Read or modify React component state
• Access browser APIs like localStorage, sessionStorage, or cookies
• Trigger UI updates, animations, or transitions
• Show alerts, toasts, or notifications
• Interact with third-party frontend libraries
• Perform anything that requires the user's immediate browser context

How it works in code#

Register a frontend tool with useFrontendTool. Give it a name, a Zod schema for parameters, and a handler. The agent can then call it like any other tool and your frontend runs it in the browser.

import React, { useState } from "react";import {  CopilotKit,  CopilotSidebar,  useFrontendTool,} from "@copilotkit/react-core/v2";import { z } from "zod";import { Background, DEFAULT_BACKGROUND } from "./background";import { useFrontendToolsSuggestions } from "./suggestions";function Chat() {  const [background, setBackground] = useState<string>(DEFAULT_BACKGROUND);  useFrontendTool({    name: "change_background",    description:      "Change the page background. Accepts any valid CSS background value — colors, linear or radial gradients, etc.",    parameters: z.object({      background: z        .string()        .describe("The CSS background value. Prefer gradients."),    }),    handler: async ({ background }) => {      setBackground(background);      return { status: "success" };    },  });

The handler receives the parsed, type-safe parameters and can do anything the browser can: update state, call an API, touch the DOM. Its return value is sent back to the agent as the tool result so the model can reason about what happened.

    handler: async ({ background }) => {      setBackground(background);      return { status: "success" };    },