Frontend Tools
Create frontend tools and use them within your Strands agent.
What is this?
Frontend tools enable you to define client-side functions that your Strands agent can invoke, with execution happening entirely in the user's browser. When your agent calls a frontend tool, the logic runs on the client side, giving you direct access to the frontend environment.
This can be utilized to let your agent control the UI, generative UI, or for Human-in-the-loop interactions.
In this guide, we cover the use of frontend tools driving and interacting with the UI.
When should I use this?
Use frontend tools when you need your agent to interact with client-side primitives such as:
- Reading or modifying React component state
- Accessing browser APIs like localStorage, sessionStorage, or cookies
- Triggering UI updates or animations
- Interacting with third-party frontend libraries
- Performing actions that require the user's immediate browser context
Implementation
Check out the Frontend Tools overview to understand what they are and when to use them.
Run and connect your agent
You'll need to run your agent and connect it to CopilotKit before proceeding.
If you don't already have CopilotKit and your agent connected, choose one of the following options:
Register the frontend tool in your agent
In your Strands agent, define a tool that returns None. This registers the tool with the LLM so it knows about it,
but the actual execution will happen on the frontend.
import os
from strands import Agent, tool
from strands.models.openai import OpenAIModel
from ag_ui_strands import StrandsAgent, create_strands_app
@tool
def change_background(background: str):
"""
Change the background color of the chat. Can be anything that CSS accepts.
Args:
background: The background color or gradient. Prefer gradients.
Returns:
None - execution happens on the frontend
"""
# Return None - frontend will handle execution
return None
api_key = os.getenv("OPENAI_API_KEY", "")
model = OpenAIModel(
client_args={"api_key": api_key},
model_id="gpt-4o",
)
agent = Agent(
model=model,
tools=[change_background],
system_prompt="You are a helpful assistant.",
)
agui_agent = StrandsAgent(
agent=agent,
name="my_agent",
description="A helpful assistant",
)
app = create_strands_app(agui_agent, "/")Create the frontend tool handler
Create a frontend tool using the useFrontendTool hook. The name must match the tool name defined in your agent.
"use client";
import { useFrontendTool } from "@copilotkit/react-core";
import { CopilotSidebar, CopilotKitCSSProperties } from "@copilotkit/react-ui";
import { useState } from "react";
export default function Page() {
const [background, setBackground] = useState("#6366f1");
useFrontendTool({
name: "change_background",
description: "Change the background color of the chat.",
parameters: [
{
name: "background",
type: "string",
description: "The background color or gradient. Prefer gradients.",
required: true,
},
],
handler: async ({ background }) => {
setBackground(background);
return `Background changed to ${background}`;
},
});
return (
<main
style={{
background,
transition: "background 0.3s ease",
}}
className="h-screen"
>
<CopilotSidebar />
</main>
);
}Give it a try!
Try asking the agent to change the background:
Change the background to a sunset gradientMake the background dark purpleYou should see the background change in real-time as the agent calls your frontend tool!
Accessing React state
Frontend tools have full access to the DOM - including your React component state. For example, you can
have your agent trigger re-renders via the useFrontendTool hook.
"use client";
import { useFrontendTool } from "@copilotkit/react-core";
import { useState } from "react";
export default function Page() {
const [tasks, setTasks] = useState<string[]>([]);
useFrontendTool({
name: "add_task",
description: "Add a task to the todo list",
parameters: [
{
name: "task",
type: "string",
description: "The task to add",
required: true,
},
],
handler: async ({ task }) => {
setTasks((prev) => [...prev, task]);
return `Added task: ${task}`;
},
});
return (
<div>
<h1>Todo List</h1>
<ul>
{tasks.map((task, i) => (
<li key={i}>{task}</li>
))}
</ul>
</div>
);
}Remember to define the corresponding tool in your Strands agent that returns None!
