Frontend Tools

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.

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.

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