Reference / Hooks

useFrontendTool

React hook for registering client-side tool handlers with optional UI rendering

Overview

useFrontendTool registers a client-side tool with CopilotKit at component scope. When the agent decides to call the tool, the provided handler function executes in the browser. Optionally, you can supply a render component to display custom UI in the chat showing the tool's execution progress and results.

The hook manages the full registration lifecycle: it warns if a tool with the same name already exists, registers the tool and its render component on mount, and cleans up both registrations on unmount. In v2, parameter schemas are defined using Zod instead of plain parameter arrays.

Signature

function useFrontendTool<T extends Record<string, unknown>>(
  tool: ReactFrontendTool<T>,
  deps?: ReadonlyArray<unknown>,
): void;

Parameters

toolReactFrontendTool<T>required

The tool definition object.

namestringrequired

A unique name for the tool. The agent references this name when deciding to call the tool. If a tool with this name is already registered, a warning is logged.

descriptionstringrequired

A natural-language description that tells the agent what the tool does and when to use it.

parametersz.ZodSchemarequired

A Zod schema defining the tool's input parameters. The schema is used for both validation and type inference.

handler(args: T, context?: { toolCall, agent, signal? }) => Promise<string>required

An async function that executes when the agent calls the tool. Receives the validated, typed arguments and an optional context object: - toolCall -- the raw tool call metadata - agent -- the agent instance that invoked the tool - signal -- an AbortSignal that is aborted when the user stops the agent (via stopAgent() or agent.abortRun()). Long-running handlers can check signal.aborted to exit early.

renderReact.ComponentType<{ name: string; args: Partial<T>; status: ToolCallStatus; result: string | undefined }>

An optional React component rendered in the chat interface to visualize tool execution. The component receives: - name -- the tool name - args -- the arguments (partial while streaming, complete once execution starts) - status -- one of ToolCallStatus.InProgress, ToolCallStatus.Executing, or ToolCallStatus.Complete - result -- the string result returned by the handler (only available when status is Complete)

available"enabled" | "disabled" | "remote"
Default: ""enabled""

Controls tool availability. Set to "disabled" to temporarily prevent the agent from calling the tool, or "remote" to indicate the tool is handled server-side.

depsReadonlyArray<unknown>

An optional dependency array, similar to useEffect. When provided, the tool registration is refreshed whenever any value in the array changes. Use this when your handler or render function captures external state.

Usage

Basic Tool with Zod Parameters

function TodoManager() {
  const [todos, setTodos] = useState<string[]>([]);

  useFrontendTool(
    {
      name: "addTodo",
      description: "Add a new item to the user's todo list",
      parameters: z.object({
        text: z.string().describe("The todo item text"),
        priority: z.enum(["low", "medium", "high"]).describe("Priority level"),
      }),
      handler: async ({ text, priority }) => {
        setTodos((prev) => [...prev, text]);
        return `Added "${text}" with ${priority} priority`;
      },
    },
    [],
  );

  return (
    <ul>
      {todos.map((t, i) => (
        <li key={i}>{t}</li>
      ))}
    </ul>
  );
}

Tool with Custom Render Component

function WeatherWidget() {
  useFrontendTool(
    {
      name: "getWeather",
      description: "Fetch and display weather information for a city",
      parameters: z.object({
        city: z.string().describe("City name"),
        units: z.enum(["celsius", "fahrenheit"]).default("celsius"),
      }),
      handler: async ({ city, units }, { signal }) => {
        const response = await fetch(
          `/api/weather?city=${city}&units=${units}`,
          { signal },
        );
        const data = await response.json();
        return JSON.stringify(data);
      },
      render: ({ args, status, result }) => {
        if (status === ToolCallStatus.InProgress) {
          return (
            <div className="animate-pulse">
              Fetching weather for {args.city}...
            </div>
          );
        }
        if (status === ToolCallStatus.Complete && result) {
          const data = JSON.parse(result);
          return (
            <div className="p-4 border rounded">
              <h3>{data.city}</h3>
              <p>
                {data.temperature}&deg; {data.units}
              </p>
              <p>{data.conditions}</p>
            </div>
          );
        }
        return null;
      },
    },
    [],
  );

  return null;
}

Conditionally Available Tool

function AdminPanel({ isAdmin }: { isAdmin: boolean }) {
  useFrontendTool(
    {
      name: "deleteUser",
      description: "Delete a user account by ID (admin only)",
      parameters: z.object({
        userId: z.string().describe("The ID of the user to delete"),
      }),
      handler: async ({ userId }) => {
        await fetch(`/api/users/${userId}`, { method: "DELETE" });
        return `User ${userId} deleted`;
      },
      available: isAdmin ? "enabled" : "disabled",
    },
    [isAdmin],
  );

  return <div>{/* admin UI */}</div>;
}

Behavior

  • Duplicate detection: If a tool with the same name is already registered, the hook logs a warning. Only one tool per name is active at a time.
  • Mount/Unmount lifecycle: The tool and its optional render component are registered on mount and removed on unmount.
  • Dependency tracking: When deps is provided, the tool registration is refreshed whenever any dependency value changes, similar to useEffect.
  • Render component lifecycle: If a render function is provided, it is added to the internal render tool calls registry. It receives streaming args (partial during InProgress, complete during Executing and Complete).
  • No return value: The hook returns void.

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