CopilotKit

useFrontendTool

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

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 on the device. Optionally, you can supply a render component to display custom UI in the chat showing the tool's execution progress and results.

Re-exported from @copilotkit/react-core/v2. It is identical to the React (V2) useFrontendTool; only the import path differs.

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

import { useFrontendTool } from "@copilotkit/react-native";

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

Parameters

Prop

Type

Prop

Type

Usage

Basic Tool with Zod Parameters

import { useState } from "react";
import { Text, View } from "react-native";
import { useFrontendTool } from "@copilotkit/react-native";
import { z } from "zod";

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 (
    <View>
      {todos.map((t, i) => (
        <Text key={i}>{t}</Text>
      ))}
    </View>
  );
}

Tool with Custom Render Component

import { Text, View } from "react-native";
import { useFrontendTool, ToolCallStatus } from "@copilotkit/react-native";
import { z } from "zod";

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(
          `https://your-server/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 (
            <View>
              <Text>Fetching weather for {args.city}...</Text>
            </View>
          );
        }
        if (status === ToolCallStatus.Complete && result) {
          const data = JSON.parse(result);
          return (
            <View>
              <Text>{data.city}</Text>
              <Text>
                {data.temperature}&deg; {data.units}
              </Text>
              <Text>{data.conditions}</Text>
            </View>
          );
        }
        return null;
      },
    },
    [],
  );

  return null;
}

Conditionally Available Tool

import { View } from "react-native";
import { useFrontendTool } from "@copilotkit/react-native";
import { z } from "zod";

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(`https://your-server/api/users/${userId}`, {
          method: "DELETE",
        });
        return `User ${userId} deleted`;
      },
      available: isAdmin ? "enabled" : "disabled",
    },
    [isAdmin],
  );

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

The render type React Native needs

render is typed as ReactToolCallRenderer<T>["render"], which is a React.ComponentType. A function component's return type is ReactNode, so this compiles:

useFrontendTool({
  name: "showWeather",
  description: "Show the weather",
  parameters: z.object({ city: z.string() }),
  handler: async ({ city }) => city,
  // Compiles. `ReactNode` includes `string`.
  render: ({ args }) => `Weather in ${args.city}`,
});

and then fails on a device. The string reaches the prebuilt chat's FlatList as a bare text child of a host view, and React Native throws Text strings must be rendered within a <Text> component. The same applies to a number, an array of elements, or undefined — all of them are ReactNode, and none of them is something React Native's list renderer can draw.

@copilotkit/react-native exports an opt-in type that narrows the return to ReactElement | null, so the compiler rejects it instead:

import { useFrontendTool } from "@copilotkit/react-native";
import type { FrontendToolRenderFunction } from "@copilotkit/react-native";
import { Text } from "react-native";
import { z } from "zod";

const renderWeather: FrontendToolRenderFunction<{ city: string }> = ({
  args,
}) => <Text>{args.city}</Text>;

// `TS2322: Type 'string' is not assignable to type 'ReactElement…'`
const broken: FrontendToolRenderFunction<{ city: string }> = ({ args }) =>
  `Weather in ${args.city}`;

useFrontendTool({
  name: "showWeather",
  description: "Show the weather",
  parameters: z.object({ city: z.string() }),
  handler: async ({ city }) => city,
  render: renderWeather,
});

Prop

Type

It is opt-in, and only opt-in

This type changes no hook signature. A renderer written inline in a useFrontendTool call is still checked against react-core's ReactNode-returning contract, and a bare string still compiles there. You get the narrowing on renderers you annotate with it — declare render as a standalone FrontendToolRenderFunction<T> and pass it in, as above. It does not make the package safe on its own.

useRenderTool needs none of this: it declares its own render as (props) => ReactElement | null, so a bare string is rejected there without any annotation.

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 registration is refreshed whenever the serialised array changes — JSON.stringify(deps), not useEffect's per-element Object.is. See Dependency comparison below; the difference is load-bearing.
  • Handler and render are captured at registration: Neither closure is re-read on every render. A value your handler or render reads from component state or props is frozen at whatever it was when the tool was last registered, so it must be declared in deps (subject to the comparison rules below).
  • 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).
  • render's return type is ReactNode, not ReactElement | null: a bare string typechecks and then throws in the chat's FlatList. Annotate the renderer with FrontendToolRenderFunction<T> to have the compiler reject it.
  • No return value: The hook returns void.

Dependency comparison

The registration effect's real dependency list is [tool.name, tool.available, copilotkit, JSON.stringify(extraDeps)] (packages/react-core/src/v2/hooks/use-frontend-tool.tsx:45, where extraDeps is your deps ?? []). Your array reaches React as one serialised string, so the reference identity that useEffect compares never enters into it.

What that means in practice — the middle column is actual JSON.stringify output for the value wrapped in an array:

Value in depsSerialises toEffect
Primitives: string, number, boolean, nullthemselvesCompared correctly. Prefer these.
A functionnullNever re-registers — every function serialises identically.
undefined, a SymbolnullNever re-registers.
new Map([["a", 1]]), new Set([1, 2, 3]){}Never re-registers — an empty and a populated Map compare equal.
Class instance with own enumerable fields (this.n = n){"n":1}Compared correctly.
Class instance whose state is in #private fields{}Never re-registers.
Class instance whose state is behind a prototype getter{} (only own fields appear)Never re-registers on the getter's value.
{ x: 1, y: 2 } vs { y: 2, x: 1 }{"x":1,"y":2} vs {"y":2,"x":1}Re-registers even though the objects are equivalent — key order counts.
A circular object, a BigIntthrows TypeErrorCrashes the component — see below.

A circular value in deps is not a silent bug. JSON.stringify throws TypeError: Converting circular structure to JSON while the component is rendering, which takes the component down. Never put a value that may reference itself — a graph node, a parent-linked tree, a store instance — into deps. A BigInt throws the same way (Do not know how to serialize a BigInt).

Passing values that do not serialise

Two patterns work.

Derive a primitive that changes exactly when the thing you care about changes, and put that in deps:

// Not `[items]`, `[cache]`, `[onSubmit]` — those are invisible or unstable.
useFrontendTool({ ...tool }, [items.length, cache.size, selectedId, revision]);

Or keep the value out of deps and read it through a latest-value ref. The registration stays stable while the closure always sees the current value. This is the only option for a changing callback, which deps cannot detect at all:

import { useEffect, useRef } from "react";
import { useFrontendTool } from "@copilotkit/react-native";
import { z } from "zod";

function DraftTool({ onSubmit }: { onSubmit: () => Promise<string> }) {
  const latestOnSubmit = useRef(onSubmit);
  useEffect(() => {
    latestOnSubmit.current = onSubmit;
  });

  useFrontendTool(
    {
      name: "submitDraft",
      description: "Submit the current draft",
      parameters: z.object({}),
      handler: async () => latestOnSubmit.current(),
    },
    [],
  );

  return null;
}

The same ref pattern applies to a render component that needs live state, and to any Map, Set or class instance you would otherwise have been tempted to list in deps.

Related

  • useRenderTool: register renderer-only tool call UI (named or wildcard), and the migration notes for React Native's older local hook of the same name
  • useCopilotKit: access the core instance and run tools programmatically
  • React (V2) reference: the web equivalent this page mirrors