---
title: useAgent
description: "useAgent Hook API Reference"
---

`useAgent` is a React hook that provides access to [AG-UI](https://ag-ui.com) agents and subscribes to their state
changes. It enables components to interact with agents, access their messages, and respond to updates in real-time.
The hook always returns an agent instance. While the runtime is syncing, it returns a provisional runtime agent; once
the runtime has synced, if the agent id does not exist the hook throws an error.

## What is useAgent?

The useAgent hook:

- Retrieves an agent by ID from the CopilotKit context
- Subscribes to agent state changes (messages, state, run status)
- Automatically triggers component re-renders when the agent updates
- Handles cleanup of subscriptions when the component unmounts

## Basic Usage

```tsx
import { useAgent } from "@copilotkit/react-core";

function ChatComponent() {
  const { agent } = useAgent({ agentId: "assistant" });

  return (
    <div>
      <h2>Agent: {agent.id}</h2>
      <div>Messages: {agent.messages.length}</div>
      <div>Running: {agent.isRunning ? "Yes" : "No"}</div>
    </div>
  );
}
```

## Parameters

### agentId

`string` **(optional)**

The ID of the agent to retrieve. If not provided, defaults to `"default"`.

```tsx
const { agent } = useAgent({ agentId: "customer-support" });
```

### updates

`UseAgentUpdate[]` **(optional)**

An array of update types to subscribe to. This allows you to optimize re-renders by only subscribing to specific
changes.

```tsx
import { useAgent, UseAgentUpdate } from "@copilotkit/react-core";

const { agent } = useAgent({
  agentId: "assistant",
  updates: [UseAgentUpdate.OnMessagesChanged],
});
```

Available update types:

- `UseAgentUpdate.OnMessagesChanged` - Updates when messages are added or modified
- `UseAgentUpdate.OnStateChanged` - Updates when agent state changes
- `UseAgentUpdate.OnRunStatusChanged` - Updates when agent starts or stops running

If `updates` is not provided, the hook subscribes to all update types by default.

## Return Value

The hook returns an object with a single property:

### agent

`AbstractAgent`

The agent instance. During runtime synchronization, the hook returns a provisional runtime agent so you can bind UI
immediately. After the runtime has synced (Connected or Error), if the agent id does not exist the hook throws an
error.

## Accessing Agent Messages

The agent's message history is available through the `messages` property. You can read messages and add new ones to
create interactive conversations.

### Reading Messages

```tsx
function SimpleChat() {
  const { agent } = useAgent();

  return (
    <div>
      {agent.messages.map((message) => (
        <div key={message.id}>
          <strong>{message.role}:</strong> {message.content}
        </div>
      ))}
    </div>
  );
}
```

### Sending Messages

To send a message, add it to the agent and then run the agent:

```tsx
import { useAgent } from "@copilotkit/react-core";
import { useCopilotKit } from "@copilotkit/react-core";

function MessageSender() {
  const { agent } = useAgent({ agentId: "assistant" });
  const { copilotkit } = useCopilotKit();

  const sendMessage = async (content: string) => {
    // Add message to agent
    agent.addMessage({
      id: crypto.randomUUID(),
      role: "user",
      content,
    });

    // Run the agent to get a response
    await copilotkit.runAgent({
      agent,
      agentId: "assistant",
    });
  };

  return (
    <div>
      <button onClick={() => sendMessage("Hello!")} disabled={agent.isRunning}>
        Send Hello
      </button>
    </div>
  );
}
```

## Accessing and Updating Shared State

Shared state enables real-time collaboration between users and agents. Both can read and modify the state, creating a
synchronized workspace for interactive features.

### Understanding Shared State

The agent's state is a shared data structure that:

- Can be read by both your application and the agent
- Can be modified by both parties
- Automatically triggers re-renders when changed
- Persists throughout the conversation

State updates cause re-renders when:

- You call `agent.setState()` from your application
- The agent modifies the state during execution
- Any state change occurs, regardless of source

### Reading State

```tsx
function StateDisplay() {
  const { agent } = useAgent({
    updates: [UseAgentUpdate.OnStateChanged], // Subscribe to state changes
  });

  return (
    <div>
      <h3>Current State</h3>
      <pre>{JSON.stringify(agent.state, null, 2)}</pre>
    </div>
  );
}
```

### Updating State

```tsx
function StateController() {
  const { agent } = useAgent({
    updates: [UseAgentUpdate.OnStateChanged],
  });

  const updateState = (key: string, value: any) => {
    // Update the shared state
    agent.setState({
      ...agent.state,
      [key]: value,
    });

    // This will trigger re-renders for all components
    // subscribed to OnStateChanged
  };

  return (
    <div>
      <button
        onClick={() => updateState("counter", (agent.state.counter || 0) + 1)}
      >
        Increment Counter: {agent.state.counter || 0}
      </button>
    </div>
  );
}
```

### Collaborative Features

Shared state enables collaborative features where users and agents work together:

```tsx
function CollaborativeTodo() {
  const { agent } = useAgent({
    updates: [UseAgentUpdate.OnStateChanged],
  });
  const { copilotkit } = useCopilotKit();

  const todos = agent.state.todos || [];

  const addTodo = (text: string) => {
    agent.setState({
      ...agent.state,
      todos: [...todos, { id: crypto.randomUUID(), text, done: false }],
    });
  };

  const toggleTodo = (id: string) => {
    agent.setState({
      ...agent.state,
      todos: todos.map((todo) =>
        todo.id === id ? { ...todo, done: !todo.done } : todo,
      ),
    });
  };

  const askAgentToOrganize = async () => {
    // Add a message asking the agent to organize todos
    agent.addMessage({
      id: crypto.randomUUID(),
      role: "user",
      content: "Please organize my todos by priority",
    });

    // The agent can read and modify the todos in agent.state
    await copilotkit.runAgent({ agent });

    // After the agent runs, the state will be updated
    // and the component will re-render automatically
  };

  return (
    <div>
      <h3>Shared Todo List</h3>
      <ul>
        {todos.map((todo) => (
          <li key={todo.id}>
            <input
              type="checkbox"
              checked={todo.done}
              onChange={() => toggleTodo(todo.id)}
            />
            <span className={todo.done ? "done" : ""}>{todo.text}</span>
          </li>
        ))}
      </ul>

      <button onClick={() => addTodo(prompt("New todo:") || "")}>
        Add Todo
      </button>

      <button onClick={askAgentToOrganize}>Ask Agent to Organize</button>
    </div>
  );
}
```

## Examples

### Optimized Updates

Subscribe only to message changes to avoid unnecessary re-renders:

```tsx
import { useAgent, UseAgentUpdate } from "@copilotkit/react-core";

function MessageList() {
  const { agent } = useAgent({
    updates: [UseAgentUpdate.OnMessagesChanged],
  });

  return (
    <ul>
      {agent.messages.map((msg) => (
        <li key={msg.id}>{msg.content}</li>
      ))}
    </ul>
  );
}
```

### Run Status Indicator

Show a loading indicator when the agent is processing:

```tsx
import { useAgent, UseAgentUpdate } from "@copilotkit/react-core";

function RunStatus() {
  const { agent } = useAgent({
    agentId: "assistant",
    updates: [UseAgentUpdate.OnRunStatusChanged],
  });

  return (
    <div className={`status ${agent.isRunning ? "running" : "idle"}`}>
      {agent.isRunning ? "🔄 Processing..." : "✅ Ready"}
    </div>
  );
}
```

### Multiple Agents

Access different agents in different components:

```tsx
function DualAgentView() {
  const { agent: primaryAgent } = useAgent({
    agentId: "primary-assistant",
  });

  const { agent: supportAgent } = useAgent({
    agentId: "support-assistant",
  });

  return (
    <div className="dual-view">
      <div className="primary">
        <h3>Primary Assistant</h3>
        <div>Messages: {primaryAgent.messages.length}</div>
      </div>

      <div className="support">
        <h3>Support Assistant</h3>
        <div>Messages: {supportAgent.messages.length}</div>
      </div>
    </div>
  );
}
```

### No Updates Subscription

For static access without subscribing to updates:

```tsx
const { agent } = useAgent({
  agentId: "assistant",
  updates: [], // No subscriptions
});

// Component won't re-render on agent changes
```

### Custom Message Rendering with Optimized Updates

```tsx
import { useAgent, UseAgentUpdate } from "@copilotkit/react-core";
import { Message } from "@ag-ui/core";

function MessageRenderer() {
  const { agent } = useAgent({
    updates: [UseAgentUpdate.OnMessagesChanged], // Only re-render on message changes
  });

  if (agent.messages.length === 0) {
    return <div>No messages yet</div>;
  }

  const renderMessage = (message: Message) => {
    switch (message.role) {
      case "user":
        return (
          <div className="user-message">
            <span className="avatar">👤</span>
            <span className="content">{message.content}</span>
          </div>
        );
      case "assistant":
        return (
          <div className="assistant-message">
            <span className="avatar">🤖</span>
            <span className="content">{message.content}</span>
          </div>
        );
      default:
        return null;
    }
  };

  return (
    <div className="message-list">
      {agent.messages.map((msg) => (
        <div key={msg.id}>{renderMessage(msg)}</div>
      ))}
    </div>
  );
}
```

## Performance Considerations

### Selective Updates

By default, `useAgent` subscribes to all update types, which can cause frequent re-renders. Use the `updates` parameter
to subscribe only to the changes you need:

```tsx
// ❌ Subscribes to all updates (may cause unnecessary re-renders)
const { agent } = useAgent({ agentId: "assistant" });

// ✅ Only subscribes to message changes
const { agent } = useAgent({
  agentId: "assistant",
  updates: [UseAgentUpdate.OnMessagesChanged],
});

// ✅ Only subscribes to run status changes
const { agent } = useAgent({
  agentId: "assistant",
  updates: [UseAgentUpdate.OnRunStatusChanged],
});
```

### Component Splitting

Split components by update type to optimize rendering:

```tsx
// Message display component - only updates on message changes
function Messages() {
  const { agent } = useAgent({
    updates: [UseAgentUpdate.OnMessagesChanged],
  });

  // Render messages...
}

// Status indicator - only updates on run status changes
function StatusIndicator() {
  const { agent } = useAgent({
    updates: [UseAgentUpdate.OnRunStatusChanged],
  });

  // Render status...
}

// Parent component doesn't need to subscribe
function ChatView() {
  return (
    <>
      <StatusIndicator />
      <Messages />
    </>
  );
}
```

## Error Handling

After the runtime has synced (Connected or Error), `useAgent` throws if the requested agent id does not exist. During
runtime syncing, a provisional agent is returned. To avoid runtime errors:

- Ensure the agent id you request is registered (via `agents__unsafe_dev_only` or exposed by your runtime).
- Optionally wrap the component that calls `useAgent` in an error boundary to render a fallback UI if the agent is
  missing due to misconfiguration in development.

Example configuration with a local agent for development:

```tsx
<CopilotKitProvider agents__unsafe_dev_only={{ myAgent: new MyAgent() }}>
  <App />
</CopilotKitProvider>
```