Transport

The useChat transport system provides fine-grained control over how messages are sent to your API endpoints and how responses are processed. This is particularly useful for alternative communication protocols like WebSockets, custom authentication patterns, or specialized backend integrations.

Default Transport

By default, useChat uses HTTP POST requests to send messages to /api/chat:

import { useChat } from '@ai-sdk/react';

// Uses default HTTP transport
const { messages, sendMessage } = useChat();

This is equivalent to:

import { useChat } from '@ai-sdk/react';
import { DefaultChatTransport } from 'ai';

const { messages, sendMessage } = useChat({
  transport: new DefaultChatTransport({
    api: '/api/chat',
  }),
});

Custom Transport Configuration

Configure the default transport with custom options:

import { useChat } from '@ai-sdk/react';
import { DefaultChatTransport } from 'ai';

const { messages, sendMessage } = useChat({
  transport: new DefaultChatTransport({
    api: '/api/custom-chat',
    headers: {
      Authorization: 'Bearer your-token',
      'X-API-Version': '2024-01',
    },
    credentials: 'include',
  }),
});

Dynamic Configuration

You can also provide functions that return configuration values. This is useful for authentication tokens that need to be refreshed, or for configuration that depends on runtime conditions:

const { messages, sendMessage } = useChat({
  transport: new DefaultChatTransport({
    api: '/api/chat',
    headers: () => ({
      Authorization: `Bearer ${getAuthToken()}`,
      'X-User-ID': getCurrentUserId(),
    }),
    body: () => ({
      sessionId: getCurrentSessionId(),
      preferences: getUserPreferences(),
    }),
    credentials: () => 'include',
  }),
});

Request Transformation

Transform requests before sending to your API:

const { messages, sendMessage } = useChat({
  transport: new DefaultChatTransport({
    api: '/api/chat',
    prepareSendMessagesRequest: ({ id, messages, trigger, messageId }) => {
      return {
        headers: {
          'X-Session-ID': id,
        },
        body: {
          messages: messages.slice(-10), // Only send last 10 messages
          trigger,
          messageId,
        },
      };
    },
  }),
});

WebSocket Transport

Use WebSocketChatTransport when you want chat turns to share a persistent WebSocket connection instead of opening an HTTP request for every turn. This is useful for chats with frequent client-side tool results or other multi-step interactions.

import { useChat } from '@ai-sdk/react';
import { WebSocketChatTransport } from 'ai';
import { useMemo } from 'react';

export default function Chat() {
  const transport = useMemo(
    () =>
      new WebSocketChatTransport({
        url: '/api/chat',
        params: () => ({ token: getShortLivedToken() }),
      }),
    [],
  );

  const { messages, sendMessage } = useChat({ transport });

  // ... render the chat
}

The transport multiplexes requests by requestId, validates incoming UIMessageChunk values, forwards aborts to the server, and reuses the open socket for later turns. HTTP and HTTPS URLs are automatically converted to WS and WSS URLs.

The code that creates a transport owns its lifecycle. React and Vue chat hooks do not close caller-supplied transports when options change or components are disposed, because the same transport can be shared by multiple chats. Call transport.close() after its last consumer is gone. If a directly managed Chat instance exclusively owns the transport, call chat.dispose(); for a shared transport, stop individual chats and close the transport only after all consumers are finished.

Your WebSocket server must implement the WebSocketChatTransport protocol. The exported WebSocketChatTransportRequest and WebSocketChatTransportResponse types can be used to type server frames. Use safeValidateWebSocketChatTransportRequest before handling an untrusted client frame.

sendAutomaticallyWhen still starts a new chat turn after the preceding UI message stream finishes. WebSocketChatTransport avoids opening another HTTP connection for that turn, but it does not send tool results into an active model generation.

If the socket closes during a response, the active stream errors. A later send opens a new connection. The transport does not automatically replay requests, because replaying a generation can duplicate model output or tool effects. reconnectToStream requests a complete replay of the retained response from sequence zero. Chat rebuilds the message with a fresh parser, so the replay must include all earlier chunks and the original start chunk's messageId. That stable message ID lets Chat replace the partial assistant message. Duplicate sequence numbers are ignored within each response stream.

For complete options, protocol details, cancellation behavior, and a server example, see the WebSocketChatTransport reference.

Direct Agent Transport

For scenarios where you want to communicate directly with an Agent without going through HTTP, you can use DirectChatTransport. This transport invokes the agent's stream() method directly in-process.

This is useful for:

• Server-side rendering: Run the agent on the server without an API endpoint
• Testing: Test chat functionality without network requests
• Single-process applications: Desktop or CLI apps where client and agent run together

GatewayProviderCustom

Claude Sonnet 5.5

import { useChat } from '@ai-sdk/react';
import { DirectChatTransport, ToolLoopAgent } from 'ai';

const agent = new ToolLoopAgent({
  model: "anthropic/claude-sonnet-5.5",
  instructions: 'You are a helpful assistant.',
  tools: {
    weather: weatherTool,
  },
});

const { messages, sendMessage } = useChat({
  transport: new DirectChatTransport({ agent }),
});

How It Works

Unlike DefaultChatTransport which sends HTTP requests:

1. DirectChatTransport validates incoming UI messages
2. Converts them to model messages using convertToModelMessages
3. Calls the agent's stream() method directly
4. Returns the result as a UI message stream via toUIMessageStream()

Configuration Options

You can pass additional options to customize the stream output:

const transport = new DirectChatTransport({
  agent,
  // Pass options to the agent
  options: { customOption: 'value' },
  // Configure what's sent to the client
  sendReasoning: true,
  sendSources: true,
});

For complete API details, see the DirectChatTransport reference.

Workflow Transport

For chat apps built on the Workflow SDK, WorkflowChatTransport from @ai-sdk/workflow provides automatic stream reconnection. It handles the common scenario where a workflow function times out mid-stream — the transport detects the missing finish event and reconnects to resume from where it left off.

import { useChat } from '@ai-sdk/react';
import { WorkflowChatTransport } from '@ai-sdk/workflow/client';
import { useMemo } from 'react';

export default function Chat() {
  const transport = useMemo(
    () =>
      new WorkflowChatTransport({
        api: '/api/chat',
        maxConsecutiveErrors: 5,
        onChatEnd: ({ chatId, chunkIndex }) => {
          console.log(`Chat complete: ${chunkIndex} chunks`);
        },
      }),
    [],
  );

  const { messages, sendMessage } = useChat({ transport });

  // ... render chat UI
}

Key features:

• Automatic reconnection: Detects interrupted streams (no finish event) and reconnects via GET to {api}/{runId}/stream
• Page refresh recovery: initialStartIndex controls where the initial reconnection begins
• Configurable retries: maxConsecutiveErrors controls how many consecutive reconnection failures to tolerate
• Lifecycle callbacks: onChatSendMessage and onChatEnd for tracking chat state

Negative initialStartIndex values can fetch only the tail when the durable server stream already stores UIMessageChunk objects. For raw WorkflowAgent streams, use a non-negative cursor and follow the server-side conversion in the WorkflowAgent guide.

For the full API reference, see WorkflowChatTransport. For server-side endpoint setup, see the WorkflowAgent guide.

Building Custom Transports

To understand how to build your own transport, refer to the source code of the default implementation:

• DefaultChatTransport - The complete default HTTP transport implementation
• HttpChatTransport - Base HTTP transport with request handling
• ChatTransport Interface - The transport interface you need to implement

These implementations show you exactly how to:

• Handle the sendMessages method
• Process UI message streams
• Transform requests and responses
• Handle errors and connection management

The transport system gives you complete control over how your chat application communicates, enabling integration with any backend protocol or service.