Vercel AI SDK

The Vercel AI SDK provides React hooks and server utilities for building streaming chat interfaces with support for tool calls, file attachments, and multi-step reasoning.

Quick Reference

Basic useChat Setup

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

const { messages, status, sendMessage, stop, regenerate } = useChat({
  id: 'chat-id',
  messages: initialMessages,
  onFinish: ({ message, messages, isAbort, isError }) => {
    console.log('Chat finished');
  },
  onError: (error) => {
    console.error('Chat error:', error);
  }
});

// Send a message
sendMessage({ text: 'Hello', metadata: { createdAt: Date.now() } });

// Send with files
sendMessage({
  text: 'Analyze this',
  files: fileList // FileList or FileUIPart[]
});

ChatStatus States

The status field indicates the current state of the chat:

• ready: Chat is idle and ready to accept new messages
• submitted: Message sent to API, awaiting response stream start
• streaming: Response actively streaming from the API
• error: An error occurred during the request

Message Structure

Messages use the UIMessage type with a parts-based structure:

interface UIMessage {
  id: string;
  role: 'system' | 'user' | 'assistant';
  metadata?: unknown;
  parts: Array<UIMessagePart>; // text, file, tool-*, reasoning, etc.
}

Part types include:

• text: Text content with optional streaming state
• file: File attachments (images, documents)
• tool-{toolName}: Tool invocations with state machine
• reasoning: AI reasoning traces
• data-{typeName}: Custom data parts

Server-Side Streaming

import { streamText } from 'ai';
import { convertToModelMessages } from 'ai';

const result = streamText({
  model: openai('gpt-4'),
  messages: convertToModelMessages(uiMessages),
  tools: {
    getWeather: tool({
      description: 'Get weather',
      inputSchema: z.object({ city: z.string() }),
      execute: async ({ city }) => {
        return { temperature: 72, weather: 'sunny' };
      }
    })
  }
});

return result.toUIMessageStreamResponse({
  originalMessages: uiMessages,
  onFinish: ({ messages }) => {
    // Save to database
  }
});

Tool Handling Patterns

Client-Side Tool Execution:

const { addToolOutput } = useChat({
  onToolCall: async ({ toolCall }) => {
    if (toolCall.toolName === 'getLocation') {
      addToolOutput({
        tool: 'getLocation',
        toolCallId: toolCall.toolCallId,
        output: 'San Francisco'
      });
    }
  }
});