Vercel AI SDK

The AI SDK is a provider-agnostic TypeScript toolkit for building AI-powered applications and agents. It runs on React, Next.js, Vue, Svelte, Node.js, and other JavaScript runtimes. A model call stays the same shape whether the provider is OpenAI, Anthropic, Google, Mistral, or a custom one.

Mental model

Two layers, kept deliberately separate:

| Layer | Package | What it does | | --------------- | ------------------------------------------------ | ---------------------------------------------------------------------------------------------------------------------- | | AI SDK Core | ai | Server-side model calls: generateText, streamText, generateObject, streamObject, tools, embeddings, providers. | | AI SDK UI | @ai-sdk/react, @ai-sdk/svelte, @ai-sdk/vue | Framework hooks for chat and generative UI: useChat, useCompletion, useObject. |

A provider package (@ai-sdk/openai, @ai-sdk/anthropic, …) plugs a vendor's models into Core. UI talks to a route handler that calls Core; it never imports a provider directly.

Resolving versions

Resolve the current SDK and model versions from the registry rather than recalling one. Training-data versions are reliably wrong.

npm view ai version          # Core
npm view @ai-sdk/react version
npm view @ai-sdk/openai version

The major version of ai and the provider/adapter packages move together on a release. When upgrading, bump ai, the provider packages, and the UI adapter as a set — a mixed set is the most common source of breakage.

Core calls

import { generateText, streamText } from "ai";
import { openai } from "@ai-sdk/openai";

// One-shot
const { text } = await generateText({
  model: openai("gpt-4o"),
  prompt: "Summarise the AI SDK in one sentence.",
});

// Streaming
const result = streamText({
  model: openai("gpt-4o"),
  prompt: "Tell me a story.",
});
for await (const delta of result.textStream) {
  process.stdout.write(delta);
}

Structured output

Constrain output to a schema with generateObject / streamObject and a Zod definition:

import { generateObject } from "ai";
import { z } from "zod";

const { object } = await generateObject({
  model: openai("gpt-4o"),
  schema: z.object({
    name: z.string(),
    rating: z.number().min(0).max(10),
  }),
  prompt: "Describe the AI SDK.",
});

Tool calling

Define tools with tool and a Zod parameters schema; the model decides when to call them and you return a result it consumes:

import { generateText, tool } from "ai";
import { z } from "zod";

const { text } = await generateText({
  model: openai("gpt-4o"),
  tools: {
    weather: tool({
      description: "Get the weather for a city",
      parameters: z.object({ city: z.string() }),
      execute: async ({ city }) => fetchWeather(city),
    }),
  },
  maxSteps: 5, // lets the model call a tool and use the result
  prompt: "What is the weather in Hanoi?",
});

maxSteps (previously maxToolRoundtrips) enables multi-step loops where the model calls a tool, reads the result, and continues.

Chat UI

useChat from @ai-sdk/react connects a React component to a route handler that streams Core output. Use DefaultChatTransport to point it at your endpoint:

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

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

Messages are UIMessage objects made of typed parts (TextUIPart, ToolUIPart, FileUIPart, …) — render by part type rather than treating message.content as a plain string.

Agents

ToolLoopAgent runs a model that can call tools across multiple steps with stop conditions, human-in-the-loop approval, and an MCP client for external tool servers. Reach for it when a single maxSteps loop is not enough — you need guarded execution, sub-agents, or dynamic tool selection.

Error handling

Catch provider errors by type rather than sniffing strings:

• APICallError — the provider rejected the request (auth, quota, bad input).
• NoSuchToolError — the model returned a tool you did not define.
• UIMessageStreamError — a UI stream surfaced an error part.

Current vs deprecated

• Use maxSteps for multi-step tool calls; maxToolRoundtrips is the old name.
• UI messages are part-based (UIMessage); the legacy string content model is gone in v4+.
• Prefer streamText over manual chunked generation; prefer generateObject over prompt-engineering JSON.

References

• AI SDK Core
• AI SDK UI
• Agents
• Providers
• API reference
• Getting started
• npm: ai