🤖 Ghostwritten by Claude Opus 4.5 · Curated by Tom Hundley

This article was written by Claude Opus 4.5 and curated for publication by Tom Hundley.

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Vercel AI SDK: Streaming RAG for Modern Web Apps

When milliseconds matter and your users expect ChatGPT-level experiences.

Why Vercel AI SDK?

The frameworks we have covered so far, LangChain, LlamaIndex, Haystack, are Python-first tools designed for backend AI pipelines. They excel at document processing, complex retrieval strategies, and server-side orchestration.

Vercel AI SDK takes a fundamentally different approach. It is TypeScript-first, built for the web, and optimized for one thing above all else: streaming.

When a user asks a question in a modern AI interface, they expect to see tokens appear in real-time, not a loading spinner followed by a wall of text. This streaming-first mentality permeates every design decision in the AI SDK. It is not an afterthought bolted onto a traditional request-response model; it is the foundation.

What AI SDK Is (And Is Not)

Let me be direct: Vercel AI SDK is not a complete RAG framework. It does not have document loaders, chunking utilities, or built-in vector store integrations like LangChain or LlamaIndex.

What it provides instead are primitives: low-level building blocks for building AI-native web applications. These primitives handle the hard parts of streaming AI responses, managing conversation state in React, and deploying to edge runtimes, while leaving the RAG architecture decisions to you.

This is a feature, not a limitation. By focusing on what web frameworks do best, AI SDK avoids the abstraction overhead that frustrates developers in more comprehensive frameworks. You get exactly what you need, nothing more.

When AI SDK Excels

Vercel AI SDK is the right choice when:

• You are building with Next.js: The integration is seamless, first-class support for App Router, Server Components, and edge deployment
• Streaming UX is critical: Your users expect ChatGPT-like responsiveness
• TypeScript is your language: Full type safety across the entire stack
• You want minimal abstraction: Direct control over every aspect of the AI interaction
• Edge deployment matters: Sub-100ms cold starts on Vercel's edge network

How AI SDK Differs

If you are coming from LangChain (Part 2) or LlamaIndex (Part 3):

Aspect	LangChain/LlamaIndex	Vercel AI SDK
Language	Python-first	TypeScript-first
Focus	Complete RAG pipeline	UI primitives for AI
Streaming	Supported (add-on)	Core architecture
Document processing	Extensive built-in	Bring your own
Vector stores	50+ integrations	Manual integration
React integration	Limited	First-class hooks
Edge runtime	Difficult	Native support

Setting Up Vercel AI SDK

Installation

The AI SDK is modular. Install the core package plus your preferred AI provider:

# Core SDK
npm install ai

# Provider packages (install what you need)
npm install @ai-sdk/openai       # OpenAI
npm install @ai-sdk/anthropic    # Anthropic Claude
npm install @ai-sdk/google       # Google Gemini
npm install @ai-sdk/mistral      # Mistral AI
npm install @ai-sdk/amazon-bedrock  # AWS Bedrock

For a typical Next.js RAG application:

npm install ai @ai-sdk/openai

Environment Configuration

Create a .env.local file in your Next.js project root:

# Required: Your AI provider API key
OPENAI_API_KEY=sk-proj-...

# Optional: For other providers
ANTHROPIC_API_KEY=sk-ant-...
GOOGLE_GENERATIVE_AI_API_KEY=...

# For vector database (choose one)
PINECONE_API_KEY=...
UPSTASH_VECTOR_REST_URL=...
UPSTASH_VECTOR_REST_TOKEN=...

# For PostgreSQL with pgvector
DATABASE_URL=postgresql://user:password@host:5432/db

Project Structure

A well-organized AI SDK project:

app/
├── api/
│   └── chat/
│       └── route.ts       # Streaming chat endpoint
├── components/
│   └── chat.tsx           # Chat UI component
├── lib/
│   ├── ai/
│   │   ├── retrieval.ts   # RAG retrieval logic
│   │   └── embedding.ts   # Embedding utilities
│   └── db/
│       └── vectors.ts     # Vector store client
├── page.tsx               # Main page
└── layout.tsx

Core Primitives

AI SDK provides three categories of primitives: server-side functions for generation, client-side hooks for React, and utilities for managing AI interactions.

streamText: The Foundation

The streamText function is the core primitive for streaming text generation:

// app/api/chat/route.ts
import { streamText } from 'ai';
import { openai } from '@ai-sdk/openai';

export async function POST(req: Request) {
  const { messages } = await req.json();

  const result = streamText({
    model: openai('gpt-4o'),
    messages,
    system: 'You are a helpful assistant.',
  });

  return result.toDataStreamResponse();
}

This creates a streaming endpoint that sends tokens as they are generated. The toDataStreamResponse() method handles all the complexity of Server-Sent Events, backpressure, and proper HTTP headers.

generateText: Non-Streaming Alternative

When you do not need streaming (batch processing, background tasks):

import { generateText } from 'ai';
import { openai } from '@ai-sdk/openai';

const { text } = await generateText({
  model: openai('gpt-4o'),
  prompt: 'Summarize the key points of this document...',
});

console.log(text);

useChat: React Hook for Conversations

The useChat hook manages the entire conversation lifecycle on the client:

// app/components/chat.tsx
'use client';

import { useChat } from 'ai/react';

export function Chat() {
  const { messages, input, handleInputChange, handleSubmit, isLoading } = useChat();

  return (
    <div className="flex flex-col h-screen">
      <div className="flex-1 overflow-y-auto p-4 space-y-4">
        {messages.map((message) => (
          <div
            key={message.id}
            className={`p-3 rounded-lg ${
              message.role === 'user'
                ? 'bg-blue-100 ml-auto max-w-[80%]'
                : 'bg-gray-100 mr-auto max-w-[80%]'
            }`}
          >
            {message.content}
          </div>
        ))}
        {isLoading && (
          <div className="text-gray-500">Thinking...</div>
        )}
      </div>

      <form onSubmit={handleSubmit} className="p-4 border-t">
        <input
          value={input}
          onChange={handleInputChange}
          placeholder="Ask a question..."
          className="w-full p-2 border rounded"
        />
      </form>
    </div>
  );
}

The hook automatically:

• Maintains conversation history
• Handles form submission
• Streams responses token by token
• Manages loading and error states
• Persists messages across renders

useCompletion: Single-Turn Generation

For single-turn completions (not conversations):

'use client';

import { useCompletion } from 'ai/react';

export function Summarizer() {
  const { completion, input, handleInputChange, handleSubmit, isLoading } =
    useCompletion({
      api: '/api/summarize',
    });

  return (
    <div>
      <form onSubmit={handleSubmit}>
        <textarea
          value={input}
          onChange={handleInputChange}
          placeholder="Paste text to summarize..."
          className="w-full h-48 p-2 border rounded"
        />
        <button type="submit" disabled={isLoading}>
          {isLoading ? 'Summarizing...' : 'Summarize'}
        </button>
      </form>

      {completion && (
        <div className="mt-4 p-4 bg-gray-50 rounded">
          <h3 className="font-bold">Summary:</h3>
          <p>{completion}</p>
        </div>
      )}
    </div>
  );
}

Building RAG with AI SDK

AI SDK does not provide RAG components out of the box. Instead, you build RAG by combining AI SDK's streaming primitives with your choice of vector database and retrieval logic.

Approach 1: Simple Context Injection

The simplest RAG pattern: retrieve documents before calling streamText:

// app/api/chat/route.ts
import { streamText } from 'ai';
import { openai } from '@ai-sdk/openai';
import { retrieveDocuments } from '@/lib/ai/retrieval';

export async function POST(req: Request) {
  const { messages } = await req.json();
  const lastMessage = messages[messages.length - 1].content;

  // 1. Retrieve relevant documents
  const documents = await retrieveDocuments(lastMessage);

  // 2. Format context
  const context = documents
    .map((doc, i) => `[${i + 1}] ${doc.content}`)
    .join('\n\n');

  // 3. Build system prompt with context
  const systemPrompt = `You are a helpful assistant. Answer questions based on the following context.
If the context doesn't contain relevant information, say so.

Context:
${context}

Always cite your sources using [1], [2], etc.`;

  // 4. Stream the response
  const result = streamText({
    model: openai('gpt-4o'),
    system: systemPrompt,
    messages,
  });

  return result.toDataStreamResponse();
}

Approach 2: Tool-Based Retrieval

A more sophisticated approach uses AI SDK's tool calling feature. The model decides when to retrieve:

// app/api/chat/route.ts
import { streamText, tool } from 'ai';
import { openai } from '@ai-sdk/openai';
import { z } from 'zod';
import { searchDocuments } from '@/lib/ai/retrieval';

export async function POST(req: Request) {
  const { messages } = await req.json();

  const result = streamText({
    model: openai('gpt-4o'),
    system: `You are a helpful assistant with access to a knowledge base.
Use the searchKnowledgeBase tool to find relevant information before answering questions.
Always cite your sources.`,
    messages,
    tools: {
      searchKnowledgeBase: tool({
        description: 'Search the knowledge base for relevant documents',
        parameters: z.object({
          query: z.string().describe('The search query'),
          limit: z.number().default(5).describe('Number of results to return'),
        }),
        execute: async ({ query, limit }) => {
          const results = await searchDocuments(query, limit);
          return results.map((doc) => ({
            title: doc.metadata.title,
            content: doc.content,
            source: doc.metadata.source,
          }));
        },
      }),
    },
    maxSteps: 3, // Allow up to 3 tool calls per request
  });

  return result.toDataStreamResponse();
}

The maxSteps parameter enables multi-step reasoning: the model can search, review results, search again with a refined query, and then answer.

Approach 3: Middleware Pattern

For advanced control, use the wrapLanguageModel function to inject context automatically:

// lib/ai/rag-middleware.ts
import { wrapLanguageModel, type LanguageModelV1Middleware } from 'ai';
import { openai } from '@ai-sdk/openai';
import { retrieveDocuments } from './retrieval';

const ragMiddleware: LanguageModelV1Middleware = {
  transformParams: async ({ params }) => {
    // Extract the last user message
    const lastUserMessage = params.prompt
      .filter((p) => p.role === 'user')
      .pop();

    if (!lastUserMessage || lastUserMessage.content[0].type !== 'text') {
      return params;
    }

    const query = lastUserMessage.content[0].text;

    // Retrieve relevant documents
    const documents = await retrieveDocuments(query);

    // Inject context into system prompt
    const context = documents.map((d) => d.content).join('\n\n---\n\n');

    const enhancedSystem = `${params.prompt.find((p) => p.role === 'system')?.content || ''}

Relevant context from the knowledge base:
${context}

Use this context to inform your response. Cite sources when applicable.`;

    return {
      ...params,
      prompt: params.prompt.map((p) =>
        p.role === 'system' ? { ...p, content: enhancedSystem } : p
      ),
    };
  },
};

export const ragModel = wrapLanguageModel({
  model: openai('gpt-4o'),
  middleware: ragMiddleware,
});

Then use the wrapped model in your route:

// app/api/chat/route.ts
import { streamText } from 'ai';
import { ragModel } from '@/lib/ai/rag-middleware';

export async function POST(req: Request) {
  const { messages } = await req.json();

  const result = streamText({
    model: ragModel,
    system: 'You are a helpful assistant.',
    messages,
  });

  return result.toDataStreamResponse();
}

Integrating Vector Databases

AI SDK does not include vector store clients. Here are integration patterns for popular options.

Pinecone Integration

// lib/db/pinecone.ts
import { Pinecone } from '@pinecone-database/pinecone';
import { openai } from '@ai-sdk/openai';
import { embed } from 'ai';

const pinecone = new Pinecone();
const index = pinecone.index('rag-documents');

export async function searchDocuments(query: string, limit: number = 5) {
  // Generate embedding for query
  const { embedding } = await embed({
    model: openai.embedding('text-embedding-3-small'),
    value: query,
  });

  // Search Pinecone
  const results = await index.query({
    vector: embedding,
    topK: limit,
    includeMetadata: true,
  });

  return results.matches.map((match) => ({
    content: match.metadata?.content as string,
    metadata: {
      title: match.metadata?.title as string,
      source: match.metadata?.source as string,
    },
    score: match.score,
  }));
}

export async function indexDocument(
  id: string,
  content: string,
  metadata: Record<string, string>
) {
  const { embedding } = await embed({
    model: openai.embedding('text-embedding-3-small'),
    value: content,
  });

  await index.upsert([
    {
      id,
      values: embedding,
      metadata: { content, ...metadata },
    },
  ]);
}

Upstash Vector Integration

Upstash Vector is particularly well-suited for edge deployments:

// lib/db/upstash.ts
import { Index } from '@upstash/vector';
import { openai } from '@ai-sdk/openai';
import { embed } from 'ai';

const index = new Index({
  url: process.env.UPSTASH_VECTOR_REST_URL!,
  token: process.env.UPSTASH_VECTOR_REST_TOKEN!,
});

export async function searchDocuments(query: string, limit: number = 5) {
  const { embedding } = await embed({
    model: openai.embedding('text-embedding-3-small'),
    value: query,
  });

  const results = await index.query({
    vector: embedding,
    topK: limit,
    includeMetadata: true,
  });

  return results.map((result) => ({
    content: result.metadata?.content as string,
    metadata: result.metadata as Record<string, string>,
    score: result.score,
  }));
}

pgvector with Drizzle ORM

For PostgreSQL-based applications:

// lib/db/pgvector.ts
import { drizzle } from 'drizzle-orm/node-postgres';
import { pgTable, text, vector, uuid } from 'drizzle-orm/pg-core';
import { cosineDistance, desc, sql } from 'drizzle-orm';
import { embed } from 'ai';
import { openai } from '@ai-sdk/openai';
import { Pool } from 'pg';

// Schema definition
export const documents = pgTable('documents', {
  id: uuid('id').primaryKey().defaultRandom(),
  content: text('content').notNull(),
  title: text('title'),
  source: text('source'),
  embedding: vector('embedding', { dimensions: 1536 }),
});

const pool = new Pool({ connectionString: process.env.DATABASE_URL });
const db = drizzle(pool);

export async function searchDocuments(query: string, limit: number = 5) {
  const { embedding } = await embed({
    model: openai.embedding('text-embedding-3-small'),
    value: query,
  });

  const results = await db
    .select({
      id: documents.id,
      content: documents.content,
      title: documents.title,
      source: documents.source,
      similarity: sql<number>`1 - (${cosineDistance(documents.embedding, embedding)})`,
    })
    .from(documents)
    .orderBy(desc(sql`1 - (${cosineDistance(documents.embedding, embedding)})`))
    .limit(limit);

  return results;
}

export async function indexDocument(
  content: string,
  title: string,
  source: string
) {
  const { embedding } = await embed({
    model: openai.embedding('text-embedding-3-small'),
    value: content,
  });

  await db.insert(documents).values({
    content,
    title,
    source,
    embedding,
  });
}

Real-Time Streaming UI

AI SDK's streaming capabilities shine in the UI layer. Here are patterns for building responsive, real-time interfaces.

Token-by-Token Rendering

The default useChat behavior streams tokens automatically:

'use client';

import { useChat } from 'ai/react';

export function StreamingChat() {
  const { messages, input, handleInputChange, handleSubmit, isLoading } = useChat();

  return (
    <div className="max-w-2xl mx-auto p-4">
      <div className="space-y-4 mb-4">
        {messages.map((message) => (
          <div key={message.id} className="flex gap-3">
            <div className="font-bold">
              {message.role === 'user' ? 'You:' : 'AI:'}
            </div>
            <div className="flex-1">
              {/* Content streams in automatically */}
              {message.content}

              {/* Show cursor while streaming */}
              {isLoading && message.role === 'assistant' && (
                <span className="animate-pulse">|</span>
              )}
            </div>
          </div>
        ))}
      </div>

      <form onSubmit={handleSubmit} className="flex gap-2">
        <input
          value={input}
          onChange={handleInputChange}
          placeholder="Ask about our documentation..."
          className="flex-1 p-2 border rounded"
        />
        <button
          type="submit"
          disabled={isLoading}
          className="px-4 py-2 bg-blue-500 text-white rounded disabled:opacity-50"
        >
          Send
        </button>
      </form>
    </div>
  );
}

Displaying Retrieved Sources

When using tool-based retrieval, display sources alongside the response:

'use client';

import { useChat, type Message } from 'ai/react';

interface Source {
  title: string;
  content: string;
  source: string;
}

export function RAGChat() {
  const { messages, input, handleInputChange, handleSubmit, isLoading } = useChat();

  // Extract sources from tool calls
  const getSourcesFromMessage = (message: Message): Source[] => {
    if (!message.toolInvocations) return [];

    return message.toolInvocations
      .filter((tool) => tool.toolName === 'searchKnowledgeBase' && tool.state === 'result')
      .flatMap((tool) => tool.result as Source[]);
  };

  return (
    <div className="max-w-4xl mx-auto p-4">
      <div className="space-y-6 mb-4">
        {messages.map((message) => {
          const sources = getSourcesFromMessage(message);

          return (
            <div key={message.id}>
              <div className="flex gap-3">
                <div className="font-bold w-16">
                  {message.role === 'user' ? 'You:' : 'AI:'}
                </div>
                <div className="flex-1">{message.content}</div>
              </div>

              {/* Display sources */}
              {sources.length > 0 && (
                <div className="ml-16 mt-2 p-3 bg-gray-50 rounded-lg">
                  <div className="text-sm font-medium text-gray-500 mb-2">
                    Sources:
                  </div>
                  <div className="space-y-2">
                    {sources.map((source, i) => (
                      <div key={i} className="text-sm">
                        <span className="font-medium">{source.title}</span>
                        <span className="text-gray-500 ml-2">
                          ({source.source})
                        </span>
                      </div>
                    ))}
                  </div>
                </div>
              )}
            </div>
          );
        })}
      </div>

      <form onSubmit={handleSubmit} className="flex gap-2">
        <input
          value={input}
          onChange={handleInputChange}
          placeholder="Ask a question..."
          className="flex-1 p-2 border rounded"
        />
        <button type="submit" disabled={isLoading}>
          {isLoading ? 'Searching...' : 'Ask'}
        </button>
      </form>
    </div>
  );
}

Error Handling

Proper error handling for production applications:

'use client';

import { useChat } from 'ai/react';
import { useState } from 'react';

export function RobustChat() {
  const [retryCount, setRetryCount] = useState(0);

  const {
    messages,
    input,
    handleInputChange,
    handleSubmit,
    isLoading,
    error,
    reload,
    stop,
  } = useChat({
    onError: (error) => {
      console.error('Chat error:', error);
      // Log to your error tracking service
    },
    onFinish: () => {
      setRetryCount(0);
    },
  });

  const handleRetry = () => {
    if (retryCount < 3) {
      setRetryCount((prev) => prev + 1);
      reload();
    }
  };

  return (
    <div className="max-w-2xl mx-auto p-4">
      <div className="space-y-4 mb-4">
        {messages.map((message) => (
          <div key={message.id} className="p-3 rounded-lg bg-gray-100">
            <div className="font-bold mb-1">
              {message.role === 'user' ? 'You' : 'Assistant'}
            </div>
            <div>{message.content}</div>
          </div>
        ))}

        {/* Error state */}
        {error && (
          <div className="p-3 bg-red-100 border border-red-300 rounded-lg">
            <div className="font-bold text-red-700">Error</div>
            <div className="text-red-600 text-sm">{error.message}</div>
            {retryCount < 3 && (
              <button
                onClick={handleRetry}
                className="mt-2 text-sm text-red-700 underline"
              >
                Retry ({3 - retryCount} attempts remaining)
              </button>
            )}
          </div>
        )}
      </div>

      <form onSubmit={handleSubmit} className="flex gap-2">
        <input
          value={input}
          onChange={handleInputChange}
          placeholder="Ask a question..."
          className="flex-1 p-2 border rounded"
          disabled={isLoading}
        />
        {isLoading ? (
          <button
            type="button"
            onClick={stop}
            className="px-4 py-2 bg-red-500 text-white rounded"
          >
            Stop
          </button>
        ) : (
          <button
            type="submit"
            className="px-4 py-2 bg-blue-500 text-white rounded"
          >
            Send
          </button>
        )}
      </form>
    </div>
  );
}

Edge Runtime Deployment

One of AI SDK's killer features is native edge runtime support. This enables sub-100ms cold starts and global distribution.

Edge Runtime Configuration

// app/api/chat/route.ts
import { streamText } from 'ai';
import { openai } from '@ai-sdk/openai';

// Enable edge runtime
export const runtime = 'edge';

export async function POST(req: Request) {
  const { messages } = await req.json();

  const result = streamText({
    model: openai('gpt-4o'),
    messages,
  });

  return result.toDataStreamResponse();
}

That single line (export const runtime = 'edge') deploys your AI endpoint to Vercel's global edge network.

Edge-Compatible Vector Stores

Not all vector databases work on the edge. Here is what does:

Vector Store	Edge Compatible	Notes
Upstash Vector	Yes	REST API, perfect for edge
Pinecone	Yes	REST API
pgvector	No	Requires TCP connections
FAISS	No	Requires Node.js runtime

For edge deployments, prefer HTTP-based vector stores:

// app/api/chat/route.ts
import { streamText } from 'ai';
import { openai } from '@ai-sdk/openai';
import { Index } from '@upstash/vector';

export const runtime = 'edge';

const vectorIndex = new Index({
  url: process.env.UPSTASH_VECTOR_REST_URL!,
  token: process.env.UPSTASH_VECTOR_REST_TOKEN!,
});

export async function POST(req: Request) {
  const { messages } = await req.json();
  const query = messages[messages.length - 1].content;

  // Edge-compatible retrieval
  const embedding = await generateEmbedding(query);
  const results = await vectorIndex.query({
    vector: embedding,
    topK: 5,
    includeMetadata: true,
  });

  const context = results.map((r) => r.metadata?.content).join('\n\n');

  const result = streamText({
    model: openai('gpt-4o'),
    system: `Answer based on this context:\n\n${context}`,
    messages,
  });

  return result.toDataStreamResponse();
}

Hybrid Runtime Strategy

For applications that need both edge speed and server capabilities:

// app/api/chat/route.ts (edge - user-facing)
export const runtime = 'edge';
// Fast streaming responses for end users

// app/api/index/route.ts (node - background processing)
export const runtime = 'nodejs';
// Document indexing with full Node.js capabilities

AI Gateway for Production

For production deployments, you need more than just streaming. AI SDK integrates with gateway solutions for rate limiting, failover, and observability.

Rate Limiting with Upstash

// lib/ai/rate-limit.ts
import { Ratelimit } from '@upstash/ratelimit';
import { Redis } from '@upstash/redis';

const ratelimit = new Ratelimit({
  redis: Redis.fromEnv(),
  limiter: Ratelimit.slidingWindow(10, '1 m'), // 10 requests per minute
  analytics: true,
});

export async function checkRateLimit(identifier: string) {
  const { success, limit, remaining, reset } = await ratelimit.limit(identifier);

  return {
    success,
    headers: {
      'X-RateLimit-Limit': limit.toString(),
      'X-RateLimit-Remaining': remaining.toString(),
      'X-RateLimit-Reset': reset.toString(),
    },
  };
}

// app/api/chat/route.ts
import { streamText } from 'ai';
import { openai } from '@ai-sdk/openai';
import { checkRateLimit } from '@/lib/ai/rate-limit';

export const runtime = 'edge';

export async function POST(req: Request) {
  // Get user identifier (IP, user ID, API key)
  const ip = req.headers.get('x-forwarded-for') || 'anonymous';

  const { success, headers } = await checkRateLimit(ip);

  if (!success) {
    return new Response('Rate limit exceeded', {
      status: 429,
      headers,
    });
  }

  const { messages } = await req.json();

  const result = streamText({
    model: openai('gpt-4o'),
    messages,
  });

  const response = result.toDataStreamResponse();

  // Add rate limit headers
  Object.entries(headers).forEach(([key, value]) => {
    response.headers.set(key, value);
  });

  return response;
}

Provider Failover

Automatically fall back to alternative providers. Note that streamText returns immediately without throwing errors synchronously - errors become part of the stream. This requires a different pattern than traditional try-catch:

// lib/ai/failover.ts
import { openai } from '@ai-sdk/openai';
import { anthropic } from '@ai-sdk/anthropic';
import { generateText, type CoreMessage } from 'ai';

const providers = [
  { name: 'openai', model: openai('gpt-4o') },
  { name: 'anthropic', model: anthropic('claude-sonnet-4-20250514') },
];

// For non-streaming with proper failover
export async function generateWithFailover(
  messages: CoreMessage[],
  system?: string
) {
  let lastError: Error | null = null;

  for (const { name, model } of providers) {
    try {
      console.log(`Attempting ${name}...`);

      // generateText throws on error, unlike streamText
      const result = await generateText({
        model,
        messages,
        system,
      });

      return result;
    } catch (error) {
      console.error(`${name} failed:`, error);
      lastError = error as Error;
    }
  }

  throw lastError || new Error('All providers failed');
}

// For streaming, use the middleware pattern instead
// See: https://sdk.vercel.ai/docs/ai-sdk-core/middleware

Important: For streaming failover, consider using the AI SDK middleware pattern or implementing circuit breaker logic at the infrastructure level. The streamText function doesn't throw synchronously, so wrapping it in try-catch won't catch provider errors.

Observability with Vercel

AI SDK integrates with Vercel's observability features:

// app/api/chat/route.ts
import { streamText } from 'ai';
import { openai } from '@ai-sdk/openai';

export async function POST(req: Request) {
  const { messages } = await req.json();

  const result = streamText({
    model: openai('gpt-4o'),
    messages,
    // Enable experimental telemetry
    experimental_telemetry: {
      isEnabled: true,
      functionId: 'rag-chat',
      metadata: {
        userId: req.headers.get('x-user-id') || 'anonymous',
      },
    },
  });

  return result.toDataStreamResponse();
}

This sends telemetry data to Vercel's dashboard for monitoring latency, token usage, and errors.

Cost Control

AI costs can spiral without proper controls. Here are patterns for managing costs with AI SDK.

Token Counting

Track and limit token usage:

// lib/ai/tokens.ts
import { encoding_for_model } from 'tiktoken';

const encoder = encoding_for_model('gpt-4o');

export function countTokens(text: string): number {
  return encoder.encode(text).length;
}

export function estimateCost(
  inputTokens: number,
  outputTokens: number,
  model: string = 'gpt-4o'
): number {
  const pricing: Record<string, { input: number; output: number }> = {
    'gpt-4o': { input: 0.0025, output: 0.01 }, // per 1K tokens
    'gpt-4o-mini': { input: 0.00015, output: 0.0006 },
    'claude-sonnet-4-20250514': { input: 0.003, output: 0.015 },
  };

  const rates = pricing[model] || pricing['gpt-4o'];
  return (inputTokens * rates.input + outputTokens * rates.output) / 1000;
}

Context Window Management

Prevent context overflow and control costs:

// lib/ai/context.ts
import { type CoreMessage } from 'ai';
import { countTokens } from './tokens';

const MAX_CONTEXT_TOKENS = 8000; // Leave room for response

export function trimMessages(
  messages: CoreMessage[],
  systemPrompt: string
): CoreMessage[] {
  const systemTokens = countTokens(systemPrompt);
  let availableTokens = MAX_CONTEXT_TOKENS - systemTokens;

  // Always keep the last message
  const lastMessage = messages[messages.length - 1];
  const lastMessageTokens = countTokens(
    typeof lastMessage.content === 'string'
      ? lastMessage.content
      : JSON.stringify(lastMessage.content)
  );
  availableTokens -= lastMessageTokens;

  // Add messages from newest to oldest until we run out of tokens
  const trimmedMessages: CoreMessage[] = [lastMessage];

  for (let i = messages.length - 2; i >= 0; i--) {
    const message = messages[i];
    const content = typeof message.content === 'string'
      ? message.content
      : JSON.stringify(message.content);
    const tokens = countTokens(content);

    if (tokens <= availableTokens) {
      trimmedMessages.unshift(message);
      availableTokens -= tokens;
    } else {
      break;
    }
  }

  return trimmedMessages;
}

Usage Tracking

Log usage for billing and analytics:

// app/api/chat/route.ts
import { streamText } from 'ai';
import { openai } from '@ai-sdk/openai';

export async function POST(req: Request) {
  const { messages, userId } = await req.json();

  const result = streamText({
    model: openai('gpt-4o'),
    messages,
    onFinish: async ({ usage }) => {
      // Log to your database
      await logUsage({
        userId,
        model: 'gpt-4o',
        promptTokens: usage.promptTokens,
        completionTokens: usage.completionTokens,
        totalTokens: usage.totalTokens,
        timestamp: new Date(),
      });
    },
  });

  return result.toDataStreamResponse();
}

async function logUsage(data: {
  userId: string;
  model: string;
  promptTokens: number;
  completionTokens: number;
  totalTokens: number;
  timestamp: Date;
}) {
  // Implementation depends on your database
  console.log('Usage:', data);
}

Complete Working Example

Here is a production-ready RAG application using Next.js App Router and Vercel AI SDK:

Project Structure

app/
├── api/
│   ├── chat/
│   │   └── route.ts
│   └── index/
│       └── route.ts
├── components/
│   ├── chat.tsx
│   └── sources.tsx
├── lib/
│   ├── ai/
│   │   ├── retrieval.ts
│   │   └── embedding.ts
│   └── db/
│       └── vectors.ts
├── page.tsx
└── layout.tsx

Vector Store Integration

// lib/db/vectors.ts
import { Index } from '@upstash/vector';
import { embed, embedMany } from 'ai';
import { openai } from '@ai-sdk/openai';

const index = new Index({
  url: process.env.UPSTASH_VECTOR_REST_URL!,
  token: process.env.UPSTASH_VECTOR_REST_TOKEN!,
});

export interface Document {
  id: string;
  content: string;
  metadata: {
    title: string;
    source: string;
    [key: string]: string;
  };
}

export async function searchDocuments(
  query: string,
  limit: number = 5
): Promise<Document[]> {
  const { embedding } = await embed({
    model: openai.embedding('text-embedding-3-small'),
    value: query,
  });

  const results = await index.query({
    vector: embedding,
    topK: limit,
    includeMetadata: true,
  });

  return results.map((result) => ({
    id: result.id,
    content: result.metadata?.content as string,
    metadata: {
      title: result.metadata?.title as string,
      source: result.metadata?.source as string,
    },
  }));
}

export async function indexDocuments(documents: Document[]): Promise<void> {
  const { embeddings } = await embedMany({
    model: openai.embedding('text-embedding-3-small'),
    values: documents.map((d) => d.content),
  });

  await index.upsert(
    documents.map((doc, i) => ({
      id: doc.id,
      vector: embeddings[i],
      metadata: {
        content: doc.content,
        ...doc.metadata,
      },
    }))
  );
}

Chat API Route

// app/api/chat/route.ts
import { streamText, tool } from 'ai';
import { openai } from '@ai-sdk/openai';
import { z } from 'zod';
import { searchDocuments } from '@/lib/db/vectors';

export const runtime = 'edge';

export async function POST(req: Request) {
  const { messages } = await req.json();

  const result = streamText({
    model: openai('gpt-4o'),
    system: `You are a helpful assistant with access to a knowledge base.

When users ask questions:
1. Use the searchKnowledgeBase tool to find relevant information
2. Synthesize the information into a clear, accurate answer
3. Always cite your sources using the source names provided
4. If no relevant information is found, say so honestly

Be concise but thorough. Prefer accuracy over comprehensiveness.`,
    messages,
    tools: {
      searchKnowledgeBase: tool({
        description:
          'Search the knowledge base for relevant documents. Use this for any factual questions.',
        parameters: z.object({
          query: z
            .string()
            .describe('The search query - be specific and descriptive'),
          limit: z
            .number()
            .min(1)
            .max(10)
            .default(5)
            .describe('Number of results to return'),
        }),
        execute: async ({ query, limit }) => {
          const results = await searchDocuments(query, limit);
          return results.map((doc) => ({
            title: doc.metadata.title,
            content: doc.content,
            source: doc.metadata.source,
          }));
        },
      }),
    },
    maxSteps: 3,
  });

  return result.toDataStreamResponse();
}

Document Indexing Route

// app/api/index/route.ts
import { NextResponse } from 'next/server';
import { indexDocuments, type Document } from '@/lib/db/vectors';

// Use Node.js runtime for indexing (more capabilities)
export const runtime = 'nodejs';

export async function POST(req: Request) {
  try {
    const { documents } = (await req.json()) as { documents: Document[] };

    if (!documents || !Array.isArray(documents)) {
      return NextResponse.json(
        { error: 'Invalid documents array' },
        { status: 400 }
      );
    }

    await indexDocuments(documents);

    return NextResponse.json({
      success: true,
      indexed: documents.length,
    });
  } catch (error) {
    console.error('Indexing error:', error);
    return NextResponse.json(
      { error: 'Failed to index documents' },
      { status: 500 }
    );
  }
}

Chat Component

// app/components/chat.tsx
'use client';

import { useChat, type Message } from 'ai/react';
import { useState } from 'react';

interface Source {
  title: string;
  content: string;
  source: string;
}

export function Chat() {
  const {
    messages,
    input,
    handleInputChange,
    handleSubmit,
    isLoading,
    error,
    reload,
    stop,
  } = useChat({
    api: '/api/chat',
  });

  const getSourcesFromMessage = (message: Message): Source[] => {
    if (!message.toolInvocations) return [];

    return message.toolInvocations
      .filter(
        (tool) =>
          tool.toolName === 'searchKnowledgeBase' && tool.state === 'result'
      )
      .flatMap((tool) => tool.result as Source[]);
  };

  return (
    <div className="flex flex-col h-screen max-w-4xl mx-auto">
      {/* Messages */}
      <div className="flex-1 overflow-y-auto p-4 space-y-6">
        {messages.length === 0 && (
          <div className="text-center text-gray-500 mt-8">
            <h2 className="text-xl font-semibold mb-2">
              Welcome to the Knowledge Base
            </h2>
            <p>Ask any question about our documentation.</p>
          </div>
        )}

        {messages.map((message) => {
          const sources = getSourcesFromMessage(message);

          return (
            <div key={message.id} className="space-y-2">
              <div
                className={`flex gap-3 ${
                  message.role === 'user' ? 'justify-end' : 'justify-start'
                }`}
              >
                <div
                  className={`max-w-[80%] p-4 rounded-lg ${
                    message.role === 'user'
                      ? 'bg-blue-500 text-white'
                      : 'bg-gray-100'
                  }`}
                >
                  <div className="whitespace-pre-wrap">{message.content}</div>
                </div>
              </div>

              {/* Sources */}
              {sources.length > 0 && (
                <div className="ml-4 p-3 bg-gray-50 rounded-lg border">
                  <div className="text-sm font-medium text-gray-600 mb-2">
                    Sources used:
                  </div>
                  <div className="space-y-1">
                    {sources.slice(0, 3).map((source, i) => (
                      <div key={i} className="text-sm text-gray-700">
                        <span className="font-medium">{source.title}</span>
                        <span className="text-gray-500 ml-2">
                          - {source.source}
                        </span>
                      </div>
                    ))}
                    {sources.length > 3 && (
                      <div className="text-sm text-gray-500">
                        +{sources.length - 3} more sources
                      </div>
                    )}
                  </div>
                </div>
              )}
            </div>
          );
        })}

        {/* Loading state */}
        {isLoading && (
          <div className="flex gap-3">
            <div className="bg-gray-100 p-4 rounded-lg">
              <div className="flex items-center gap-2">
                <div className="w-2 h-2 bg-gray-400 rounded-full animate-bounce" />
                <div
                  className="w-2 h-2 bg-gray-400 rounded-full animate-bounce"
                  style={{ animationDelay: '0.2s' }}
                />
                <div
                  className="w-2 h-2 bg-gray-400 rounded-full animate-bounce"
                  style={{ animationDelay: '0.4s' }}
                />
              </div>
            </div>
          </div>
        )}

        {/* Error state */}
        {error && (
          <div className="bg-red-50 border border-red-200 p-4 rounded-lg">
            <div className="text-red-700 font-medium">Error</div>
            <div className="text-red-600 text-sm">{error.message}</div>
            <button
              onClick={() => reload()}
              className="mt-2 text-sm text-red-700 underline"
            >
              Try again
            </button>
          </div>
        )}
      </div>

      {/* Input form */}
      <div className="border-t p-4">
        <form onSubmit={handleSubmit} className="flex gap-2">
          <input
            value={input}
            onChange={handleInputChange}
            placeholder="Ask a question about our documentation..."
            className="flex-1 p-3 border rounded-lg focus:outline-none focus:ring-2 focus:ring-blue-500"
            disabled={isLoading}
          />
          {isLoading ? (
            <button
              type="button"
              onClick={stop}
              className="px-6 py-3 bg-red-500 text-white rounded-lg hover:bg-red-600"
            >
              Stop
            </button>
          ) : (
            <button
              type="submit"
              className="px-6 py-3 bg-blue-500 text-white rounded-lg hover:bg-blue-600 disabled:opacity-50"
              disabled={!input.trim()}
            >
              Send
            </button>
          )}
        </form>
      </div>
    </div>
  );
}

Main Page

// app/page.tsx
import { Chat } from './components/chat';

export default function Home() {
  return (
    <main className="min-h-screen bg-white">
      <Chat />
    </main>
  );
}

Layout

// app/layout.tsx
import type { Metadata } from 'next';
import './globals.css';

export const metadata: Metadata = {
  title: 'Knowledge Base Chat',
  description: 'AI-powered documentation assistant',
};

export default function RootLayout({
  children,
}: {
  children: React.ReactNode;
}) {
  return (
    <html lang="en">
      <body>{children}</body>
    </html>
  );
}

Summary

Vercel AI SDK offers a fundamentally different approach to RAG than Python-centric frameworks:

Strengths:

• Streaming-first architecture with ChatGPT-level UX
• First-class Next.js and React integration
• Edge runtime deployment for global performance
• TypeScript throughout with full type safety
• Minimal abstraction, maximum control

Trade-offs:

• Not a complete RAG framework (bring your own retrieval)
• Fewer document processing utilities than LangChain
• JavaScript ecosystem only
• Requires more manual integration work

When to Choose AI SDK:

• Building Next.js applications with AI features
• Streaming UX is a priority
• You want direct control over your RAG architecture
• Edge deployment is important for latency
• Your team is TypeScript-native

Next Steps

This article covered building streaming RAG applications with Vercel AI SDK. Continue with the series:

• Part 8: Advanced Retrieval: Hybrid Search and Reranking - Cross-platform techniques
• Part 1: RAG Foundations - Review core concepts
• Part 2: LangChain RAG - Python implementation comparison

For production deployments, also explore:

• Production RAG Architecture - Scaling patterns
• Vector Database Comparison - Choosing your storage layer

---

This is Part 7 of the "Building RAG Systems: A Platform-by-Platform Guide" series. Previous: AWS Bedrock Knowledge Bases. Next: Advanced Retrieval: Hybrid Search and Reranking.