Anthropic's Claude 3.5 Sonnet is widely regarded as the best model for complex coding tasks. Here's a complete guide to integrating it into your Next.js 14+ application using the App Router, streaming responses, and keeping your costs under control.
Setting Up the Anthropic SDK
Start by installing the official Anthropic TypeScript SDK and adding your API key to your environment file. The SDK handles rate limiting, retries, and streaming out of the box — don't roll your own fetch logic.
Create a server-side API route at app/api/claude/route.ts. Because Claude responses can take several seconds for complex tasks, you should always use streaming. Next.js App Router supports this natively through ReadableStream and the StreamingTextResponse helper from the Vercel AI SDK.
Managing SSE Streaming
Server-Sent Events (SSE) allow the server to push tokens to the client as they are generated, giving users the familiar "typewriter" effect. The Anthropic SDK provides a messages.stream() method that wraps SSE complexity. Pipe this to a ReadableStream in your route handler, and on the client use the useChat hook from the AI SDK to manage state and rendering automatically.
"Streaming is not optional for production AI apps — users will bounce if they stare at a loading spinner for 8 seconds."
— Product UX research, AlgoBoost 2026
Optimizing Token Costs
Claude 3.5 Sonnet is priced at $3/million input tokens and $15/million output tokens. For a high-volume application, this adds up fast. Key optimization strategies: cache system prompts using Anthropic's prompt caching beta (saves up to 90% on repeated system prompt tokens), truncate conversation history to the last N messages, and use claude-haiku for simple classification or routing tasks before escalating to Sonnet.