Claude 4 to Sonnet 4.6: One-Line Migration Guide

Claude 4 Sonnet is shutting down June 15. If you're using it, you need to migrate — and the fix is a single line of code. Claude Sonnet 4.6 is Anthropic's direct replacement: same price, same quality, and a 5x larger context window.

This guide shows you exactly what to change, what it costs, and how to test before the deadline.

The Code Change: One Line

Here's the complete diff — change the model ID and nothing else:

// Before (Claude 4 Sonnet — will break June 15)
const response = await anthropic.messages.create({
  model: "claude-sonnet-4-20250514",  // ← CHANGE THIS
  max_tokens: 4096,
  messages: [{ role: "user", content: "Hello" }]
});

// After (Claude Sonnet 4.6 — works immediately)
const response = await anthropic.messages.create({
  model: "claude-sonnet-4-6-20260101",  // ← NEW MODEL ID
  max_tokens: 4096,
  messages: [{ role: "user", content: "Hello" }]
});

That's it. The API endpoint, request parameters, response format, streaming behavior, and tool use are all identical. No other code changes needed.

The Cost: Same Price, Better Value

Claude Sonnet 4.6 costs exactly the same as Claude 4 Sonnet. But you get more for your money:

Model	Input (per 1M tokens)	Output (per 1M tokens)	Context Window
Claude 4 Sonnet (deprecated)	$3.00	$15.00	200K
Claude Sonnet 4.6 (replacement)	$3.00	$15.00	1M

You pay the same price but get 5x the context window (1M vs 200K tokens). That means you can process longer documents, bigger codebases, and more complex conversations without hitting limits.

Model ID Reference

Deprecated Model	Replacement	New Model ID
Claude 4 Sonnet	Claude Sonnet 4.6	claude-sonnet-4-6-20260101
Claude 4 Opus	Claude Opus 4.8	claude-opus-4-8-20260101

Step-by-Step Migration

1. Search your codebase for claude-sonnet-4-20250514 or any reference to Claude 4 Sonnet model IDs. Use grep -r "claude-sonnet-4" . to find all occurrences.
2. Update the model ID to claude-sonnet-4-6-20260101 in each file.
3. Run your test suite to verify everything works. The response format is identical, but test edge cases like long context, streaming, and tool use.
4. Deploy to production before June 15. After that date, the old model ID returns HTTP 404 immediately.

Why Sonnet 4.6 Is an Upgrade

• 5x larger context: 1M tokens vs 200K. Process entire codebases, long documents, or extended conversations without chunking.
• Same price: $3/$15 per 1M tokens — identical to Claude 4 Sonnet.
• Same quality: Matches Claude 4 Sonnet on all benchmarks, with improvements in instruction-following and code generation.
• Identical API: Drop-in replacement. Change the model ID and everything else stays the same.

Beyond Sonnet 4.6: Cheaper Alternatives

If you want to cut costs further, consider these alternatives to Claude Sonnet 4.6:

Model	Input/Output	Savings vs Sonnet	Best For
GPT-5 (OpenAI)	$1.25/$10	58% cheaper input	General purpose, strong coding
Gemini 2.0 Flash (Google)	$0.10/$0.40	97% cheaper	Budget workloads, high volume
DeepSeek V4 Pro	$0.435/$0.87	85% cheaper	Budget premium quality
Llama 4 Maverick	$0.27/$0.85	91% cheaper	Cheapest 1M context
GPT-5 mini (OpenAI)	$0.25/$2	92% cheaper	High-volume, cost-sensitive

Frequently Asked Questions

What happens if I don't migrate by June 15?

All API calls using Claude 4 Sonnet model IDs will return HTTP 404 errors immediately. There is no grace period, no partial shutdown, and no extensions. Your application will break.

Does Sonnet 4.6 produce the same outputs as Claude 4 Sonnet?

For most use cases, yes. Claude Sonnet 4.6 matches or exceeds Claude 4 Sonnet on all benchmarks. You may see slightly different outputs due to model improvements, but quality will be equal or better.

Can I use Sonnet 4.6 as a drop-in replacement?

Yes. Change the model ID and everything else stays the same. The API, parameters, response format, streaming, and tool use are all identical.

What about Claude 4 Opus users?

Claude 4 Opus is also deprecated on June 15. Its replacement is Claude Opus 4.8 ($5/$25 per 1M tokens, 1M context). See our Opus 4.8 migration guide.

Is there a migration cost?

No. Changing the model ID is free. In fact, you get more value — Claude Sonnet 4.6 has a 5x larger context window at the same price.

Will my existing API key work?

Yes. If you have an Anthropic API key, it works with Claude Sonnet 4.6. No new API key or account needed.