Claude 1M Context Window Migration: What Breaks Today and How to Fix It

If your application sends the anthropic-beta: context-1m-2025-08-07 header to claude-sonnet-4-5 or claude-sonnet-4, your requests started failing or silently truncating as of April 30, 2026. Anthropic retired the 1M context beta for those two models today, and the fix is a one-line model ID change — but there are a few gotchas worth knowing before you deploy.

This guide covers exactly what changed, what errors you'll see, and the fastest path to a stable migration.

What Anthropic Changed and Why

Back in 2025, the 1 million token context window was an experimental feature gated behind a beta header. Developers opted in by adding anthropic-beta: context-1m-2025-08-07 to their API requests alongside older models like claude-sonnet-4 (released May 2025) and claude-sonnet-4-5 (released September 2025).

On March 13, 2026, Anthropic graduated the 1M context window to general availability on claude-sonnet-4-6 and claude-opus-4-6 at standard pricing — no beta header, no long-context premium. That was the signal that the beta era was ending.

The underlying reason for retirement is straightforward: Anthropic wants developers off models that are heading toward deprecation. Sonnet 4 and Opus 4 are scheduled to retire June 15, 2026, so retiring the beta header on them first is part of the staged wind-down.

The Migration: Two Model Swaps

The migration is simple at the API level. You are changing one string.

Option 1: Migrate to Claude Sonnet 4.6 (Recommended for most workloads)

python# Before
response = client.messages.create(
    model="claude-sonnet-4-5-20250929",
    max_tokens=8096,
    extra_headers={"anthropic-beta": "context-1m-2025-08-07"},
    messages=[{"role": "user", "content": very_long_document}]
)

# After — remove the beta header, update model ID
response = client.messages.create(
    model="claude-sonnet-4-6-20260101",
    max_tokens=8096,
    messages=[{"role": "user", "content": very_long_document}]
)

The context-1m-2025-08-07 header is simply gone. Sonnet 4.6 accepts up to 1,000,000 tokens with no special configuration.

Option 2: Migrate to Claude Opus 4.6 (For highest-quality long-context work)

pythonresponse = client.messages.create(
    model="claude-opus-4-6-20260101",
    max_tokens=8096,
    messages=[{"role": "user", "content": very_long_document}]
)

Opus 4.6 runs slower and costs more than Sonnet 4.6, but delivers meaningfully better reasoning on tasks like legal document review, codebase-wide refactoring, or multi-document synthesis. If your use case justifies the cost, this is the upgrade path.

TypeScript / Node.js version

typescriptimport Anthropic from "@anthropic-ai/sdk";

const client = new Anthropic();

const response = await client.messages.create({
  model: "claude-sonnet-4-6-20260101", // was claude-sonnet-4-5-20250929
  max_tokens: 8096,
  // no anthropic-beta header needed
  messages: [
    {
      role: "user",
      content: veryLongDocument,
    },
  ],
});

Four Things to Test After Migrating

Swapping the model ID is not a purely mechanical change. Sonnet 4.6 and Opus 4.6 are different models than 4.5, and they can behave differently on the same prompt. Before you ship:

Why You Should Not Just Remove the Header and Stay on Sonnet 4.5

Some developers may be tempted to simply drop the beta header and stay on claude-sonnet-4-5-20250929. That works only if your prompts are consistently under 200K tokens. If you are already using the 1M window in production, that means staying on an old model that:

• Retires June 15, 2026 alongside Sonnet 4 and Opus 4
• Has no path to 1M tokens after today
• Will be replaced by an alias that routes to a newer model after retirement, potentially breaking your expected behavior

Migrating now to Sonnet 4.6 or Opus 4.6 gives you a GA feature at standard pricing on models that are the current generation.

The Bigger Picture: Long Context is Now a Standard Feature

The retirement of the beta header reflects a broader shift. When Anthropic first shipped the 1M context window, it was genuinely experimental — a technical proof of concept that not all use cases warranted. By promoting it to GA in March 2026 on the 4.6 generation, Anthropic is signaling that long context is now table stakes, not a premium add-on.

The practical implication: if you are building on the Claude API today, you should be on Sonnet 4.6 or Opus 4.6 as your baseline. The 4.6 generation also introduced improved instruction following, better tool use, and (in the case of Opus 4.6) the foundation for the extended thinking capabilities that Opus 4.7 builds on.

For context on what else changed in this generation, Anthropic's migration guide has a full model comparison table.

Key Takeaways

• The anthropic-beta: context-1m-2025-08-07 header stopped working for Sonnet 4.5 and Sonnet 4 on April 30, 2026
• Requests over 200K tokens on those models now return a 400 invalid_request_error
• The fix is changing your model ID to claude-sonnet-4-6-20260101 or claude-opus-4-6-20260101 and removing the beta header
• 1M context is now GA on the 4.6 generation at standard pricing — no header, no premium
• Both Sonnet 4 and Sonnet 4.5 retire June 15, 2026, so migrating now avoids a second forced migration in six weeks

Next Steps for Your Claude Integration

If you are actively building on the Claude API and want to stay current on deprecations, model releases, and platform changes, the AI for Anything certification prep tracks Claude Certified Architect (CCA-F) exam topics — including the API lifecycle and model selection criteria that come up in the exam.

For a deeper look at how the 1M context window works and when to use it versus chunked RAG, see our guide on Claude Sonnet 4.6 and the 1M context window.