Anthropic closes 1M context beta for Sonnet 4.5 and Sonnet 4 — migration to 4.6 required

Anthropic announced on April 30, 2026, in platform release notes the closure of beta access to the million-token context window for older models.

Which models are affected and what exactly changes?

Affected models are Claude Sonnet 4.5 and Claude Sonnet 4 — both used the beta header context-1m-2025-08-07 to activate the extended context window of one million tokens.

From April 30 onward, requests sending that header and exceeding 200,000 tokens return an error. The standard limit of 200k tokens remains functional and is not affected.

How to migrate to a supported solution?

Migration is straightforward: simply change the model name in your API call to claude-sonnet-4-6 or claude-opus-4-6. On these models, the million-token context window (1M context window — the ability to process up to one million tokens in a single request) is available as a generally available feature without any beta headers.

Teams using automation or infrastructure-as-code should check all configuration files and environment variables where the model name is hardcoded.

Context: routine retirement of beta features

Anthropic regularly closes beta access on older models when a new generation delivers the same capabilities in a stable form. In this case, the 1M context window moves from an experimental phase on Sonnet 4.5 to production support on Sonnet 4.6 — a standard feature maturity cycle.

For developers just building systems with long contexts, Sonnet 4.6 is now the recommended starting point.