A quiet but consequential change is coming to the Claude API at the end of this month. Anthropic is retiring the 1 million token context window beta for Claude Sonnet 4 and Claude Sonnet 4.5 on April 30, 2026. After that date, the context-1m-2025-08-07 beta header stops working entirely, and any request that exceeds the standard 200,000 token limit returns an error, not a warning.
What Changed
The 1M context window has been available in beta for Sonnet 4 and Sonnet 4.5 via the context-1m-2025-08-07 header. That beta is closing. The models themselves aren't going anywhere, Sonnet 4 and 4.5 remain available. What dies on April 30 is the extended context capability on those specific models.
The migration path is straightforward: move to Claude Sonnet 4.6 or Claude Opus 4.6. Both support 1 million token context windows at standard pricing with no beta header required, it's part of their standard capability set, not a beta feature. No header changes, no special configuration.
The Bigger Picture
This is part of Anthropic's ongoing cleanup of beta features that have graduated to standard offerings on newer models. The pattern across the 4.6 generation is consolidation: features that required beta headers (extended context, extended thinking, effort parameters) are either being deprecated or promoted to GA. For developers on older model versions, it means the migration pressure is building — not just for context windows, but across the API surface.
What This Means for You
If you have the context-1m-2025-08-07 header anywhere in your codebase and you're calling Sonnet 4 or Sonnet 4.5, you have 21 days to migrate. The swap is a model string change — replace claude-sonnet-4-5 or claude-sonnet-4-20250514 with claude-sonnet-4-6. Test in a development environment first, particularly for any prompts that depend on long context behaviour, as Sonnet 4.6 may handle edge cases differently.
If you're not sure whether you're using the header, search your codebase for context-1m before April 30.
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