Anthropic released Claude API version 2026-10-21, the first new API version since 2023-06-01. New SDKs and the ant CLI send it by default. Nothing changes for you until you send the new version header or upgrade to the new SDK major version.
Move to it
- With an SDK: install the new major version (Python 2.0.0, TypeScript 1.0.0, C# 13.0.0, Go 2.0.0, Java 3.0.0, PHP 1.0.0, Ruby 2.0.0, ant CLI 2.0.0).
- Without an SDK: send the header anthropic-version: 2026-10-21.
- In Claude Code 2.1.307 or later, run /claude-api api-version to move your Claude API code to the new version.
- Fix the breaking changes below, then test.
What breaks
- temperature, top_p and top_k are removed from the Messages API (also token counting and Batches). Any request that sends them gets a 400 error on every model, even at the default value.
- A document block placed directly in an assistant message returns 400.
- A text block with non-empty citations in a user message (also inside search_result) returns 400.
- Admin API and Enterprise Analytics API renamed some fields, for example roles became role_ids and actor.email became actor.email_address.
- On Amazon Bedrock the new version is available only for Claude 4.7 and later models.
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