Claude API version 2026-10-21 removes temperature, top_p and top_k

Anthropic released Claude API version 2026-10-21, its first since 2023, and new SDK majors default to it, so an unpinned upgrade can break requests.

Anthropic has released Claude API version 2026-10-21, its first new API version since 2023-06-01, and new major versions of its SDKs and the ant CLI send it by default, according to the ClaudeDevs announcement. The change most code will hit: on the new version, a request that sends temperature, top_p or top_k returns a 400 error "even at the parameter's default value," per Anthropic's migration guide. If a dependency bump crosses a major version, you switch over without touching a header.

Anthropic says 2023-06-01 is not deprecated. The guide says requests without the three parameters sample with the defaults, including a temperature of 1.0. Claude 4.7 and later models sample as before, but on a model like Claude Sonnet 4.6, a request that lowered temperature for steadier output loses that effect, and "no other setting replaces it." Those requests can stay on 2023-06-01.

Other breaks, from the guide: document blocks directly in assistant messages and non-empty citations on user text blocks return 400s; Admin and Enterprise Analytics API fields are renamed, with workspace_id on API keys replaced by scope; and the SDKs now send your key as Authorization: Bearer instead of X-Api-Key. The API accepts either header, but any proxy or gateway that reads X-Api-Key needs changing. Developers Digest lists the first versions on the new API, including Python 2.0.0 and TypeScript 1.0.0, and notes the new SDKs move from MIT to Apache 2.0. AlphaSignal reports that on Amazon Bedrock the new version works only with Claude 4.7 and later.

What to do: pin your SDK major until you've searched for the three sampling parameters, checked proxy header handling, and tested against 2026-10-21. Anthropic also points to a /claude-api skill in Claude Code for the update. Its exact scope isn't documented.

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