THE ESSENTIALS IN 30 SECONDS

  • Under 2026-10-21, sending temperature, top_p or top_k to the Messages API returns a 400 error, even at default values.
  • New major versions include Python 2.0.0, TypeScript 1.0.0, Go 2.0.0, Java 3.0.0 and C# 13.0.0, licensed under Apache 2.0.
  • Teams can stay on 2023-06-01 by keeping their current SDK or header; no sunset date has been published.

Anthropic published a new version of the Claude API on October 21, the first change to the anthropic-version header since 2023-06-01. It ships together with new major releases of every official SDK and of the ant command-line tool, which send the new version by default.

What breaks in the Messages API

The most visible change concerns sampling. Requests that include temperature, top_p or top_k are now rejected with a 400 error, including when they are set to their default values. The migration guide notes that code which lowered temperature to get more consistent answers loses that effect, and that no replacement setting exists; teams that need it must keep those calls on the old version.

Two message-structure rules also tighten. A document block can no longer sit directly in an assistant message, and user-side text blocks may no longer carry a non-empty citations array. The Admin and Enterprise Analytics APIs rename several fields, for example workspace_id becoming a scope object on API keys.

SDK and CLI changes

According to the release notes, the new versions are 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 and the ant CLI 2.0.0. Some breaking changes apply regardless of API version: API keys are now sent as Authorization: Bearer instead of X-Api-Key, which affects proxies and log filters; a client can hold only one credential; the legacy Text Completions interface is removed from several SDKs; and the Python MessageParam type is split by role. The SDKs also move from the MIT licence to Apache 2.0.

What teams should do

Upgrading is opt-in. Direct API callers choose the version through the header, and SDK users opt in by moving to the new major version. Anthropic advises testing against 2026-10-21 and shipping the SDK upgrade together with code fixes. On Amazon Bedrock, the new version works only with Claude 4.7 and later models.

The guide does not give a retirement date for 2023-06-01, so there is no immediate deadline. The removal of sampling controls is the change most likely to need a product decision rather than a code fix, particularly for applications that relied on low temperature for repeatable output.