Claude's API gets its first new version since 2023, and the temperature setting is gone.

On Wednesday, Oct 21, Anthropic released Claude API version 2026-10-21, its first official API version since 2023 (Claude Developers on X). The old version, 2023-06-01, isn't deprecated, so nothing breaks unless you switch. Anthropic says it plans to release versions more often from now on, so developers can deal with breaking changes before a new model requires them.

The biggest change is that the temperature, top_p and top_k settings are removed. On the new version, any request that sends one of them gets an error, even at the default value (migration guide). Requests then use the defaults, including a temperature of 1.0. For Claude 4.7 and newer models, which already accept only the defaults, nothing changes. On older models that still allow other values, such as Claude Sonnet 4.6, a request that lowered temperature to get more consistent answers loses that effect, and no other setting replaces it. Two smaller changes also return errors: putting a document directly in an assistant message, and sending citations on text in a user message.

Anthropic also shipped new major versions of all its SDKs that use the new version by default, including Python 2.0.0, TypeScript 1.0.0 and Go 2.0.0, plus version 2.0.0 of its ant command-line tool (release notes). Those releases bring their own changes. They send your API key in the Authorization header instead of X-Api-Key, they drop the old Text Completions API from several SDKs, and they switch the license from MIT to Apache 2.0. On Amazon Bedrock, the new version works only with Claude 4.7 and later models.

What this means for you: if you build on Claude, upgrading the SDK now means a code update, not just a version bump. If your project doesn't pin its SDK version, the next upgrade can pull in the new major version. Before you upgrade, search your code for temperature, top_p and top_k. If you run Claude traffic through a proxy or gateway that reads X-Api-Key, update it to read the Authorization header. If a request truly depends on a low temperature, Anthropic's advice is to keep that request on 2023-06-01 for now. If you only use Claude through apps like claude.ai or Claude Code, this doesn't affect you.