Claude API version 2026-10-21: temperature, top_p and top_k now return a 400 error

ClaudeProduct changeConfirmed

Anthropic released API version 2026-10-21.

In this version the sampling parameters temperature, top_p and top_k are removed and return a 400 error.
New versions of the client SDKs and the ant CLI send API version 2026-10-21 by default: Python SDK 2.0.0, TypeScript SDK 1.0.0, C# SDK 13.0.0, Go SDK 2.0.0, Java SDK 3.0.0, PHP SDK 1.0.0, Ruby SDK 2.0.0 and ant CLI 2.0.0.
Anthropic says these SDK and CLI versions also have breaking changes that don't depend on the API version, and are licensed under Apache 2.0 instead of the MIT License.

Who can use it: Developers calling the Claude API with API version 2026-10-21, including through the new SDKs and CLI that send it by default. Earlier API versions return the error only for non-default values on Claude 4.7 and later models and Claude Mythos Preview.
When: Released October 21, 2026. The source gives a date, not an exact time.

Read the official announcement for access details. Availability can depend on your plan, region or workplace settings.