Short answer: Anthropic released API version 2026-10-21 on October 21, 2026. Requests on the new version that send temperature , top_p or top_k , a document in an assistant message, or citations in a user message return 400. New SDK majors send the new version by default and pass your key as Authorization: Bearer . Requests that stay on 2023-06-01 keep working.

What changed on October 21?

Anthropic released API version 2026-10-21 on October 21, 2026, the first new version since 2023-06-01 . Eight official SDK and CLI libraries released new major versions simultaneously to send 2026-10-21 by default. These releases include 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 CLI 2.0.0. The new versions are licensed under Apache 2.0 instead of MIT.

Direct API callers opt in explicitly by passing the header anthropic-version: 2026-10-21 . Older SDK major releases remain functional and continue pinning requests to the 2023-06-01 version.…