Claude API version 2026-10-21: new SDKs drop temperature, top_p and top_k by default

Anthropic released Claude API version 2026-10-21 on 21 October, its first new API version since 2023-06-01. The old version is not going away: the ClaudeDevs account says “2023-06-01 is not deprecated,” and that Anthropic plans to release versions more often so developers can handle breaking changes before a new model forces them.

The catch is that you may move to the new version without meaning to. The new major SDKs send anthropic-version: 2026-10-21 by default: Python 2.0.0, TypeScript 1.0.0, Go 2.0.0, Java 3.0.0, C# 13.0.0, PHP 1.0.0, Ruby 2.0.0 and the ant CLI 2.0.0. An unpinned dependency bump is a migration.

What breaks in the Messages API

Per the versions page and migration guide, under 2026-10-21:

• temperature, top_p and top_k are removed. Sending any of them returns a 400 error on every model, “even at the parameter’s default value”. The new SDKs no longer have the fields.
• A document block placed directly in an assistant message returns a 400. Send documents in user messages.
• A text block with a non-empty citations array in a user message returns a 400. Citations in assistant messages are still accepted.

The same rules apply to token counting and to each request in a Message Batch created with the new version.

The sampling change matters most for older models. Anthropic says requests now sample with the defaults, including a temperature of 1.0. Claude 4.7 and later models already accept only the defaults, so they behave as before. On a model such as Claude Sonnet 4.6, a request that lowered temperature for steadier output loses that effect, and “no other setting replaces it”. Anthropic’s advice is to drop the parameter or keep that request on 2023-06-01.

Other changes

• Admin and Analytics APIs drop old field names: roles becomes role_ids, group_type becomes group.type, RBAC role name becomes display_name, and API keys lose workspace_id in favour of scope.workspace_id. Code that treated a null workspace_id as the Default Workspace needs a rewrite.
• SDK code changes: in Python, MessageParam becomes a union of per-role types, so calling MessageParam(...) raises a TypeError. Go moves to a /v2 module path.
• License: the new SDK and CLI versions are Apache 2.0 instead of MIT. Earlier versions stay MIT.
• Bedrock: 2026-10-21 works only with Claude 4.7 and later models there. Older Bedrock models should stay on today’s SDK.

How to move safely

1. Pin your current SDK major version now.
2. Search your code for temperature, top_p and top_k, documents in assistant turns, and citations in user turns.
3. Upgrade the SDK and test against the new version before release, as the guide recommends. If you call the API directly, set the header yourself.