Claude API version 2026-10-21 drops temperature, top_p and top_k

Anthropic released Claude API version 2026-10-21 on October 21, 2026, together with new SDK and CLI majors that send it by default. On that version, a request that sets temperature, top_p or top_k returns a 400 error on every model.

What changed

• temperature, top_p and top_k are removed from the Messages API. Sending any of them returns a 400 error.
• A document block placed directly in an assistant message returns a 400 error, as does a text block with a non-empty citations array in a user message.
• These SDK versions send 2026-10-21 by default: Python 2.0.0, TypeScript 1.0.0, C# 13.0.0, Go 2.0.0, Java 3.0.0, PHP 1.0.0, Ruby 2.0.0 and the ant CLI 2.0.0. They also carry breaking changes unrelated to the API version and move from the MIT License to Apache 2.0.
• Admin API responses rename fields, for example roles becomes role_ids and API keys return scope.workspace_id instead of workspace_id.
• The version is available on the Claude API, Claude Platform on AWS, Claude Models on Google Cloud and Microsoft Foundry. On Amazon Bedrock it covers Claude 4.7 and later models.

What it means

An SDK upgrade is now a code change. Wrappers, retry helpers and framework defaults that send temperature on every call will start failing. Anthropic's versioning policy keeps existing input parameters within a given version, so calls pinned to 2023-06-01 are not affected by these removals.

What to do

# Before upgrading the Anthropic SDK, find sampling parameters
grep -rnE "temperature|top_p|top_k" src/

# Remove them from Messages and Message Batch requests,
# or stay on API version 2023-06-01 until you do.

Also in Anthropic's release notes: on October 30 it deprecated fast mode for Claude Opus 4.8, with removal on November 30, 2026. After that, claude-opus-4-8 requests with speed: "fast" return an error.

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Sources

1. Anthropic: Claude Platform release notes
2. Anthropic: API versions