The Claude API, which went more than three years without a new version, just had a generational rewrite: on October 21, 2026, Anthropic released version 2026-10-21, its first since 2023-06-01, and the first thing it did was remove the temperature, top_p and top_k sampling parameters outright — any request carrying any of them now returns a 400.

Sampling parameters are removed, not ignored

Per the official versions page, the new version treats the three sampling parameters as removed rather than ignored: sending temperature, top_p or top_k returns a 400 error on every model, even when the value is the former default. Without them, requests run with the default sampling behavior, equivalent to a 2023-06-01 request with temperature 1.0 and no sampling parameters.

For Claude 4.7 and later models nothing changes — they already accepted only the defaults. The real pain sits with models that still accept custom values, such as Claude Sonnet 4.6: code that lowered temperature for steadier output loses that control entirely, Anthropic provides no replacement setting, and those calls must stay pinned to 2023-06-01.

The same version tightens message validation: a document block placed directly in an assistant message returns a 400, and a text block with a non-empty citations array in a user message errors too. On the Admin API side, a batch of fields are renamed: roles becomes role_ids, group_id becomes rbac_group_id, and workspace_id in API key responses becomes a scope object.

Eight official libraries break at once

This release is more than a protocol change. Seven official SDKs — Python, TypeScript, C#, Go, Java, PHP, Ruby — plus the ant CLI jumped to new majors (Python 2.0.0, TypeScript 1.0.0) that send the new version by default. Three knock-on changes stand out:

  • The auth header changed. SDKs now send keys as Authorization: Bearer. The API itself accepts both headers, but any proxy or gateway reading X-Api-Key simply stops seeing the key.
  • The license changed. The new SDKs and CLI are Apache 2.0; earlier versions remain MIT.
  • Text Completions is gone. The legacy completion endpoints were removed from the TypeScript, Go, Java and Ruby SDKs and the CLI.

On Amazon Bedrock the new version supports only Claude 4.7 and later, so older-model workloads need to stay on an older SDK combination. Anthropic also shipped a /claude-api skill that helps Claude Code locate affected call sites.

Versioning gets a cadence

The old 2023-06-01 version is not deprecated; integrations pinned to it keep running. But as AlphaSignal reports, Anthropic plans to release API versions more frequently, putting breaking changes on a defined schedule ahead of model launches — the era of one version every three years is over.

I read this as a statement of intent. In the reasoning-model era, the model decides internally how to think, and the external sampling knobs have long been nominal; rather than leave them to mislead developers, Anthropic took them away. For developers, the genuinely useful move this week is to grep the whole codebase for temperature and check which auth header the gateway reads — the next breaking change will almost certainly not wait another three years.

Reference: Anthropic API versions page