Anthropic's New Claude API Version Drops Temperature

Anthropic has released a new version of the Claude API, 2026-10-21, under which any request that sends the sampling parameters temperature, top_p or top_k fails with a 400 error, even when the value matches the default. The company’s release notes for 21 October say new versions of its seven client SDKs and its ant command-line tool now send the new version by default, and are licensed under Apache 2.0 instead of the MIT License.

For developers, the upgrade is a choice with a cost. Staying on an older SDK keeps the old behaviour, but moving to the new major versions means rewriting any code that tunes sampling, sends documents or citations in certain places, or reads renamed admin fields. Anthropic’s migration guide describes the move as one from API version 2023-06-01 to 2026-10-21.

The new SDKs switch versions automatically

Anthropic says the SDK versions that send 2026-10-21 by default are Python 2.0.0, TypeScript 1.0.0, C# 13.0.0, Go 2.0.0, Java 3.0.0, PHP 1.0.0 and Ruby 2.0.0, plus version 2.0.0 of the ant CLI. According to the guide, “If you use an SDK, your requests use the API version that was current when your SDK’s major version was released”, so the change arrives with a package upgrade rather than a setting.

The new SDK and CLI releases also carry breaking changes that have nothing to do with the API version. The guide says they now send API keys as “Authorization: Bearer” instead of in the X-Api-Key header, which matters for anyone running a proxy or gateway that reads that header; the API itself still accepts either. The legacy Text Completions API has been removed from the TypeScript, Go, Java and Ruby SDKs and from the CLI. In Python, MessageParam becomes a union of separate user, assistant and system types.

Four request patterns now return errors

The guide lists the changes to Messages API calls, token counting and Message Batches. A document block placed directly in an assistant message now returns a 400 error, as does a text block in a user message that carries a non-empty citations array. Documents nested inside other blocks, such as a web fetch result, and citations on Claude’s own earlier replies are still accepted.

The sampling change is the widest. Without the three parameters, requests sample at their defaults, including a temperature of 1.0. Anthropic says Claude 4.7 and later models already accept only the defaults and “sample as before”. The same rule arrived model by model before this: its notes for Claude Haiku 5.5 warned that temperature, top_p and top_k “can each return a 400 error”.

Responses from the Admin API and the Claude Enterprise Analytics API also drop the old names of renamed fields, and an API key’s workspace_id is replaced by a scope object.

On Amazon Bedrock, 2026-10-21 is available only for Claude 4.7 and later models. Anyone calling Opus 4.6 or earlier there is told to stay on their current SDK version until they switch models.

Fixed sampling becomes a platform rule, not a model quirk

The practical effect falls on teams still using older models that allow sampling to be tuned. On those models, the guide says, using Claude Sonnet 4.6 as its example, “a request that lowered temperature for more consistent output loses that effect. No other setting replaces it.” Its only alternatives are to stop sending the parameter or to keep that request on 2023-06-01. That turns a design choice already made for newer models into a condition of using the current SDKs at all, and Anthropic’s notes give no reason for it.

The guide also gives no date for retiring 2023-06-01, which leaves developers free to hold back. But holding back has its own cost, because the new major versions are where Anthropic is shipping its SDK changes. Teams that lowered temperature for what the guide calls “more consistent output” will need to test whether their results change at the default of 1.0 before they upgrade.

The licence switch from MIT to Apache 2.0 applies to every new SDK and CLI release, and Anthropic states it without explanation.

Platforms get a view of enforcement

The same release adds enforcement_status and enforcement_actions fields to user profiles, so a platform can see whether Anthropic has restricted or banned one of its end customers, along with a filter to list profiles with an open restriction or ban. User profiles arrived a day earlier, on 20 October, so that “a safety enforcement action can then apply to one end customer instead of your whole organization”, according to the notes. It is the latest in a run of platform changes this month, after Opus 5.5 fast mode left research preview and Anthropic described its Claude Duet agent.

Developers on Bedrock with older models have the clearest instruction of all: the new version is closed to them until they move to Claude 4.7 or later.