Claude API version 2026-10-21 removes temperature. What breaks, and how to upgrade

What changed on October 21

Anthropic released Claude API version 2026-10-21 on October 21, 2026, according to its release notes. It is the first new version since 2023-06-01: the version history lists 2023-01-01, 2023-06-01, and now 2026-10-21. Every request names its version in the anthropic-version header, and the SDKs set it for you.

The same day, Anthropic released a new major version of each client SDK and of its ant CLI. Its SDK overview says each major version sends one API version by default, and “a new major version comes with each new API version.”

SDK	Package	First version on 2026-10-21	Latest on October 22
Python	anthropic	2.0.0	2.1.0
TypeScript	@anthropic-ai/sdk	1.0.0	1.1.0
C#	Anthropic	13.0.0	13.1.0
Go	anthropic-sdk-go/v2	2.0.0	2.1.0
Java	com.anthropic:anthropic-java	3.0.0	3.1.0
PHP	anthropic-ai/sdk	1.0.0	1.1.0
Ruby	anthropic	2.0.0	2.1.0
ant CLI	anthropic-cli	2.0.0	2.1.0

Earlier versions send 2023-06-01. The new ones are licensed under Apache 2.0 instead of MIT; earlier versions stay under MIT.

The new API version works on the Claude API, Claude Platform on AWS, both Google Cloud offerings and Microsoft Foundry. On Amazon Bedrock it works only with Claude 4.7 and later models.

What breaks in the API

Three changes to the Messages API, which also apply to token counting and to Message Batches:

• temperature, top_p and top_k are removed. A request that sends any of them returns a 400 error, “even at the parameter’s default value.”
• Documents go in user messages. A document block placed directly in an assistant message returns a 400 error.
• Citations stay out of user messages. A text block with a non-empty citations array in a user message returns a 400 error.

The Admin API and the Claude Enterprise Analytics API stop returning the old names of renamed fields. RBAC groups return role_ids instead of roles, usage and cost reports return actor.email_address instead of actor.email, and API keys return scope instead of workspace_id. The migration guide has the full list.

What it means for temperature

On current models, less than it sounds. Under 2023-06-01, Claude 4.7 and later models already returned a 400 error for a non-default temperature, top_p or top_k, according to Anthropic’s deprecations page. Those models “sample as before.” What’s new is that even a default value, such as temperature: 1.0, is now an error, so the parameter has to come out of the request.

It matters on older models that still accept other values, such as Claude Sonnet 4.6, which Anthropic still offers as a legacy model. A request there that lowered temperature for more consistent output “loses that effect. No other setting replaces it.” Anthropic’s recommended replacement is prompting. If a request depends on a low temperature, the guide says to keep that request on 2023-06-01.

What breaks in the SDKs

The new SDK versions bring changes of their own:

• The API key moves header. The SDKs and the CLI send it as Authorization: Bearer <key> instead of X-Api-Key. The API accepts either, but a proxy, gateway, log filter or test stub that reads X-Api-Key has to change.
• A client holds one credential. Passing both an API key and an auth token now raises an error, a ValueError in Python and a TypeError in TypeScript, where earlier versions sent both.
• Messages have a type for each role. MessageParam is now a union of user, assistant and system types. Messages written as plain dicts or object literals mostly need no change; code that constructs or subclasses MessageParam does.
• Text Completions is gone from the TypeScript, Go, Java and Ruby SDKs and from the CLI.
• Go needs Go 1.25 or later and the /v2 import path. The CLI drops --api-key and --auth-token; use ANTHROPIC_API_KEY, --api-key-stdin or ant auth login.

Who has to act now

Nothing forces a move today. Requests that send 2023-06-01 keep working, and so do SDKs below the new major versions. Anthropic “recommends using the latest API version whenever possible,” but hasn’t announced an end date for 2023-06-01.

You do have something to do if:

1. Your dependency isn’t pinned. pip install anthropic now installs 2.x, and a requirement such as anthropic>=1.0 picks it up on the next fresh install. Pin below the new major, anthropic<2 in Python, until you’ve made the changes.
2. You rely on a low temperature on an older model. Keep those requests on 2023-06-01, or move them to a current model and steer it with the prompt.
3. You call older Claude models on Bedrock. Stay on your current SDK version for those calls, or switch them to Claude 4.7 or later first.
4. A gateway in front of the API reads X-Api-Key. Change it to read Authorization before you upgrade the SDK.
5. You use a TypeScript platform package, such as @anthropic-ai/bedrock-sdk or @anthropic-ai/vertex-sdk. Move it to its v1 in the same upgrade, or your package manager installs a second, older copy of the SDK for it.

How to upgrade

Anthropic’s migration guide gives three steps:

1. Switch to 2026-10-21. Upgrade to the SDK version in the table above, or, if you call the API without an SDK, set the anthropic-version header to 2026-10-21.
2. Update your code for each breaking change that affects you, in the API and in the SDK. Release the upgrade together with the fixes.
3. Test against 2026-10-21 before you release.

If you use Claude Code, version 2.1.307, released October 21, added /claude-api api-version “to move Claude API code to API version 2026-10-21,” in its changelog’s words. Review what it changes like any other diff, and decide for yourself about requests that set a temperature on an older model: dropping the parameter changes their output.

We’ll update this memo if Anthropic sets a date for retiring 2023-06-01.