Claude API gets its first new version since 2023, and old SDK code will break

Anthropic has released a new Claude API version, 2026-10-21, and new major versions of its SDKs and the ant CLI that use it by default, according to its Claude Platform release notes. The Apache 2.0 license replaces MIT for these releases.

Which SDK versions send the new API version

Until now, every SDK and CLI release sent anthropic-version: 2023-06-01. The new releases send 2026-10-21 instead, including their Amazon Bedrock and Google Cloud clients, according to Anthropic’s migration guide. Three of the packages land on a 2.0.0 release: the Python, Go and Ruby SDKs. The ant CLI is also at 2.0.0. The TypeScript and PHP SDKs both reach 1.0.0, while the Java SDK jumps to 3.0.0 and the C# SDK to 13.0.0.

Anyone calling the API without an SDK must set the anthropic-version header to 2026-10-21 to opt in. Anthropic’s own post on Bluesky called this its first official version release since 2023 and said it plans to release versions more often, so developers can handle breaking changes before a new one is forced on them.

Sampling controls are gone, and other requests now fail

The migration guide lists three breaking changes for the Messages API, token counting and Message Batches. A request that sends temperature, top_p or top_k returns a 400 error, even at the default value. A document block placed directly in an assistant message also returns a 400 error, and so does a text block in a user message that carries a non-empty citations array. Citations on assistant messages that you send back are still accepted.

The Admin API and the Claude Enterprise Analytics API also drop old field names. RBAC groups now use role_ids instead of roles, per-user analytics reports use actor.email_address instead of actor.email, and API keys lose workspace_id in favor of a scope object. An all-workspaces key now has a scope of type organization, where it previously showed a null workspace ID.

The SDK and CLI releases add changes that do not depend on the API version. According to the guide, they send the API key as Authorization: Bearer rather than in the X-Api-Key header, so proxies and gateways that read the old header need updating. The guide says the Claude API accepts either header. Text Completions is removed, and client-side compaction is gone from the tool runner. The Go module path for the CLI now ends in /v2.

What upgrading changes for different teams

For most application developers, the practical risk is sampling. The migration guide says models that already accept only default values, such as Claude 4.7 and later, behave as before. On models that still accept other values, such as Claude Sonnet 4.6, a request that lowered temperature for steadier output loses that effect, and Anthropic says no other setting replaces it. Teams that rely on this can keep those specific requests on 2023-06-01. That fits the direction of Anthropic’s newer models: Claude Opus 5.5 and Sonnet 5.5 already reject several older request shapes, and the same release notes show the Opus 5.5 launch on September 22 returning errors for forced tool_choice types.

Platform teams face a different job. Anyone who runs a gateway, logs headers, or parses Admin API responses has to check for the Authorization change and the renamed fields. Companies that track dependency licenses need to update entries, since earlier versions stay under MIT and only the new releases move to Apache 2.0. Anthropic says no code changes are needed for the license change. For the TypeScript and C# platform packages, the license changes only in their own 1.0.0 releases, and the guide says the Java change covers every com.anthropic artifact.

Pinning a version is the safety valve. Because the API version is chosen by the SDK major release or the header, existing integrations on older SDKs keep sending 2023-06-01 until their owners upgrade. The release notes do not give a retirement date for the old version, so how long it stays supported is not established.

Anthropic adds visibility into bans of end customers

The same notes add enforcement_status and enforcement_actions fields to user profiles, which Anthropic launched on October 20. Platforms and resellers can tag each request with the end customer it serves, so a safety action can apply to one customer instead of a whole organization. With the new enforcement_status[] filter, an operator can list the profiles that currently have an open restriction or ban. A related Admin API webhook feature, in beta, notifies organizations when Anthropic restricts or bans a profile and when it ends. Access is by request through the Anthropic account team.

The release notes do not say how many developers are affected, and the list of SDK breaking changes in the notes is a summary, so teams should read the full migration guide before upgrading. Anthropic’s claims about the benefits of more frequent versioning are its own and have not been independently verified.