New featureReleased 21 October 2026 · 4 min read
Your next Claude SDK update is a breaking change.
Anthropic has released Claude API version 2026-10-21, with new client SDKs and a new ant CLI that send it by default. The new SDK and CLI versions also carry breaking changes of their own and move from the MIT License to Apache 2.0.
At a glance
- API version
- 2026-10-21
- New SDKs and ant CLI
- Send 2026-10-21 by default
- Breaking changes
- Yes, in SDKs and CLI
- License
- MIT to Apache 2.0
- Migration guide
- Published by Anthropic
01 · What changed
What Anthropic shipped.
What changed: the Claude API has a new version, and the official SDKs now request it unless told otherwise. A team that moves an SDK to the new major version moves to API version 2026-10-21 by default. It also takes on SDK changes that apply whatever API version is sent. Anthropic publishes a migration guide for the API version and a separate page for the SDK and CLI changes, so this is planned work for whoever owns your Claude integrations.
- 1
API version 2026-10-21
A new version of the Claude API. Anthropic's guide, Migrating to API version 2026-10-21, shows how to upgrade.
- 2
Python SDK 2.0.0
Sends API version 2026-10-21 by default.
- 3
TypeScript SDK 1.0.0
Sends API version 2026-10-21 by default.
- 4
C#, Go and Java SDKs
C# SDK 13.0.0, Go SDK 2.0.0 and Java SDK 3.0.0 send API version 2026-10-21 by default.
- 5
PHP and Ruby SDKs
PHP SDK 1.0.0 and Ruby SDK 2.0.0 send API version 2026-10-21 by default.
- 6
ant CLI 2.0.0
The command-line tool also sends API version 2026-10-21 by default.
- 7
Breaking SDK changes
Anthropic says the new SDK and CLI versions have breaking changes that don't depend on the API version. They are listed on its SDK and CLI changes page.
- 8
Apache 2.0 license
The new SDK and CLI versions are licensed under Apache 2.0 instead of the MIT License.
02 · What it means
What it means for your seat.
This release concerns code that calls the Claude API: the internal tools that summarise a data room, triage inbound email or draft a weekly status report. The work sits with the people who maintain those tools, with a license question for whoever reviews open-source software at your firm.
Leadership
This is maintenance work for the people who build on Claude. Ask who owns your Claude integrations and when they plan the upgrade. Expect a testing step before anything changes in production.
IT and engineering
Pin your current SDK versions so a routine dependency update does not pull in the new major versions untested. Read Anthropic's migration guide and the SDK and CLI changes page, then upgrade one integration at a time, starting with a low-risk tool such as a weekly status report before an IC memo or credit agreement workflow.
Operations
If a tool you rely on is built on the Claude API, such as inbound email triage or a board pack drafting helper, ask its owner when it will move to the new SDK. Plan a check of its output after the change, since the new SDKs carry breaking changes.
Risk and compliance
At these versions the Claude SDKs and ant CLI move from the MIT License to Apache 2.0. If your firm keeps a register of open-source licenses in production software, update it and route the change through the review your policy requires. Treat the SDK upgrade as a production change with the usual change record.
03 · This week
What to do this week.
0 of 3 done
Sources
Facts and dates are Anthropic’s own. What it means and what to do are our view, from the firms we work with. Drafted with Claude from these sources, checked against them figure by figure, and published 22 October 2026.
The short answers.
What is Claude API version 2026-10-21?
It is a new version of the Claude API released by Anthropic. Anthropic's guide, Migrating to API version 2026-10-21, shows how to upgrade.
Which Claude SDK versions use API version 2026-10-21 by default?
Python SDK 2.0.0, TypeScript SDK 1.0.0, C# SDK 13.0.0, Go SDK 2.0.0, Java SDK 3.0.0, PHP SDK 1.0.0, Ruby SDK 2.0.0 and ant CLI 2.0.0 all send API version 2026-10-21 by default.
Do the new Claude SDKs have breaking changes?
Yes. Anthropic says the new SDK and CLI versions have breaking changes that don't depend on the API version, and lists them on its SDK and CLI changes page.
What license do the Claude SDKs use now?
The new SDK and ant CLI versions released alongside API version 2026-10-21 are licensed under Apache 2.0 instead of the MIT License.
Read next
Newsletter
Keep up with every release.
One email when something ships or retires that matters to a company: what changed, what it means, the date and what to do. Unsubscribe any time.
Want this working inside your firm?
We run Claude workshops on your own documents and stay on as your teams adopt each release.