Anthropic releases Claude API version 2026-10-21 and major versions of all its SDKs

Archive entry: this covers an event from October 21, 2026, and we published it on November 8, 2026. The details come from sources dated at the time and may have changed since.

On October 21, 2026, Anthropic released Claude API version 2026-10-21, with a migration guide, together with major versions of its client SDKs and the ant CLI. The same date appears in the Claude Code v2.1.307 release notes on GitHub.

What changed in the release

• The SDKs and the CLI sent 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.
• These SDK and CLI versions also contained breaking changes that did not depend on the API version.
• They were licensed under Apache 2.0 instead of the MIT License.
• Claude Code v2.1.307 added /claude-api api-version, a command that moves Claude API code to API version 2026-10-21.

What it means for people building with AI

Every project that uses an Anthropic SDK faced a major-version upgrade with breaking changes and a license change from MIT to Apache 2.0. Since the SDKs and CLI in this release sent the API version by default and also carried breaking changes of their own, teams had to review both the API version migration guide and the SDK changes. The Claude Code command offered a way to move Claude API code to that API version.

Where to read more

Anthropic's Claude Platform release notes cover the platform changes of that date.

Sources

• Release v2.1.307 · anthropics/claude-code · GitHub (Anthropic (GitHub), October 21, 2026, official)
• Claude Platform release notes - Claude Platform Docs (Anthropic (Claude Platform Docs), October 21, 2026, official)

Written with AI assistance, strictly from the sources listed above. Before it went live, automated checks verified its dates, sources and every language edition.