Anthropic releases Claude API version 2026-10-21, its first new version since 2023

On 21 October 2026 Anthropic released version 2026-10-21 of the Claude API. According to ClaudeDevs it is the first official version release since 2023, and Anthropic plans to publish versions more often so that developers can handle breaking changes before a new model requires them. Version 2023-06-01 is not deprecated. New major versions of the Anthropic SDKs and the ant CLI came out the same day and send 2026-10-21 by default; the Python package moves to 2.0.0. Integrations without an SDK set the anthropic-version header.

The migration guide lists the breaking changes for the Messages API, token counting and Message Batches. A request that sends temperature, top_p or top_k now returns a 400 error, even at the default value, and sampling then uses the defaults. Document blocks placed directly in assistant messages and citations on text blocks in user messages are rejected too. Admin API and Claude Enterprise Analytics API responses drop the old names of renamed fields, and API keys report a scope object instead of workspace_id.

On Amazon Bedrock the new version works only with Claude 4.7 and later models. The new SDKs also send API keys as Authorization: Bearer, and several of them drop the legacy Text Completions API.