Anthropic has released Claude API version 2026-10-21, its first official version update since 2023. The new release removes the temperature, top_p, and top_k sampling parameters from the Messages API. Passing documents in an assistant turn or citations in a user turn now returns a 400 error, and several deprecated Admin and Enterprise Analytics API fields have been replaced. The company confirmed that the legacy 2023-06-01 version remains supported and is not deprecated. Anthropic plans to ship API versions more frequently going forward, giving developers time to handle incompatible changes before new models require them. The Anthropic SDK and ant CLI have simultaneously rolled out new major versions defaulting to 2026-10-21, complete with migration guides.