Claude API 2026-10-21: Build a Migration Scanner in Python

On October 21, 2026 Anthropic shipped the first new Claude API version since 2023-06-01. The version string is 2026-10-21, and every official SDK got a new major release that sends it by default: Python 2.0.0, TypeScript 1.0.0, Go 2.0.0, Java 3.0.0, C# 13.0.0, PHP 1.0.0, Ruby 2.0.0 and the ant CLI 2.0.0. If your dependency file says anthropic>=1 with no upper bound, the next fresh install can pull the new SDK and change your wire behavior without a single line of your code changing.

The headline break is small to describe and nasty in practice: temperature, top_p and top_k now return a 400, even when you send the default value. Add a few request shapes that also 400, a Python-only change that turns MessageParam(...) into a TypeError, and an auth header switch that quietly breaks API gateways, and you have an upgrade that is easy to get wrong in a large codebase.