Sonnet 4.5 has a retirement date. Put the migration on the calendar.

Anthropic’s 30 September notice schedules claude-sonnet-4-5-20250929 for retirement on the Claude API on 30 November 2026. The recommended replacement is Sonnet 5.5. Requests still pinned to the retiring ID need a migration plan. ¹

Start with the actual model IDs in production configuration, fallback routes and eval jobs. A default changed in an editor leaves an explicitly pinned API job untouched. Keep the old baseline while testing the replacement; [issue #41](/issues/2026-09-29/) covered Sonnet 5.5’s migration breaks.

Assign an owner and a rollout date, then run the same acceptance tasks against both models. Check tool selection and thinking settings as well as answer quality. A retirement deadline gives you time to measure; it does not establish compatibility.