Anthropic announced on September 30 that Claude Sonnet 4.5 will be retired on November 30, 2026, and recommends Claude Sonnet 5.5 as the replacement for the claude-sonnet-4-5-20250929 model.

Migration is not drop-in. Moving from Sonnet 4.5 means thinking budgets, non-default temperature, top_p and top_k, and assistant prefill all return 400 errors — and thinking is on by default, which changes cost and output. Coming from Sonnet 5, five more things break: disabled thinking, forced tool use, replaying thinking blocks across accounts, the old computer_20251124 tool on the API and Google Cloud, and older models as advisors in the advisor tool.

On price, Sonnet 5.5 costs $2/$10 per million tokens — a third less per token than Sonnet 4.5's $3/$15 — though Anthropic notes the new model uses about 30% more tokens for the same text, so real bills depend on settings. Teams should re-test on real traffic before the November 30 cutoff.