Claude Sonnet 4.5 (claude-sonnet-4-5-20250929) retires on the Claude API on November 30, 2026, and Anthropic recommends claude-sonnet-5-5.
Five settings that Sonnet 4.5 accepts return a 400 error on Sonnet 5.5: thinking budgets, sampling parameters, assistant prefill, forced tool choice and thinking disabled.
Sonnet 5.5 lists at $2 and $10 per million tokens, 13 percent cheaper with no thinking, but default thinking can erase the saving on short requests.