September 22, 2026 — Claude Opus 5.5 is the first model in the new Claude 5.5 family. It performs at the level of Claude Fable 5.1 on most work and costs 40% less to run than Opus 5 (20 per MTok).
Four breaking API changes from Opus 5: thinking cannot be disabled, forced tool use returns error, thinking blocks tied to model/conversation, computer_20251124 tool deprecated on Claude API and Google Cloud.
Sonnet 5.5 and Haiku 5.5 expected in coming weeks. Tested by Frontier Design and METR before release.