Claude Sonnet 4.5
Agents, coding, computer use, and long-running tasks from the 2025 Sonnet line.
Claude Sonnet 4.5, announced September 29, 2025 on the Sonnet timeline, is described there as a strong model for agents, coding, and computer use, with extra weight on long-running tasks and domain knowledge in coding, finance, and cybersecurity. It is part of the public Sonnet history, not a model I would pick over Sonnet 5 for a greenfield agent in 2026.
If you still have prompts tuned to 4.5, migrate through 4.6 notes into Claude Sonnet 5 and watch the thinking-parameter change. Catalog: all Sonnet versions.