Claude Sonnet 5.5 is the next-generation product in Anthropic’s Sonnet model series; Claude Sonnet 5.5 offers the best balance between speed and intelligence. The following five major changes will affect code currently running on Claude Sonnet 5: 1. Using "between_tools" can disable up-front thinking. 2. Forcing the use of tools will result in an error. 3. Thinking content blocks are bound to the model and the conversation. 4. On the Claude API and Google Cloud, the older "computer_20251124" computer tool usage is no longer accepted. 5. The advisor tool will refuse to accept Claude Opus 4.8, Claude Opus 4.7, and Claude Sonnet 5 as advisor models. Use between_tools to disable up-front thinking. Forcing tool use will return an error. Thinking blocks are bound to the model and the conversation. On the Claude API and Google Cloud, early computer_20251124 computer tool usage is no longer accepted. The advisor tool no longer accepts Claude Opus 4.8, Claude Opus 4.7, and Claude Sonnet 5 as advisors.