Anthropic releases Claude Sonnet 5.5 and documents API migration changes
Anthropic says Claude Sonnet 5.5 is available on several named API and cloud platforms. It also documents compatibility changes affecting thinking settings, forced tool use, thinking blocks, computer use, and advisor-tool requests.
SessionWatcher editorial · Published · Updated · Source announcement: 2026-09-28
Where Sonnet 5.5 is available
Anthropic says Claude Sonnet 5.5 (claude-sonnet-5-5) is available on the Claude API, Claude in Amazon Bedrock, Claude Platform on AWS, Claude on Google Cloud, and Claude in Microsoft Foundry. The release notes direct developers to the model page for context window, output limits, and prices; this announcement text does not provide those figures.
Sources: Anthropic release notes: 2026-09-28
Compatibility changes to check
Anthropic warns that code written for Claude Sonnet 5 can break in five ways on Sonnet 5.5. At high effort or below, turn off up-front thinking with thinking: {"type": "between_tools"} instead of "disabled". Forced tool use with tool_choice types any or tool returns a 400 error. Thinking blocks are tied to the model and conversation.
The earlier computer_20251124 computer use tool is not accepted on the Claude API and Google Cloud. The advisor tool rejects Claude Opus 4.8, Claude Opus 4.7, and Claude Sonnet 5 as advisors. Anthropic points to its change overview and migration guide for details and before-and-after requests.
Sources: Anthropic release notes: 2026-09-28
Thinking blocks and account boundaries
Thinking blocks produced by Sonnet 5.5 work only in the account that produced them or an account linked to it. If another account sends one, the API drops the block before the model sees it and the request succeeds. Anthropic says blocks from earlier models are unaffected.
Sources: Anthropic release notes: 2026-09-28
AI assisted reporting, checked against the linked official sources. Source pages checked 2026-09-30. Editorial process and corrections.