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Claude Opus 5.5 breaking changes: thinking can't be disabled, forced tool_choice removed

UpdateVerifiedAdded Sep 22, 2026

On Claude Opus 5.5, disabling or manually enabling thinking returns a 400; control depth with the effort parameter instead. tool_choice any and tool also return 400, so use auto with strict tool use, and computer use requires the computer_toolset_20260801 toolset.

Topics: LLMs, Developer tools

Anthropic's release notes

Summaries of vendors' own notes. Product names and logos belong to their owners; logos via logo.dev.

More Claude Platform releases

Every Claude Platform release
DateRelease
Sep 22Claude Opus 5.5 launches with a 1M token context window and 128k output
GA
Sep 22Fast mode research preview available for Claude Opus 5.5 on the Claude API
Preview
Sep 22Claude API beta lets tools be defined inside mid-conversation system messages
Beta
Sep 18Compliance API adds Claude in Chrome session transcripts in beta
Beta
Sep 14Messages API adds on-demand conversation compaction in beta
Beta
Sep 14Claude API input_transformations adds thinking_mismatch_allowed entries in beta
Beta
Sep 10Claude Managed Agents permission policies add an auto mode
Update
Sep 10ant CLI 1.32.0 adds sessions connect for live Managed Agents sessions
Beta

Also shipped on Sep 22, 2026

Anthropic in September 2026
DateRelease
Sep 22Shopify connector (Beta)
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Sep 22Lakebase backup schedule API (Beta)
DatabricksLakebaseBeta
Sep 22OpenAI GPT-6 Sol and GPT-6 Luna are now generally available on Amazon Bedrock
AWSAmazon BedrockGA
Sep 22Drives for Vercel Sandbox are now in public beta
VercelAI SDK and AI GatewayBeta
Sep 22Deprecation notice: All-platform CodeQL bundle
GitHubGitHub Advanced SecurityDeprecation
Sep 22Refreshed repository pull requests page generally available
GitHubGitHub platformGA

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