Claude Opus 5.5 changelog

Claude Opus 5.5 now powers Standard effort in Perplexity Computer

Claude Opus 5.5 became the model behind Standard effort in Computer for eligible paid personal accounts. GPT-6.1 Sol now powers Light effort.

Claude Code defaults to Opus 5.5, including on Pro plans

Claude Code 2.1.280 made Opus 5.5 the default Opus model and changed the default on Pro and Team Standard plans from Sonnet to Opus.

Who is affected:

Claude Code users on Pro and Team Standard plans, whose usage limits may now be consumed by Opus.

What to do:

Use /model to pick Sonnet if you want to stretch usage limits.

Claude Opus 5.5 cannot turn thinking off

On Opus 5.5, thinking disabled and manual thinking both return a 400 error, forced tool_choice returns a 400 error, and the older computer use tool is rejected on the Claude API and Google Cloud.

Who is affected:

Code moving from Claude Opus 5 to Opus 5.5.

Action needed:

Omit the thinking field, control depth with effort, and use tool_choice auto.

New API accounts can no longer edit context around thinking blocks

To counter distillation, the API now checks that a thinking block is sent back with the same system prompt, tools and messages. On Fable 5.1, Opus 5.5, Sonnet 5.5 and Haiku 5.5, accounts created on or after August 31, 2026 get an error unless they opt into dropping the block.

Who is affected:

Agents that compact or rewrite earlier turns, especially on new API accounts.

Action needed:

Use server-side compaction or mid-conversation tool changes, or opt into non-strict mode.