Upgrade to Claude Fable 5 for 1M Token Context Windows
Access 1M tokens and adaptive thinking by upgrading models.
Migrate legacy Claude Sonnet/Opus 4 integrations to Claude Fable 5 to take advantage of the 1M token context window and built-in adaptive thinking.
The Scenario
You are hitting context window limits with older models or your existing Opus 4 integration has started returning errors due to model retirement.
Before & after
Users stay on older models like Sonnet or Opus 4, which have smaller context windows and lack adaptive thinking. Manually managing these older models' limitations takes hours of prompt engineering.
Upgrade your API calls to 'claude-fable-5' or 'claude-mythos-5' via the Claude Platform on AWS. The upgrade takes minutes to update the model string in your environment variables.
The Prompt
I am currently using Claude Opus/Sonnet 4. Provide a checklist for migrating my integration to Claude Fable 5, specifically regarding adaptive thinking and tokenizer changes.
Claude Fable 5 and Mythos 5 offer 1M token context windows and mandatory adaptive thinking. Note that Opus 4 and Sonnet 4 were retired on June 15, 2026.
Source
Claude Platform - Claude API Docs"We recommend upgrading to Claude Sonnet 4.6 and Claude Opus 4.8 respectively... We've launched Claude Fable 5."