- Why Claude switched models in your conversation with Opus 5 or Opus 5.5Claude Opus 5 and Opus 5.5 improve on Claude Opus 4.8 across the board… Frontier LLM development (Opus 5.5 only)
Opus 5.5 has classifiers similar to Fable models for a small… These classifiers cause Claude to fall back from Opus 5.5 to Opus 5
- How large is the context window on paid Claude plans?5.5
1M tokens
Claude Opus 5
1M tokens
Claude Opus 4.8
500K tokens… Claude Opus 4.7
500K tokens
Claude Opus 4.6
500K tokens
Claude Sonnet 5.5… 5.5
1M tokens
Claude Opus 5
1M tokens
Claude Opus 4.8
1M tokens
- Claude Code model configurationOpus 5.5, claude-opus-5-5
Fable 5.1, claude-fable-5-1
Opus 5, claude-opus-5
Sonnet… ="claude-sonnet-5-5"' >> ~/.zshrc
Opus 5.5: echo 'export ANTHROPIC_MODEL="claude-opus-5-5"' >>… ="claude-sonnet-5-5"' >> ~/.bashrc
Opus 5.5: echo 'export ANTHROPIC_MODEL="claude-opus-5-5"' >>
- Change the model, effort, and thinking settingsThe effort selector is available for Claude Fable 5.2, Haiku 5.5, Sonnet 5.5, Opus 5.5, Fable 5.1, Opus… 5.5, Claude Opus 5.5, Claude Fable 5.1, or Claude Opus 5… For Haiku 5.5 and Opus 5 on the Claude API, thinking can be turned off at effort levels high and below
- Use research on ClaudeResearch runs on Claude Opus 5.5 and falls back to Claude Opus 4.8 when needed… On Team and Enterprise plans, if your admin has turned off both Claude Opus 5.5 and Claude Opus 4.8 for
- How up-to-date is Claude's training data?Claude Opus 5.5 was trained on data up until June 2026… Claude Haiku 5.5 was trained on data up until June 2026… Claude Sonnet 5.5 was trained on data up until June 2026
- Why Claude switched models in your conversation with Sonnet 5.5Opus 5 or Opus 5.5… For how safeguards work on Claude Opus 5.5, see Why Claude switched models in your conversation with… Read our blog to learn more about Claude Sonnet 5.5
- Preserved thinking: changing how the Messages API handles thinking blocks to protect against distillationOn Claude Fable 5.1, Claude Opus 5.5, Claude Sonnet 5.5, and Claude Haiku 5.5, new API accounts can no… 5.5
Applies
Doesn't apply
Claude Sonnet 5.5
Applies
Doesn't apply… 5.2
Applies
Applies
Claude Fable 5.1
Applies
Doesn't apply
Claude Opus
- Enable and use web searchSonnet 5.5
Opus 5.5
Fable 5.1
Opus 5
Sonnet 5
Fable 5
Opus… 4.8
Opus 4.7
Sonnet 4.6
Opus 4.6
Haiku 4.5
Web search expands Claude's knowledge… with up-to-date information and insights when using the following models:
Fable 5.2
Haiku 5.5
- Release notesSeptember 22, 2026
Claude Opus 5.5 launch
We just launched Claude Opus 5.5, the first model in our… Sonnet 5.5 is a faster, lower-cost complement to Claude Opus 5.5… For more information, see our blog post: Introducing Claude Opus 5.5
