Anthropic releases Claude Opus 5.5, the first model in its Claude 5.5 family

On September 22, 2026 Anthropic introduced Claude Opus 5.5, which it calls the first model in its new Claude 5.5 family, and said it was available on all platforms with the API model ID claude-opus-5-5. Anthropic says external evaluators, including Frontier Design and METR, tested the model before release, and that because Opus 5.5 is comparable to Claude Mythos 5.1 in biology and cybersecurity, it is deployed with safeguards similar to those on Claude Fable 5.1. The release notes say thinking cannot be disabled on Opus 5.5 and that the tool_choice types any and tool return a 400 error. Anthropic later released Claude Sonnet 5.5 and Claude Haiku 5.5, which USASI covered separately. Opus 5.5 is a hosted model; no release of weights is described. USASI has not tested it.12

USASI. (2026, October 13). Anthropic releases Claude Opus 5.5, the first model in its Claude 5.5 family. United States of America Superintelligence. https://unitedstatesofamericasuperintelligence.com/news/anthropic-releases-claude-opus-5-5/

@misc{usasi_news_anthropic_releases_claude_opus_5_5,
  author = {{USASI}},
  title = {Anthropic releases Claude Opus 5.5, the first model in its Claude 5.5 family},
  year = {2026},
  month = oct,
  howpublished = {\url{https://unitedstatesofamericasuperintelligence.com/news/anthropic-releases-claude-opus-5-5/}},
  note = {United States of America Superintelligence. Last updated 2026-10-13}
}