You'll noticeTier: how much it should matter to you
4Useful: my rating, 1 to 5
4Signal: worth watching, 1 to 5
ModelsArea: what it touches
ImprovementsKind: in v2.1.280,
What probably matters to youSection of the release
What
The system prompt Claude Code sends to the model describes fast mode, a setting toggled with /fast that makes Claude Opus respond faster. Previously the prompt said fast mode "is available on Opus 5/4.8"; that restriction has been dropped, and the prompt now just says it can be toggled with /fast. The model catalog line in the same prompt now lists claude-opus-5-5 (Opus 5.5) where it used to list claude-opus-5.
Why
Dropping the model-restriction wording means the model is no longer told fast mode only works on specific Opus versions, in step with the newly listed Opus 5.5 model.
The entry above is what we published on the day. These lines were added later, as Anthropic's own pages caught up, and they sit beside the original rather than replacing it.
Confirmed sinceAnthropic's documentation has since written up /fast, on overview.* [Fast mode](https://platform.claude.com/docs/en/build-with-claude/fast-mode) (research preview) is available for Claude Opus 5.5 on the Claude API.release-notes/overviewsee the edit
Added sinceA small documentation edit on API usage primer for Claude touched a line naming claude-opus-5-5 after this was published.* Claude Opus 5.5 (`claude-opus-5-5`, adaptive thinking only, always on)claude_api_primersee the edit
Added sinceA small documentation edit on fast-mode touched a line naming /fast after this was published.Fast mode is not available on Claude Opus 4.6. Requests to `claude-opus-4-6` with `speed: "fast"` do not return an error: they run at standard speed and are billed at [standard rates](https://platform.claude.com/docs/en/about-claude/pricin…build-with-claude/fast-modesee the edit
Confirmed sinceAnthropic's documentation has since written up claude-opus-5-5, on claude-api-skill.When the scope is ambiguous (for example, a bare `/claude-api migrate to claude-opus-5-5`), the skill asks you to choose between the entire working directory, a specific subdirectory, or an explicit file list before editing any files. This…agents-and-tools/agent-skills/claude-api-skillsee the edit
How sure we are
Two sources agreeTwo things we can check say the same as this entry.
Anthropic's documentation agreesAnthropic's documentation has since written up claude-opus-5-5, on claude-api-skill.
Anthropic's release notes agreeAdded Claude Opus 5.5 (claude-opus-5-5), now the default Opus model — 1M context, $4/$20 per Mtok with $0.20/Mtok cache reads
