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Written by AI from the official notes. Check them for exact details.Claude Opus 4.8 introduces mid-conversation system messages and updates various features for improved performance.
- Mid-conversation system messages can now be sent after user turns.
- The
stop_detailsfield on refusal responses is now documented. - Effort parameter defaults to high across all surfaces.
- Minimum cacheable prompt length reduced to 1,024 tokens.
- Adaptive thinking triggers reasoning only when needed.
Why it matters: Developers using Claude Opus 4.8 should care for enhanced flexibility and performance in their applications.
Full release notes16 changes
- We've launched mid-conversation system messages. On Claude Opus 4.8, you can send
role: "system"messages after a user turn (subject to placement rules) in themessagesarray, preserving prompt cache hits when instructions change during a long-running session. No beta header is required. - The
stop_detailsfield on refusal responses is now publicly documented; it returns acategory(cyber,bio, ornull) and a human-readableexplanation, so your application can route different classes of refusal to the right next step. No beta header is required. - On Claude Opus 4.8, the effort parameter defaults to
highacross all surfaces, including Claude Code and the Messages API. - On Claude Opus 4.8, the minimum cacheable prompt length for prompt caching is 1,024 tokens, lower than on Claude Opus 4.7.
- With adaptive thinking enabled, Claude Opus 4.8 triggers reasoning only when a turn needs it, reducing wasted thinking tokens compared to Claude Opus 4.7 at the same effort level.
- Claude Opus 4.8 supports high-resolution image input (up to 2576 pixels on the long edge), same as Claude Opus 4.7.
- Task budgets now support Claude Opus 4.8.
- The advisor tool now supports Claude Opus 4.8.
- Computer use now supports Claude Opus 4.8.
- Fast mode for Claude Opus 4.8 is available as a research preview on the Claude API only.
- Setting the sampling parameters
temperature,top_p, ortop_kto a non-default value returns a 400 error on Claude Opus 4.8, same as on Claude Opus 4.7. See the migration guide for details. - In Claude Code, we've expanded Auto mode to more users for long-running tasks. See the Claude Code documentation.
- In Claude Code, Max plan users now default to fast mode on Claude Opus 4.8. See the Claude Code documentation.
- In Claude Code, Workflows are available as a research preview, letting you define and run multistep agentic plans. See the Claude Code documentation.
- We've deprecated fast mode for Claude Opus 4.6, with removal approximately 30 days after launch. Migrate to fast mode for Claude Opus 4.8 or Claude Opus 4.7. Read more in Fast mode.
- For updates to claude.ai, Cowork, Claude for Microsoft 365, and other Claude apps in this release, see the release notes for Claude Apps.