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Anthropic made Fast mode for Claude Opus 4.8 available as a Claude API research preview.
The read
The constraint matters: Fast mode is for API evaluation, not a general Claude surface.
The frame
For Claude API builders, Opus 4.8 Fast mode sits behind a research preview, not every Claude surface.
Primary source
Anthropic →Keep reading
See what Nova3 builds →Related updates
- Jun 12, 2026Anthropic publicly documented stop_details on refusal responses, with category and explanation fields for routing.
- Jun 12, 2026Claude Opus 4.8 defaults the effort parameter to high across all surfaces.
- Jun 12, 2026Claude Opus 4.8 lowers prompt caching’s minimum cacheable prompt length to 1,024 tokens versus Opus 4.7.
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