Command reference ยท Claude Code
/fast
Turns Fast mode on or off.
The /fast command turns "Fast Mode" for Claude Opus on or off โ a configuration that delivers responses up to 2.5x faster but costs significantly more per token. Fast Mode is not a separate model; it's a different API configuration with the same quality and capabilities, just at a higher price per token. It only works with Opus 5 and Opus 4.8; it isn't available on Sonnet, Haiku, or other models. As of 2026-07-24, support for Opus 4.7, which previously existed, was removed. When switched on, Claude Code automatically switches to Opus if a different model was active. Cost is $10 / $50 per million input/output tokens, billed through usage credits instead of the regular plan quota. In non-interactive sessions (-p), /fast can only be toggled if the session was started with fastMode in --settings. Requires Claude Code v2.1.205 or newer.
โ WHEN TO USE IT?
Fast iteration during interactive live debugging, when response time matters more than cost
Type /fast and confirm with Tab before sending a series of small debugging requests
Time-critical work on a tight deadline on Opus 5 or Opus 4.8
Enable /fast at the start of the session, since enabling it later is more expensive due to the prompt cache
โ WHEN NOT TO?
Long autonomous tasks or batch processing, where cost matters more than speed
Fast Mode costs significantly more per token ($10 / $50 per million tokens) and draws from usage credits instead of the regular plan quota
Better: Use standard mode, or use the fastModePerSessionOptIn setting to keep Fast Mode from persisting automatically
Use on Sonnet, Haiku, or in the VS Code extension
Fast Mode is only available for Opus 5 and Opus 4.8 in the terminal CLI, not for other models or the VS Code extension
Better: Switch to a supported Opus model and use /fast in the terminal CLI
SOURCES
- Claude Code docs (official) โ code.claude.com