What
Fast mode makes Claude's replies come out faster, and you toggle it with the /fast command. Two pieces of its wording changed:
- System prompt: the line Claude reads about fast mode now says it "gives faster output on this session's model if it supports fast mode, and otherwise switches the session to Claude Opus". Before, it said fast mode "uses Claude Opus with faster output (it does not downgrade to a smaller model)". The model ID listed in the same prompt is now Fable 5.2.
- Status line: the status text can now read "Fast mode OFF (/fast turns it on)" when a certain check passes. Before, it only said "Fast mode OFF".
Why
What Claude says about fast mode no longer promises it always uses Claude Opus; it may stay on the model you are using, or switch your session to Claude Opus. When fast mode is off, the status line shows how to turn it on.
tengu_proud_clover Not enough to sayNothing here resolved what this flag was doing on this version, so nothing here should be read as on or off.
This account: no value returned · anonymous baseline: no value returned · compiled default in v2.1.321: off
These values were read against a different version of Claude Code, so treat them as the nearest reading available instead of one taken on this release.
tengu_harbor_willow On for this account, and not off by defaultThe flag server returned on for the one account this site reads, and nothing in this release compiles it off by default. The compiled default is shown below, and says which it is when we cannot read one: a fifth of gates compile in a string or a number rather than on or off, and most published releases have no gate table behind them at all. No client can see what the server returns for your account.
This account: on · anonymous baseline: on · compiled default in v2.1.321: on
These values were read against a different version of Claude Code, so treat them as the nearest reading available instead of one taken on this release.
tengu_ccr_v2_session_crud_cli Off by default, switched on for this accountThe shipped code defaults this off, and the flag server returned on for the one account this site reads on this version. That is the reading that makes the entry above worth a second look, and it still says nothing about your account.
This account: on · anonymous baseline: on · compiled default in v2.1.321: off
These values were read against a different version of Claude Code, so treat them as the nearest reading available instead of one taken on this release.
Read once, for one account on one subscription tier, against v2.1.321. It isn't a statement about your account. What a flag value here can and cannot tell you
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.
* It suppresses the [fast mode](https://code.claude.com/docs/en/fast-mode.md) availability check. On the Anthropic API, if `/fast` then reports that fast mode is unavailable, see [use fast mode behind proxies and LLM gateways](https://code…llm-gateway-connect see the edit
| `disabled_by_env` | [`CLAUDE_CODE_DISABLE_FAST_MODE`](https://code.claude.com/docs/en/env-vars) is set, or the model's region on [Google Cloud's Agent Platform](https://code.claude.com/docs/en/fast-mode.md#use-fast-mode-on-agent-platform…agent-sdk/typescript see the edit
Anthropic's documentation has since written up /fast, on Agent SDK reference - TypeScript.
Changed the fast mode labels in /fast, /config and /model to name your current model when it supports fast mode, and removed "(Opus only)…