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DisclaimerUnofficial, and not affiliated with Anthropic. Nearly all of this is read straight out of what ships: npm bundles, captured prompts, published docs. Anthropic's own notes go in verbatim, marked as theirs. The rest is my reading, and every entry carries the strings behind it. If one looks wrong, vote it down and say why.

All of v2.1.251 Home All releases olderv2.1.250
Claude Code v2.1.251

Fast mode now vets the model change through a hook before applying

Use it now
Useful3 Signal3
Models

Turning Fast mode on now runs your model-switch hook and can be refused.

/fast/config
What

Turning Fast mode on or off, from /config, /fast or the fast-mode picker, now checks for in-flight turns and runs your PreModelSwitch hook against the model it would move to. If the hook denies it or answers "ask", fast mode stays off and you get an error telling you to run /model to switch first, then enable fast mode. This used to be an immediate state flip with no checks.

Details
  • The toggle now shows a cancellable "Applying fast mode…" / "Waiting for the workspace…" state while the hook runs.
  • If the model changes underneath the hook, the toggle is abandoned with a message asking you to try again.
  • The fast mode option itself only appears when its own two internal checks pass.
Evidence

Fast mode was not enabled: the model changed while PreModelSwitch hooks ran; try again, Model switch blocked by a PreModelSwitch hook

Strings lifted out of the shipped bundle, so the claim above can be checked against them.

Related

Other releases about the same thing. Found by shared names or similar wording; neither means one caused the other.

See this entry in the whole of v2.1.251 →