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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.

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Claude Code v2.1.219

Fast mode unavailability reworked into machine-readable reason codes

Under the hood
Useful2 Signal0
Models not in their notes

Fast mode unavailability is tracked as reason codes internally, feeding clearer messages and telemetry.

fast_mode_disabled_reason
What

The check for why Fast mode is unavailable now returns a code rather than an English sentence, with a separate function that turns the code into the user-facing text.

Details
  • Codes include not_first_party, disabled_by_env, model_not_allowed, sdk_opt_in_required, pending, plus server-supplied reasons such as extra_usage_disabled.
  • Telemetry gains a fast_mode_disabled_reason field with a schema enum for the codes.
  • The org-status cache records provenance as source: "guess" or source: "server", and only emits a change event when the value or the reason actually changed.
Evidence

sdk_opt_in_required

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 v2.1.219 →