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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_disabled_reason on turn-completion telemetry

Under the hood
Useful2 Signal0
Telemetry

Turn logs now record why fast mode was skipped, such as env setting or model not allowed.

What

Turn-completion events now report why fast mode was not used, alongside the existing state field.

Details
  • New fast_mode_disabled_reason sits next to fast_mode_state and is populated from the fast-mode eligibility check.
  • Values include not_first_party, disabled_by_env, model_not_allowed, sdk_opt_in_required, pending and unknown.
  • Present at 18 sites in this build; did not exist in the previous one.
Evidence

fast_mode_disabled_reason

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

See this entry in the whole of v2.1.219 →