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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

SDK reports why fast mode is unavailable

Use it now
Useful3 Signal0
SDK not in their notes

SDK messages now tell you exactly why fast mode was unavailable for a request.

fast_mode_disabled_reason
What

An optional fast_mode_disabled_reason field now travels on SDK init and result messages, so a client can tell why a request could not use fast mode rather than guessing.

Details
  • Added to the init message and to both the success and error result message schemas.
  • Populated on every result path, including "tool_deferred_unavailable", "budget_exhausted" and "turn_setup_failed".
  • Enum values: free, preference, extra_usage_disabled, network_error, unknown, not_first_party, disabled_by_env, model_not_allowed, sdk_opt_in_required, pending.
  • A companion Set of the first five values normalizes server-supplied reasons: unrecognized values map to unknown, null maps to preference.
  • The field is absent when nothing blocks fast mode; a request may still choose standard speed.
Evidence

Why fast mode can't serve right now. Absent when nothing blocks it (a request may still choose standard speed).

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 →