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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 v2.1.51

Smart Tool Deferral [Gradual Rollout]

What

An intelligent tool deferral system that tracks your tool usage patterns and only defers tools you haven't used recently, keeping frequently-used tools immediately available.

Status

Feature-flagged behind tengu_coral_whistle (default: true)

Details
  • Tracks tool usage count and last-used timestamp in config (toolUsage)
  • Uses a 7-day half-life decay function to compute a "recency-weighted usage score"
  • Tools below the threshold are deferred (loaded on demand via ToolSearch)
  • MCP tools are always deferred regardless of this flag
  • When enabled, the shouldDefer flag on a tool triggers the usage-based check
Evidence

tengu_coral_whistle feature flag, functions h84 (track usage), I84 (compute score with decay), b84 (check threshold) (search for "tengu_coral_whistle")

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 v2.1.51 →