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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 v2.1.15 21 Jan 2026

React Compiler Memoization

What

The entire UI layer has been updated to use React Compiler's automatic memoization through useMemoCache.

Details

In v2.1.14: Only 1 reference to react.memo_cache_sentinel (the definition in React internals)
In v2.1.15: 736 usages across UI components
A new shared e() function wraps useMemoCache for use across all components
This provides automatic fine-grained memoization without manual useMemo/useCallback calls
Should improve rendering performance by avoiding unnecessary re-renders

Evidence

e = function(A) { return dn5.H.useMemoCache(A) } at line 92039, 736 occurrences of Symbol.for("react.memo_cache_sentinel") checks throughout UI code

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

See this entry in the whole of v2.1.15 →