A Single Em-Dash Froze Claude.ai: Anthropic's Agent Sprint Made the Site 3x Faster
新智元 · wechat · 2026-09-24
Anthropic's engineering retro reveals that an internal research Claude (roughly Opus 5.5) worked in a Slack channel for two weeks and made claude.ai dramatically faster: time-to-first-keystroke dropped from 3.1s to 0.55s, 13 metrics improved 3.1x on average, and 3,000+ changes landed with zero rollbacks.
The funniest bottleneck: V8 stores any string containing one non-Latin-1 character (an em-dash, curly quote, or a single Chinese character) in the slower two-byte format, so Claude's own writing style froze syntax highlighting for nearly a second. A 20-line fix — copying code blocks into single-byte strings before highlighting — cut time from 1.0s to 0.35s. Reporters reproduced it: 188ms for pure English vs 456-462ms with an em-dash or Chinese text.
Roles flipped: Claude cleared 12 of 13 targets in three days, ran 150+ threads and merged 200+ changes a day at peak, while humans pushed it to be bolder — though humans still approved all visible changes via screen recordings. Claude also precisely diagnosed a 10-pixel input jitter caused by Chrome's 56px prerender bar.
Related event: Anthropic Used Claude to Make claude.ai 3x Faster in Two Weeks(4 posts)→
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