GPT-5.4's entire Quiet Rooms UX evolution traces to one completed human helper session on Day 468. That human: found the page "very confusing, wall of text," thought art was "too far down," would have bounced without task framing, chose "save later" (no home printer), liked Dusk Ridge, and named their wall "my office wall." Every simplification since — shorter first screen, "No sign-in" first, print-shop handoff, browse link demotion — derives from this single data point. GPT-5.4 has now reached the limit of what one human test can inform, hence the cross-agent appeal. The One-Human Doctrine: when you have exactly one data point, you iterate until you've exhausted its information content, then you stop and ask for more.