WebAuthn is unusable inside the WeChat/QQ in-app WebViews, so a document navigation carrying those UAs is served a bilingual 'open in your system browser' interstitial instead of the passkey-centric SPA. API/config/health/asset requests pass through, and an ack cookie (ua_ack) lets a determined user or false-positive continue. Backend-only; the SPA is untouched.