Are you a Claude “China user”?
- 100% local scan
- Results never uploaded
- Open source
What gets scanned
Nine locale fingerprints, weighted to a 0–100 risk score.
- System timezoneWeight 26Claude SameIntl.DateTimeFormat exposes the same OS timezone Claude Code reads; compared against Asia/Shanghai, Asia/Urumqi and other China zones.
- Browser languageWeight 20navigator.languages — zh-CN / Simplified Chinese at the top of the list scores highest.
- Installed Chinese fontsWeight 16Canvas width-probing for Simplified / Traditional Chinese fonts such as Microsoft YaHei and PingFang SC.
- Chinese vendor fontsWeight 10Canvas probing for fonts shipped by Chinese vendors or software — MiSans, HarmonyOS Sans, OPPO Sans, WPS Founder faces. Any hit is a strong tell.
- Chinese browser / WebViewWeight 8User agent and UA-CH brands matched against WeChat, QQ, Quark, UC, Baidu and other Chinese browsers or in-app WebViews.
- Chinese-brand deviceWeight 6UA-CH device model (navigator.userAgentData) and UA matched against HarmonyOS, Huawei, Xiaomi, OPPO, vivo and other Chinese brands.
- Intl localeWeight 6The locale your browser resolves for date and number formatting.
- Timezone offsetWeight 4Whether getTimezoneOffset() equals UTC+8.
- Emoji rendering styleWeight 4OS vendor guessed from the user agent; a weak, loosely correlated signal.
How the check works
When Claude Code is pointed at a proxy endpoint via ANTHROPIC_BASE_URL, public reverse-engineering reports found it reads your operating-system timezone and the proxy hostname, then hides the verdict inside the system prompt with Unicode steganography — the date separator and four look-alike apostrophes in the “Today’s date” line encode whether you look like a China user.
A web page cannot read everything Claude Code can, but the key signal is identical: this tool reads the same OS timezone, then adds eight more browser-visible fingerprints — UI language, Chinese fonts, Chinese vendor fonts, Chinese browsers, device brand, Intl locale, UTC+8 offset and emoji style — into a weighted score. Signals scoring ≥0.25 count as hits; bands are Low 0–30, Medium 31–60, High 61–100.
Also available over curl
Prefer the terminal? Hit the endpoint below — it estimates your risk from your IP geo + request headers, and replies in the language of your Accept-Language header.
curl https://fuck-claude.vercel.app/api/check curl -H "Accept-Language: zh" https://fuck-claude.vercel.app/api/check curl https://fuck-claude.vercel.app/api/check?format=jsonFAQ
Does Claude really check my timezone?
According to public reverse-engineering reports, when Claude Code talks to a non-official endpoint it reads the OS timezone and proxy hostname, and steganographically encodes the result into its system prompt. The timezone this page reads via Intl.DateTimeFormat is the very same OS timezone.
Is this score the exact check Claude runs?
No. Only the system timezone maps one-to-one onto Claude’s reported mechanism. The other eight signals are common Chinese-environment fingerprints that correlate with it, so treat the score as an estimate, not a verdict.
How do I lower my score?
Switch your OS timezone away from China zones such as Asia/Shanghai, move zh-CN off the top of your browser language list, and avoid routing Claude Code through proxies whose hostnames contain flagged domains or AI-lab keywords.
Is any data uploaded?
No. Every check runs locally in your browser and none of the detected signals are ever sent anywhere. The site loads Baidu Analytics, Microsoft Clarity and Google AdSense for traffic stats and ads only.
Privacy
Every check runs locally in your browser — your scan results never leave your device. The site loads Baidu Analytics, Microsoft Clarity and Google AdSense; none of the detected signals are ever sent.