Are you a Claude “China user”?
- 100% local scan
- Results never uploaded
- Open source
What gets scanned
Ten locale & network fingerprints, weighted to a 0–100 risk score.
- System timezoneWeight 24Claude SameIntl.DateTimeFormat exposes the same OS timezone Claude Code reads; compared against Asia/Shanghai, Asia/Urumqi and other China zones.
- Browser languageWeight 18navigator.languages — zh-CN / Simplified Chinese at the top of the list scores highest.
- Installed Chinese fontsWeight 14Canvas 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.
- WebRTC IP leakWeight 10RTCPeerConnection probing for local or real public IP candidate leaks through STUN.
- 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 4The locale your browser resolves for date and number formatting.
- Timezone offsetWeight 3Whether getTimezoneOffset() equals UTC+8.
- Emoji rendering styleWeight 3OS 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 nine more browser-visible fingerprints — UI language, Chinese fonts, Chinese vendor fonts, WebRTC IP leaks, 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=jsonClaude Anti-Ban & Safety Knowledge Base
In-depth guides on Anthropic risk mechanisms, OS/browser environment cleanup, payment safety, API safety, and appeal SOPs.
Claude Code Steganography & Anthropic 4-Layer Risk Model
Deep dive into how Claude Code embedded hidden Unicode markers into system prompts and how Anthropic flags accounts based on IP, timezone, BIN, and usage patterns.
Read Guide →OS & Browser Environment Cleanup & Residential IP Guide
Step-by-step guide to syncing OS timezones, isolating Chinese font fingerprints, preventing WebRTC/DNS leaks, and setting up anti-detect browsers.
Read Guide →Safe Account Registration, Virtual Card BIN & Payment Risk Avoidance
How to choose physical SIM verification services, pick high-rate virtual credit card BINs, align billing addresses, and avoid chain bans.
Read Guide →Chinese models are simply better
FAQ
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 nine signals are common Chinese-environment or network 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, block WebRTC leaks, 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 Google Analytics for anonymous page-view stats and Google AdSense for advertising (ads may use cookies). WebRTC leak detection may briefly contact a public STUN server to gather ICE candidates.
Privacy
Every check runs locally in your browser — your scan results never leave your device. The site loads Google Analytics for anonymous page-view stats and Google AdSense for ads; none of the detected signals are ever sent. WebRTC leak detection may briefly contact a public STUN server.