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Will Claude Code flag you as a China user?

ClaudeFlag is a local Claude Code China user detector for timezone, locale, browser language, Chinese fonts and ANTHROPIC_BASE_URL proxy-risk clues.

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What gets scanned

Six browser-visible fingerprints that map to common Claude Code China user, security and privacy questions.

  1. System timezoneWeight 30Claude SameIntl.DateTimeFormat exposes the same OS timezone Claude Code reads; compared against Asia/Shanghai, Asia/Urumqi and other China zones.
  2. Browser languageWeight 24navigator.languages — zh-CN / Simplified Chinese at the top of the list scores highest.
  3. Installed Chinese fontsWeight 20Canvas width-probing for Simplified / Traditional Chinese fonts such as Microsoft YaHei and PingFang SC.
  4. Intl localeWeight 10The locale your browser resolves for date and number formatting.
  5. Timezone offsetWeight 8Whether getTimezoneOffset() equals UTC+8.
  6. Emoji rendering styleWeight 8OS 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 five more browser-visible locale fingerprints — UI language, installed Chinese fonts, 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.

Claude Code router, proxy and ANTHROPIC_BASE_URL risk

Most China-user questions are not just about geography. They are about proxy endpoints, router tools and environment fingerprints.

Claude Code router and proxy setups usually change the endpoint hostname while leaving your operating-system timezone, browser language and locale untouched. That mismatch is why a user can look overseas at the network layer but still look China-like at the local environment layer.

This detector focuses on the local side of that equation. It does not inspect your real Claude account, subscription, API key, IP address or router configuration. It shows the browser-visible signals that can explain why Claude Code security or privacy checks may feel inconsistent.

FAQ

Does Claude Code check 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 five 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, none of the detected signals are ever sent anywhere, and no third-party analytics scripts are loaded.

What is ANTHROPIC_BASE_URL and why does it matter?

ANTHROPIC_BASE_URL is the environment variable many Claude Code users set when routing traffic through a non-default API endpoint. Reports suggest Claude Code can compare that proxy hostname with local environment signals such as timezone.

Can a Claude Code router or proxy increase China-user risk?

A router or proxy can change where requests are sent, but it does not automatically change your OS timezone, browser language, Intl locale or installed fonts. Those local signals can still look China-like.

Is Claude Code China user detection the same as IP geolocation?

No. IP geolocation is only one possible network signal. This page focuses on local signals: timezone, locale, browser language, fonts, UTC offset and emoji rendering style.

Privacy

Every check runs locally in your browser — your scan results never leave your device. No third-party analytics scripts are loaded.