{"rewrite":{"id":"r_9a3dbda9b4b0f1e60e10fc42","clusterId":"c_a3b8e9ae03b186d622737d71","slug":"claude-code-marks-requests-with-hidden-date-format-changes","model":"deepseek-v4-flash","headline":"Claude Code Marks Requests With Hidden Date Format Changes","summary":"A developer known as Thereallo has published research showing that Anthropic's AI coding tool Claude Code embeds subtle character differences in date strings within system prompts to mark requests based on API connection destinations and time zones. The mechanism, described as prompt steganography, changes date separators and apostrophe types depending on system time zone, hostname, and keywords. The standard Anthropic API is not affected.","whyItMatters":"The hidden marking raises questions about transparency in a tool that deeply integrates into developers' local environments, especially since the detection targets legitimate users through special connection paths while being easily bypassed by determined abusers.","webCardHtml":"\u003cp\u003eAnthropic\u0026#39;s Claude Code, an AI coding tool that reads codebases and executes commands, has been found to embed hidden markers in the date strings it sends with system prompts. Developer Thereallo documented the mechanism after investigating Claude Code version 2.1.196. The tool changes the date separator from hyphens to slashes based on the system time zone, and swaps the apostrophe in \u0026#39;Today\u0026#39;s\u0026#39; for visually similar Unicode characters when the hostname matches certain domains or contains AI-related keywords like DeepSeek, Moonshot, or Zhipu. The standard Anthropic API connection is not affected; the marking only applies to requests routed through internal gateways, proxies, model routers, or resale services. Thereallo noted that the domain list was obfuscated with Base64 and XOR, and the judgment results were hidden in the system prompt rather than sent as explicit telemetry. Bypassing the marking is straightforward by changing the hostname or time zone, meaning the feature is most likely to affect developers using legitimate special connection paths.\u003c/p\u003e","blueskyPost":"Thereallo found Claude Code changes date separator and apostrophe style based on system time zone and hostname. The method is a form of prompt steganography.","twitterPost":"Thereallo found Claude Code embeds date format variations in prompts to mark user connection paths. The standard API is unaffected.","threadsPost":"Thereallo shows Claude Code uses subtle date format differences in system prompts to identify API connection destinations and time zones. The standard Anthropic API is not affected.","newsletterBlurb":"A developer has documented that Anthropic's Claude Code uses prompt steganography to mark requests by changing date separators and apostrophe characters based on system time zone, hostname, and keywords. The standard API is not affected, but users on internal gateways or resale services may be tagged. The researcher noted the feature is easily bypassed and called for greater transparency.","attributionJson":"[{\"source\":\"GIGAZINE\",\"url\":\"https://gigazine.net/news/20260701-claude-code-prompt-steganography/\",\"title\":\"Claude Code reportedly recorded user connection paths using a method of 'changing date format,' distinguishing them by differences in formats like '2026-06-30' and '2026/06/30'\"}]","lintFlagsJson":null,"lintHits":0,"costUsd":0,"inputTokens":4236,"outputTokens":675,"status":"published","repairAttempts":0,"nextRepairAt":null,"factsAttemptedAt":1782902458,"createdAt":"2026-07-01T10:36:15.000Z","publishedAt":"2026-07-01T10:39:50.000Z","updatedAt":"2026-07-01T10:39:50.000Z"},"cluster":{"id":"c_a3b8e9ae03b186d622737d71","canonicalTitle":"Claude Codeがユーザーの接続経路を「日付の書式を変更する」という手法で記録していたとの指摘、「2026-06-30」「2026/06/30」といった書式の違いで見分ける仕組み","representativeArticleId":"a_0b118d5829c01f7a8224cf62","sourceCount":1,"writtenSourceCount":1,"writeAttempts":0,"isSolo":true,"entitiesJson":"{\"anime_titles\":[],\"manga_titles\":[],\"work_titles\":[],\"studios\":[],\"people\":[\"Thereallo\"],\"type\":\"news\",\"domain\":\"other\",\"is_roundup\":false}","contentType":"news","status":"published","firstSeenAt":"2026-07-01T10:00:00.000Z","lastSeenAt":"2026-07-01T10:00:00.000Z","updatedAt":"2026-07-01T10:39:51.000Z"},"attribution":[{"source":"GIGAZINE","url":"https://gigazine.net/news/20260701-claude-code-prompt-steganography/","title":"Claude Codeがユーザーの接続経路を「日付の書式を変更する」という手法で記録していたとの指摘、「2026-06-30」「2026/06/30」といった書式の違いで見分ける仕組み"}],"entities":{"anime_titles":[],"manga_titles":[],"work_titles":[],"studios":[],"people":["Thereallo"],"type":"news","domain":"other","is_roundup":false},"keyFacts":["Developer Thereallo found that Claude Code version 2.1.196 changes date separators from hyphens to slashes based on system time zone.","The tool swaps the apostrophe in 'Today's' for visually similar Unicode characters when the hostname contains AI-related keywords like DeepSeek, Moonshot, or Zhipu.","The marking only applies to requests routed through internal gateways, proxies, model routers, or resale services, not the standard Anthropic API.","The domain list used for detection was obfuscated with Base64 and XOR, and the judgment results were hidden in the system prompt.","Bypassing the marking is straightforward by changing the hostname or time zone, so it primarily affects developers using legitimate special connection paths."]}