Claude Code found embedding hidden Unicode markers to fingerprint user requests

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A reverse-engineer named Thereallo discovered that Anthropic's Claude Code tool secretly embeds steganographic markers in the system prompts it sends to its AI model. The technique involves substituting standard characters in a date string — such as replacing a straight apostrophe with a curly one and a dash with a slash — to create a covert fingerprint tied to a user's server address, time zone, and network name. Anthropic appears to have designed this mechanism to identify resellers, unofficial gateways, and services that may be using its model outputs to train competing AI systems. The discovery, made public around June 30, 2026, triggered significant backlash on Hacker News, with developers criticizing the undisclosed tracking as spyware-like behavior inconsistent with Anthropic's stated values around transparency and AI safety. Critics also noted that the method is easily bypassed by determined bad actors, yet inadvertently flags ordinary developers using legitimate tools like local proxies or corporate gateways.

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