Anthropic removed hidden tracking markers from Claude Code after a security researcher discovered the AI coding assistant was using undisclosed markers to identify some users’ location, proxy use, and possible links to Chinese AI labs. The feature, discovered in June by developer “Thereallo,” embedded signals in Claude Code’s system prompts that could flag users Anthropic believed were bypassing restrictions or attempting to extract model capabilities. Thereallo criticized how it was done, noting that Claude Code hid tracking signals inside system prompts using Unicode markers and encoded domain lists rather than disclosing the system through documentation or release notes. After the tracker was revealed online, Anthropic engineer Thariq Shihipar said on X that it was introduced in March as an “experiment” to stop account abuse by unauthorized resellers and protect Claude from distillation attacks. Shihipar wrote last week that the team had landed stronger mitigations and merged the pull request, with the rollback expected in the next release. The discovery comes as Anthropic pushes lawmakers to crack down on unauthorized copying of frontier AI models.