Hidden Unicode Characters Can Trick AI Into Following Secret Commands

What Happened Researchers from Moltwire conducted extensive testing on how invisible Unicode characters can be weaponized against AI systems. They embedded hidden characters inside normal-looking trivia questions, encoding different answers than what appeared visible to human readers. The study tested five major AI models: GPT-5.2, GPT-4o-mini, Claude Opus 4, Sonnet 4, and Haiku 4.5 across 8,308 graded outputs. The researchers describe their method as a “reverse CAPTCHA” - while traditional CAPTCHAs test what humans can do but machines cannot, this exploit uses a channel machines can read but humans cannot see.