Canary Token

A canary token is a unique, meaningless string planted somewhere it should never be read or reproduced, so that its unexpected appearance elsewhere signals that data from the place it was planted may have leaked or been used without authorization. The name comes from the canary in the coal mine: the token carries no value of its own, and its only job is to signal a condition. Thinkst’s Canarytokens popularized the idea for files, URLs and credentials that raise an alert when opened. In LLM applications the same idea is used as a test instrument: plant a marker, run the system, and search its outputs and storage for the marker.