I love this idea, but have a hypothesis that 90% of agents that people actually use today would fail this test inadvertently (false negative).
Industry best practice + standard implementation for most agents right now is to do web browsing / fetching via subagents. Their output is summarized using a cheaper model and then passed back to the parent. It's very unlikely that without preserving the actual content the subagents see that the `CANARY-` strings would be found in the output.
Any thoughts on how you'd change the test structure with this in mind?