Function canary_prompt 

pub fn canary_prompt(token: &str) -> String

The throwaway prompt: dispatch one trivial background task, wait for its completion notification, and only then report.

The DEVFLOW_RESULT: line and the bare token line are separate on purpose. The marker line is what a pipe-owning supervisor’s close rule watches for (it must parse as the existing marker grammar, so nothing may be added inside its JSON body); the bare token line is what agent_result::token_reported_in_capture matches. Folding the token into the marker’s JSON would couple this prompt to AgentResult’s schema for no gain.