A method of controlling an engagement state of an agent during a human-machine dialog is provided. The method can include receiving a spoken request that is a conditional locking request, wherein the conditional locking request uses a natural language expression to explicitly specify a locking condition, which is a predicate, storing the predicate in a format that can be evaluated when needed by the agent, entering a conditionally locked state in response to the conditional locking request, in the conditionally locked state, receiving a multiplicity of requests without a need for a wakeup indicator, and for a request from the multiplicity of requests evaluating the predicate upon receiving the request, and processing the request if the predicate is true.