A delegated child copies the parent's prompt_caching.cache_ttl. The 1h tier
is priced for a person who steps away between turns (2x write vs 1.25x for
5m); a subagent calls every few seconds for minutes and is gone, so it paid
2x on every tool result and never collected the retention. Live measurement
(Sep 5, 40 concurrent Fable 5.1 children via OpenRouter, 1h markers on the
wire): cache writes billed at $20/M against $12.51/M for the same run at
5m, ~60% of a write-dominated bill.
_apply_child_cache_ttl runs right after child construction: 1h -> 5m,
5m stays, disabled stays disabled, parent untouched. Tests: unit (markers
on the wire drop ttl; parent's 1h layout still differs) and the real spawn
path with cache_ttl: 1h in a temp HERMES_HOME.
Teknium