A scheduler whose tasks survive crashes and restarts. Heidi's tasks persist with their fire-at times, from minutes to months out, so 'follow up Thursday' still happens even if everything restarted on Wednesday.
Durability is what separates a runtime from a chat product. Without it, every deploy silently kills promised work; with it, delegation is real.
Completion, failure, and timeout all emit signals, so scheduled work that goes wrong becomes a visible question instead of a silent gap.