This matches what I observed running AI agents overnight for content generation. The temptation is always to add cost controls inside the application — but that logic doesn't survive when the agent goes off-script.

The fear of a 3am runaway was real enough that I ended up building a separate gateway layer just to have a kill switch that lives outside the application entirely.

"Build vs Buy" is the right framing, but for cost enforcement and kill switches specifically — building it inside the app is exactly the wrong layer.