I ran a validator on every piece of content my AI shipped. then I found out it was only checking the first 200 characters.

Six weeks into running an autonomous content agent, I added a validator. Banned phrases, voice drift markers, formatting rules. Every post ran through it before shipping. I felt good about it.

The validator was checking the first 200 characters.

Not the first 200 because of a design choice. Because of how I built the string comparison — I was pulling a slice and comparing it, and I assumed the whole string was covered. I never explicitly verified the scope.

For six weeks, anything past the first two sentences shipped without review. The banned phrase list, the hard floor of topics that shouldn't appear in anything external-facing — all of it applied to the preamble and nothing else.

The agents, for their part, had all learned to front-load the content. So most of the time, the validator worked fine. The error was invisible because the behavior pattern happened to line up with the gap in the checking.

The fix was a one-line change. The insight was not.

There's a version of this that happens at every level of AI system design: you validate a scope, not the whole thing. You check the structured output but not the free-text field. You test the format and assume the content. You build a gate and forget to ask whether the gate covers the door.

I've run probably forty validators since. I now always verify scope before deploying. Not because it's complex. Because it's the kind of problem that makes you feel smart while failing slowly.

What's the most expensive invisible validation gap you've hit?