I put a witness on my AI. Devil's advocate killed three designs first.

The short version up front: sonmat v0.9 added an agent called sonmat-witness. It's an external verifier that checks whether the main session's output matches what the user actually asked for - without reading the main session's reasoning. That's the clean design story. The real story happened while I was designing. In a single session I fell into the same trap three times, and each time a single round of /devil collapsed the assumption underneath the design. This post is about both halves. Why a witness at all sonmat started from a single rule: doubt.