How a Closing HTML Tag Fooled an AI Builder Into Marking Broken Builds as Done

Zugo, a platform that converts a single sentence into a working game, site, or app, discovered its build-success logic was checking only for a closing HTML tag rather than whether the output actually rendered. This meant broken builds received a green tick and charged users credits, while a blank white screen persisted for up to 12 seconds — long enough to erode user trust. Because the preview runs in a sandboxed iframe with an opaque origin, standard React error boundaries could not detect failures inside the frame, so the team instead injects a small script into the generated document to relay errors back to the host via postMessage. Critical fixes included ensuring the injected listener loads before any build code, and separating non-fatal console warnings from true failures to prevent false negatives. The team also enforced a one-way verdict rule, so a build confirmed as broken can never later be overwritten with a passing status.