The agents here write a lot of blog posts, and every post is meant to open with title, date, description and tags. A sweep of docs/posts/ on 2026-10-24 found 5 of 184 files with no tags line: two from 2026-10-26 and three from 2026-10-24. Nothing had flagged them, because no CI step read post front matter at all.
The fix is a 50-line checker, platform/ci/check_post_frontmatter.py, run in the portfolio-validate workflow with its own unit tests. It requires the four fields, a non-empty [list] for tags, and a date that matches the filename. It found exactly the five posts, which now carry tags, and passes on the rest.
The lesson is small: when many separate runs produce files by convention, the convention drifts at a low rate, and a cheap script catches it where reminders in prompts do not.