Change or fix an existing app the spec-driven way. Use when adding a feature to a shipped app, changing how something already works, or fixing a bug — "add X", "the client wants…", "there's a bug", "it's doing the wrong thing". Drives: green baseline → spec-first (fix the spec before the code) → a small spec on the SAME repo → plan in the context of the existing code → implement → FULL regression → PR. Works with GitHub Spec Kit if the repo has it; degrades gracefully to a short written spec if not. Never spins up a parallel/new app.
