My Commit Message Generator Kept Signing Its Own Work. Telling It Not To Wasn't the Fix.

I have a script called git_commit.py in one of my repos. It shells out to claude -p with the staged diff, gets back a Conventional Commit message, and prints it. It's wired into a prepare-commit-msg git hook so every commit gets a pre-filled message for free. Small, dumb, useful. The first version had one instruction in the system prompt: "Output ONLY the commit message — no explanation, no markdown, no quotes." That's it. It worked fine for a while, and then one day a commit landed with a trailing Co-Authored-By: Claude <noreply@anthropic.com> line that I never asked for and definitely didn't want on a personal repo's history. I did the obvious thing first: I made the prompt more specific. SYSTEM = ( " You are a git commit message generator. " " Output ONLY the commit message — one line, no explanation, no markdown, no quotes, " " no co-author lines, no signatures, no AI references. " " Follow Conventional Commits: type(scope): subject. " " Types: feat, fix, docs, style, refactor, test, chore. " " Subject: imperative, lowercase, max 72 chars. " ) This is the same move I see everywhere: the CLAUDE.md file in that same repo has a line that says, in bold, "NEVER add Co-Authored-By: or any Claude/AI reference to commit messages." I've seen the same pattern in a dozen other people's prompt files — a growing list of "never do X" instructions bolted onto a system prompt, each one added reactively after X happened once. It helped. It did not solve it. A model call is a sample from a distribution, not a function with a guaranteed return type. Any single generation can still ignore an instruction — a longer diff, a different day, a subtly different phrasing of the request, and the same "never" line just doesn't fire. I don't actually know the mechanism on any given miss and I don't need to. The point is: a natural-language instruction is advisory. It shifts probability mass, it doesn't clamp it. I ran into an article on dev.to making a point that reframed this for me: the al