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. "