pre-commit-format-guard

name	pre-commit-format-guard
description	Set up a git pre-commit hook that rejects commits when staged files aren't formatted. Detects the repo's formatter and writes a check-only hook.
user-invocable	true
allowed-tools	Bash Glob Grep Read Write

Pre-Commit Format Guard

Set up a git pre-commit hook that runs the project's code formatter in check-only mode on staged files only, rejecting the commit if any staged file is not properly formatted.

Steps

1. Detect the formatter. Inspect the repo for formatter config files, dependency manifests, and tool configs (e.g. .prettierrc, biome.json, pyproject.toml, Cargo.toml, go.mod, deno.json, etc.). Use your knowledge of the ecosystem to determine which formatter the project uses, what the correct check-only command is, and which file extensions it handles.

2. If no formatter is detected, tell the user you couldn't find one and stop. Don't guess.

3. Check for an existing pre-commit hook. If .git/hooks/pre-commit already exists, show the user its contents and ask how to proceed — don't overwrite silently.

4. Write the hook. Create .git/hooks/pre-commit as a shell script that:

   • Lists staged files with git diff --cached --name-only --diff-filter=d
   • Filters to file extensions relevant to the detected formatter
   • Exits 0 (allows commit) if no matching files are staged
   • Runs the formatter's check command on only the matched staged files
   • On success: prints nothing — the hook should be completely silent when everything is fine
   • On failure: prints the formatter output followed by a clear message explaining what happened and how to fix it
   • Keeps it minimal — just the check, no extra logic

5. Make it executable. chmod +x .git/hooks/pre-commit

6. Report. Tell the user what formatter was detected, what check command the hook runs, and how to bypass it (git commit --no-verify) if needed.