Commit message drafting

The commit history of this repository doubles as its change documentation: each message explains what changed and how. Writing that by hand for every change is tedious, so a hook drafts it, and you review and edit it in the editor before committing.

How it worksCopy Anchor Link

One script, modules/home/vcs/hooks/prepare-commit-msg, serves both version-control systems:

• Git runs it as a global prepare-commit-msg hook. It skips messages that already exist (-m, templates, merges, squashes, amends) and drafts from the staged diff.
• jj runs it through jj-commit-editor, the configured ui.editor. It only drafts when the description is still empty and the editor buffer contains jj's diff below JJ: ignore-rest; that diff comes from the builtin_draft_commit_description_with_diff template. The draft is placed above the existing buffer content.

The script sends the diff to claude --print with the write-commit-msg agent (see AI tooling). The agent runs on Haiku with no tools, no MCP servers, no permission prompts, and no saved session.

Message styleCopy Anchor Link

The prompt asks for:

• An imperative subject of at most 72 characters, preferably type: summary with feat, fix, or chore.
• A body that explains what user-visible behaviour or repository state changed and how the implementation achieves it, including important constraints.
• Lines wrapped at 80 characters, with no Markdown, trailers, or validation results.

When drafting failsCopy Anchor Link

Failures never block a commit; the normal editor opens either way. The reason is written to stderr and also inserted as a comment at the top of the editor buffer (# for Git, JJ: for jj), because the editor clears the terminal. This covers a missing claude executable, a non-zero exit (with Claude Code's own output, such as "Not logged in"), and an empty response.