Cm

Commit Message Formatting

Invocation

• With commit hash: generate from that commit's diff.
• Otherwise: generate from the current staged/unstaged diff.

Always derive the message from the diff only.

Arguments

Detect from the user's words; they combine (e.g. micro apply). Default (none given): full-length message, presented without committing.

Verbosity — mutually exclusive, default full length:

• micro → summary line only, no body. Add ! + BREAKING CHANGE: footer

only when the change is breaking.

• mini → summary line plus one short paragraph explaining the single

most important why. Refs: only if a breaking change applies.

Commit control:

• apply → commit the generated message directly via heredoc without asking —

git commit, or git commit --amend for a hash invocation.

Describe changes only

Write for any reader of git log, not for someone who sat in planning or review. They only see the diff and the message.

• State what changed and why in product/code terms (bugs fixed,

behavior, API, tests).

• Match detail to reader impact. Describe user-facing changes (behavior, API,

bug fixes, CLI/output) precisely; give mechanical cleanups (lint, formatting, renames, dep bumps, test tweaks) a one-sentence summary, not a per-edit account. When a commit mixes both, lead with the user-facing change and fold the cleanup into one closing sentence.

• A body is optional. If the summary line already conveys the change and

there is nothing user-facing to explain, omit the body entirely rather than manufacturing detail.

• Do not reference internal process the reader cannot know — remediation

phases, review plans, skill names, "as discussed", ticket/session context, project milestones ("phase 1"), or tmp/*-plan.md paths — unless the diff itself only touches those files.

• Prefer concrete symbols and files: ` TargetNameFromContext `,

` Prepare `, not umbrella slogans that hide the actual edits.

If the diff mixes unrelated edits (e.g. IDE config + library fix), say so in the body or ask to split commits—still without process jargon.

Structure

[()][!]: 

[body]

[BREAKING CHANGE: ]
[Refs: , ...]

Summary Line

• Type: feat, fix, refactor, perf, test, docs, style, build,

ci, chore, revert

• Scope: optional, short noun
• Description: imperative, lowercase, no period, ideally ≤ 50 chars (hard 72)

Breaking Changes

If the change affects exported symbols or observable behavior for callers:

• Add ! in the summary.

• Add mandatory BREAKING CHANGE: footer describing the impact and migration.

Body (Kernel Style)

• Wrap at 72 columns.
• Imperative mood.
• Explain why the change was made (the diff shows what).
• Use backticks for symbol references: ` xrr.FieldErrors `,

` WithCause `.

• When referencing prior commits: Commit ("summary") ...

High-clarity standard: Write bodies with godoc-level precision — the smallest body that fully explains the user-facing change and its motivation.

Footers

Only two are allowed:

• BREAKING CHANGE: ... (when ! is used)
• Refs: [, ...]

Omit all other trailers.

Never add an AI Co-Authored-By trailer (e.g. Co-Authored-By: ).

Output

Present the full message in a fenced code block with zero leading whitespace on every line.

The message is deliverable. Run git commit / git commit --amend (via heredoc) only when the invocation asks to commit or amend — the apply argument, or wording like "and amend it". Otherwise, never propose committing; the user decides when.

Report tersely: no preamble or narration; state each fact once; don't restate output the user can already see.

Self-learning

Read this skill's lessons and obey them: sibling LESSONS.md, else $HOME/.agent-data/ctx42-skills/lessons/craft/cm.md when this directory is read-only. On a correction or self-caught mistake, append a one-line rule to whichever is writable (creating it) and report where.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: ctx42
• Source: ctx42/skills
• License: MIT

Install and usage instructions live in the source repository linked above.