Produce the part of a commit message that is not recoverable from the diff. Whoever reads the commit later already has the diff; what they do not have is what the running code does that forced the change, which call reaches the broken path, what breaks for someone who updates, when the defect was introduced, and what was noticed and deliberately left alone.

Output the body text only — no subject line, no preamble, no commentary. The caller owns the subject.

git status --short
git diff
git diff --cached

Read the actual hunks, not just the stat. You need the semantics of the edit to weigh it.

Derive (Step 3) when the change alters behaviour, a contract, a public type, or a default, or when it fixes a defect.

Report (Step 4) when the change is mechanical (a rename, a mass import rewrite, a formatting pass), generated (lockfiles, build output, snapshots, regenerated trees), or purely additive scaffolding with no behaviour to describe.

Weight decides, not diff size. A one-line semantic fix derives. A rename across 30 files is one fact and reports.

A mixed change is weighed on its heaviest part: derive for the behaviour change, and give the mechanical remainder one line.

Read and follow it. It carries the seven questions, the table of which code is authoritative for each kind of change, the evidence rules, the writing rules, and three worked bodies.

Three things from it that decide whether the output is worth its cost:

When the caller hands over design rationale pulled out of source comments — a cleanup pass reporting "Suggested for commit message" — that text answers why the code is built this way. Fold it into the first paragraph. Do not append it as a block at the end.

Output paragraphs, blank-line separated, wrapped at 80.

For mechanical and generated changes, one sentence per line, two or three lines, past tense, no blank lines, no bullets:

Renamed `useToggle` to `useToggleGroup` across 31 call sites.
Regenerated the Codex wrapper symlink trees.

Name what a reader cannot see from the file list — the count, the mechanism, the tool that generated it. Do not invent a rationale paragraph for a change that has none.

Check the project's , , or for commit conventions. A house style that caps body width, forbids paragraphs, prescribes a template, or requires another language overrides both formats above. Answer as many questions as it has room for, in order, and drop the rest.

Add only when it applies, after a blank line:

The body ends on its last paragraph. Emit no "Drafted with AI" or "Generated with" line, no session or transcript link, no line, no rule, no badge, and no trailer crediting an assistant. Never copy one forward from an existing message — a trailer already in the log is not licence to repeat it.

This skill returns text and never runs , so it cannot be the only guard. The skill that assembles the final message strips these on the way to the commit — see step 5 and Step 3.