Diff Skill

Analyze the current branch's diff against its merge-base with main.

Usage

/diff             # Stat overview (default)
/diff full        # Full patch output
/diff files       # List changed files only
/diff commits     # List commits on this branch
/diff overlap feature/other-branch  # Check for file overlap with another branch

⚠️ Critical: Always Use Merge-Base

# CORRECT — only shows YOUR changes
git diff $(git merge-base main HEAD)..HEAD

# WRONG — includes main's forward progress as noise
git diff main

Instructions

Determine action from $ARGUMENTS:

Argument	Action
stat or (empty)	git --no-pager diff $(git merge-base main HEAD)..HEAD --stat
full	git --no-pager diff $(git merge-base main HEAD)..HEAD | head -200
files	git --no-pager diff $(git merge-base main HEAD)..HEAD --name-only
commits	git --no-pager log $(git merge-base main HEAD)..HEAD --oneline
overlap <branch>	Cross-branch file overlap check (see below)

Cross-Branch Overlap Check:

BRANCH_A_FILES=$(git --no-pager diff $(git merge-base main HEAD)..HEAD --name-only)
BRANCH_B_FILES=$(git --no-pager diff $(git merge-base main <other-branch>)..<other-branch> --name-only)
# Show files that appear in both
comm -12 <(echo "$BRANCH_A_FILES" | sort) <(echo "$BRANCH_B_FILES" | sort)

Report Format

For stat/default mode, report:

1. Files changed (count)
2. Lines added / removed
3. Any files that look unexpectedly large (>200 lines changed)
4. Any test files missing for changed source files