format-on-edit

PostToolUse hook that formats the file Claude just edited or wrote.

Trigger

• Event: PostToolUse
• Matcher: Edit|Write|MultiEdit

What it does

Detects the file extension and runs the matching formatter when available:

• .ts/.tsx/.js/.jsx/.mjs/.cjs/.json/.md/.css/.scss/.html/.yml/.yaml → prettier --write
• .py → ruff format (preferred) or black --quiet
• .go → gofmt -w
• .rs → rustfmt --quiet

Other extensions are left alone.

Exit codes

Always 0. Failures are logged to stderr but never block the session.

Kill switches

• CLAUDE_HARNESSES_DISABLE=1

Notes

Requires jq to extract the file path from the tool input envelope. If jq is missing, the hook silently no-ops.

Pack: verification-pack