auto-formatter

name	auto-formatter
description	Generate PostToolUse auto-formatter hook. Triggers: /harn:format, 'auto format', 'formatter hook', 'PostToolUse formatter'

Auto-Formatter PostToolUse Hook Generator

Generate scripts/harness/auto_formatter.sh — a PostToolUse hook that auto-formats files after every Edit or Write tool call.

Steps

1. Detect the project formatter

Scan the project root to determine which formatter to use:

Check	Formatter command
.prettierrc, prettier.config.*, or "prettier" in package.json	npx prettier --write
pyproject.toml containing [tool.black], or .black config file	black
rustfmt.toml or Cargo.toml	rustfmt
go.mod	gofmt -w
None of the above	Skip formatting (echo to stderr and exit 0)

Use Glob and Read to check for these files. Pick the first match in the order listed above.

2. Create the formatter script

Create scripts/harness/auto_formatter.sh with the following structure. Replace {{FORMATTER_CMD}} with the detected formatter command from step 1.

#!/usr/bin/env bash
# Auto-formatter PostToolUse hook — generated by harn:auto-formatter
# Formats files after Edit/Write tool calls

set -euo pipefail

# Read hook JSON payload from stdin
PAYLOAD="$(cat)"

# Extract tool name
TOOL_NAME="$(echo "$PAYLOAD" | jq -r '.tool_name // empty')"

# Only run for Edit or Write tools
if [[ "$TOOL_NAME" != "Edit" && "$TOOL_NAME" != "Write" ]]; then
  exit 0
fi

# Extract file path from parameters
FILE_PATH="$(echo "$PAYLOAD" | jq -r '.parameters.file_path // empty')"

if [[ -z "$FILE_PATH" || ! -f "$FILE_PATH" ]]; then
  exit 0
fi

# Run formatter with timeout
timeout 5 {{FORMATTER_CMD}} "$FILE_PATH" 2>/dev/null && \
  echo "Harn: Formatted $FILE_PATH" >&2 || true

exit 0

Make the script executable:

chmod +x scripts/harness/auto_formatter.sh

3. Wire into .claude/settings.json

Read the existing .claude/settings.json (or create it if missing). Add the PostToolUse hook configuration:

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Edit|Write",
        "hooks": [
          {
            "type": "command",
            "command": "bash scripts/harness/auto_formatter.sh",
            "timeout": 5
          }
        ]
      }
    ]
  }
}

If settings.json already has other hooks, merge this entry into the existing PostToolUse array without overwriting other hook types.

4. Verify

Run the script with a test payload to confirm it exits cleanly:

echo '{"tool_name":"Edit","parameters":{"file_path":"/dev/null"}}' | bash scripts/harness/auto_formatter.sh

Report which formatter was detected and confirm the hook is wired.