Auto Format Hook (auto-format)

Core Concepts / How It Works

The PostToolUse hook runs immediately after Claude uses a specific tool. When file-modifying tools such as Write, Edit, or MultiEdit complete, the hook is triggered and the specified command is automatically executed.

Exit code behavior:

• 0: Success, Claude task continues
• 1: Error, stderr delivered to Claude (task not interrupted)
• 2: Task blocked (not used in this hook)

One-Line Summary

Automatically runs Prettier and ESLint every time Claude modifies a file, keeping code style consistently clean.

Getting Started

Step 1: Create the Script File

mkdir -p scripts
cat > scripts/auto-format.sh << 'EOF'
#!/usr/bin/env bash
# Automatically apply formatting to files modified by Claude
set -e

FILE="$1"
if [ -z "$FILE" ] || [ ! -f "$FILE" ]; then
  exit 0
fi

export PATH="./node_modules/.bin:$PATH"

case "$FILE" in
  *.ts|*.tsx|*.js|*.jsx|*.json|*.css|*.md)
    prettier --write "$FILE" 2>&1 || true
    ;;
  *) exit 0 ;;
esac

case "$FILE" in
  *.ts|*.tsx|*.js|*.jsx)
    eslint --fix "$FILE" 2>&1 || true
    ;;
esac

exit 0
EOF
chmod +x scripts/auto-format.sh

Step 2: Configure .claude/settings.json

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Write|Edit|MultiEdit",
        "hooks": [
          {
            "type": "command",
            "command": "bash scripts/auto-format.sh \"$CLAUDE_TOOL_INPUT_PATH\""
          }
        ]
      }
    ]
  }
}

Step 3: Verify Behavior

When you ask Claude to modify a file, Prettier + ESLint will run automatically.

Practical Example

Scenario: Eliminating the inconvenience of having to manually run pnpm format after every component Claude generates for the Next.js 15 "Student Club Notice Board" project.

Request to Claude:
"Create a notice list component for the Student Club Notice Board.
File: components/notices/NoticeList.tsx"

The moment Claude saves the file → auto-format.sh runs automatically → Prettier + ESLint applied. The developer no longer needs to manually run pnpm format.

# Path setup for pnpm workspace monorepo
# Add to the top of scripts/auto-format.sh:
export PATH="./node_modules/.bin:$PATH"

Learning Points / Common Pitfalls

• File path injection method: In a PostToolUse hook, the modified file path is passed via the CLAUDE_TOOL_INPUT_PATH environment variable. Always verify the environment variable list in the official documentation.
• No infinite loop concern: Even if Prettier modifies the file, PostToolUse is not triggered again. Hooks only respond to Claude tool executions.
• Beware of slow formatters: ESLint --fix can take several seconds on large files. If it's slow, restrict to Write only or add a condition to exclude large files.
• Windows Git Bash environment: Path separator \ issues can occur. Add slash conversion handling to the FILE variable.

Related Resources

• Block Dangerous Commands (block-dangerous) — Hook to block destructive commands
• Auto Test (auto-test) — Hook to automatically run tests
• Completion Notification (notify-complete) — Desktop notification when a task completes
• Full Hooks Recipe List — All Hooks recipes

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Field	Value
Source URL	https://docs.anthropic.com/en/docs/claude-code/hooks
Author / Source	Anthropic
License	CC BY 4.0
Translation Date	2026-04-12