Auto-Format Hook

Automatically formats files after Claude edits them, keeping code consistently styled without manual intervention.

What It Does

This PostToolUse hook fires after every Edit, MultiEdit, or Write tool call. It detects the file type and runs the appropriate formatter:

Behavior

Python files:

1. Runs ruff format to format the code
2. Runs ruff check --fix to auto-fix linting issues
3. Preserves unused imports (allows Claude to add imports before using them)

Go files:

1. Runs goimports -w to format and manage imports

Other files:

1. Runs prettier --write for supported file types
2. Prettier auto-detects supported formats

The hook exits successfully even if formatting fails — it won't block Claude's work.

Prerequisites

Install the formatters you need:

# Python
pip install ruff

# Go
go install golang.org/x/tools/cmd/goimports@latest

# JavaScript/TypeScript/etc
npm install -g prettier

Installation

Step 1: Copy the hook

cp auto-format.sh ~/.claude/hooks/auto-format.sh
chmod +x ~/.claude/hooks/auto-format.sh

Step 2: Configure in settings.json

Add to ~/.claude/settings.json:

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Edit|MultiEdit|Write",
        "hooks": [
          {
            "type": "command",
            "command": "$HOME/.claude/hooks/auto-format.sh"
          }
        ]
      }
    ]
  }
}

Configuration

The hook uses mise for tool version management if available. It also checks common paths:

• $HOME/.local/share/mise/shims
• /usr/local/bin
• /opt/homebrew/bin
• $HOME/go/bin

If your tools are installed elsewhere, add the paths to the script.

Customization

Adding more formatters

Edit the script to add support for other file types:

# Example: Add Rust formatting
if [[ "$file_path" == *.rs ]]; then
  rustfmt "$file_path" 2>&1
  exit $?
fi

Changing formatter options

Modify the formatter commands in the script. For example, to change ruff's line length:

ruff format --line-length 100 "$file_path" 2>&1

Disabling for specific file types

Add early returns for file types you don't want formatted:

# Skip formatting for generated files
if [[ "$file_path" == *_generated.* ]]; then
  exit 0
fi

Troubleshooting

Formatter not found

The hook silently skips if a formatter isn't installed. Check that the formatter is in your PATH:

which ruff
which goimports
which prettier

Wrong formatter version

If using mise, ensure the correct version is activated. The hook runs eval "$(mise activate bash)" to load mise.

Formatting conflicts with project config

The hook uses default formatter settings. If your project has specific config (.prettierrc, ruff.toml, etc.), the formatter should pick it up automatically.

Related Hooks

• change-summary — Runs TypeScript type checking at session end (complements this hook by checking types after all edits are done)