Claude Code Hooks — Automate Pre/Post Task Actions

What it is

Claude Code hooks let you define custom scripts that run automatically before or after specific tool calls. When Claude Code writes a file, you can trigger ESLint to fix it. When Claude Code finishes a task, you can run your test suite. Hooks are configured in ~/.claude/settings.json and apply to all projects or per-project in .claude/settings.json.

Hooks are designed for developers who want to enforce code quality standards, automate repetitive checks, and integrate external tools into their Claude Code workflow without manual intervention.

How it saves time or tokens

Without hooks, you manually run linters, formatters, and tests after Claude Code modifies files. Hooks automate this entirely. Every file write triggers your configured checks, catching issues immediately rather than at the end of a long coding session.

Hooks also reduce token waste. If a linter catches an error right after the file is written, Claude Code can fix it in the next turn instead of generating more code on top of a broken file.

How to use

1. Open your Claude Code settings file:

# Global: ~/.claude/settings.json
# Per-project: .claude/settings.json

1. Add hook configurations:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Write",
        "hooks": ["eslint --fix $CLAUDE_FILE_PATH"]
      }
    ],
    "PostToolUse": [
      {
        "matcher": "Write",
        "hooks": ["prettier --write $CLAUDE_FILE_PATH"]
      }
    ]
  }
}

1. Claude Code will now run these hooks automatically when the matched tool is used.

Example

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": ["echo 'About to run a bash command'"]
      }
    ],
    "PostToolUse": [
      {
        "matcher": "Write",
        "hooks": [
          "eslint --fix $CLAUDE_FILE_PATH",
          "prettier --write $CLAUDE_FILE_PATH"
        ]
      },
      {
        "matcher": "Bash",
        "hooks": ["echo 'Bash command completed'"]
      }
    ],
    "PostTask": [
      {
        "matcher": ".*",
        "hooks": ["npm test 2>&1 | tail -20"]
      }
    ]
  }
}

This configuration lints and formats after every file write, logs bash command execution, and runs the test suite after each completed task.

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Common pitfalls

• Hook scripts must be available on the system PATH. If ESLint is installed locally in node_modules, use npx eslint instead of bare eslint in the hook command.
• Long-running hooks block Claude Code's execution. Keep hooks fast (under 5 seconds). Move heavy operations like full test suites to PostTask rather than PostToolUse.
• The $CLAUDE_FILE_PATH variable is only available for file-related tools like Write. Using it with non-file tools like Bash will produce an empty string or an error.