🤖 Ghostwritten by Claude Opus 4.5 · Curated by Tom Hundley

This article was written by Claude Opus 4.5 and curated for publication by Tom Hundley.

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Hooks & Automation Deep Dive

Hooks are Claude Code's event-driven automation system. They let you run scripts before Claude uses a tool, after it makes changes, when a session starts, or when Claude finishes a task. Think of them as automated quality gates that enforce your team's standards without manual intervention.

In Part 1, we covered CLAUDE.md for persistent context. Hooks take that further—they're not just instructions, they're executable enforcement.

Why Hooks Matter

Without hooks, you're constantly checking Claude's work manually:

• Did it format the file correctly?
• Did it run the linter?
• Did it accidentally edit a protected file?

With hooks, these checks happen automatically. A PostToolUse hook can auto-format TypeScript files after every edit. A PreToolUse hook can block writes to .env files. A Stop hook can run your test suite before Claude considers a task complete.

Hook Configuration

Hooks are defined in your settings files at any scope:

• ~/.claude/settings.json — User-level (all projects)
• .claude/settings.json — Project-level (team-shared)
• .claude/settings.local.json — Local project (personal, gitignored)

The structure follows this pattern:

{
  "hooks": {
    "EventName": [
      {
        "matcher": "ToolPattern",
        "hooks": [
          {
            "type": "command",
            "command": "your-script-here"
          }
        ]
      }
    ]
  }
}

Hook Events: The Complete List

Claude Code fires hooks at specific lifecycle points. Here's every event you can tap into:

Tool-Related Events

Event	When It Fires	Matchers	Use Case
PreToolUse	Before a tool executes	Yes	Block dangerous operations, validate inputs
PostToolUse	After a tool succeeds	Yes	Auto-format, run linters, validate output
PermissionRequest	When permission dialog shows	Yes	Auto-approve or auto-deny based on context

Session Events

Event	When It Fires	Matchers	Use Case
SessionStart	On startup, resume, clear, compact	By trigger type	Load context, install dependencies
SessionEnd	When session terminates	No	Cleanup, logging

Agent Events

Event	When It Fires	Matchers	Use Case
Stop	When main agent finishes a turn	No	Run tests, quality gates, summarize status
SubagentStop	When a subagent finishes	No	Validate subagent output
UserPromptSubmit	Before processing user input	No	Add context, validate, block unsafe prompts

Special Events

Event	When It Fires	Matchers	Use Case
Notification	On system notifications	By notification type	Custom alerts, logging
PreCompact	Before context is compacted	By trigger (manual/auto)	Preserve critical context

Matcher Patterns

Matchers determine which tools trigger your hooks. They only apply to PreToolUse, PostToolUse, and PermissionRequest events.

Simple Matching

Match a specific tool exactly:

{
  "matcher": "Write"
}

This triggers only for the Write tool.

Multiple Tools

Use pipe syntax to match multiple tools:

{
  "matcher": "Write|Edit"
}

Triggers for either Write or Edit.

Wildcard

Match all tools:

{
  "matcher": "*"
}

Or simply omit the matcher field for the same effect.

MCP Tool Matching

For MCP server tools, match the full tool name:

{
  "matcher": "mcp__github__create_pull_request"
}

Two Hook Types

Command Hooks

Run shell commands when triggered:

{
  "type": "command",
  "command": "/path/to/script.sh",
  "timeout": 60
}

The timeout is in seconds (default: 60). Commands receive JSON input via stdin and communicate results via exit codes and stdout.

Prompt Hooks

Use an LLM to evaluate the situation:

{
  "type": "prompt",
  "prompt": "Evaluate if this code change follows our security guidelines: $ARGUMENTS",
  "timeout": 30
}

Prompt hooks are powerful for nuanced decisions that can't be codified in a simple script.

Hook Input and Output

Input (stdin)

Every hook receives a JSON object via stdin:

{
  "session_id": "abc123",
  "transcript_path": "/path/to/transcript.jsonl",
  "cwd": "/working/directory",
  "permission_mode": "default",
  "hook_event_name": "PreToolUse",
  "tool_name": "Write",
  "tool_input": {
    "file_path": "/src/app/page.tsx",
    "content": "..."
  },
  "tool_use_id": "toolu_..."
}

Your script can parse this to make decisions about what to do.

Output (exit codes)

Exit Code	Meaning	Behavior
0	Success	Continue normally; stdout shown in verbose mode
2	Blocking error	Block the action; stderr shown to Claude
Other	Non-blocking error	Continue; error shown in verbose mode

Structured JSON Output

For advanced control, return JSON in stdout:

{
  "decision": "block",
  "reason": "File is in protected directory",
  "continue": false,
  "systemMessage": "Warning: Attempted to modify protected file"
}

For PreToolUse, you can even modify the tool input:

{
  "hookSpecificOutput": {
    "updatedInput": {
      "file_path": "/corrected/path.tsx"
    }
  }
}

Practical Examples

Auto-Format TypeScript After Edits

Run Prettier automatically after any TypeScript file is edited:

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Edit|Write",
        "hooks": [
          {
            "type": "command",
            "command": "jq -r '.tool_input.file_path' | { read file_path; if echo \"$file_path\" | grep -qE '\\.tsx?$'; then npx prettier --write \"$file_path\"; fi; }"
          }
        ]
      }
    ]
  }
}

Block Protected Files

Prevent edits to sensitive files:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Edit|Write",
        "hooks": [
          {
            "type": "command",
            "command": "python3 -c \"import json, sys; d=json.load(sys.stdin); p=d.get('tool_input',{}).get('file_path',''); sys.exit(2 if any(x in p for x in ['.env','.git/','package-lock.json']) else 0)\""
          }
        ]
      }
    ]
  }
}

Exit code 2 blocks the action and tells Claude why it couldn't proceed.

Log All Bash Commands

Create an audit trail of every command Claude runs:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": [
          {
            "type": "command",
            "command": "jq -r '\"[\\(.session_id)] \\(.tool_input.command) - \\(.tool_input.description // \"No description\")\"' >> ~/.claude/bash-command-log.txt"
          }
        ]
      }
    ]
  }
}

End-of-Turn Quality Gate

Run tests and linting before Claude considers a task complete:

{
  "hooks": {
    "Stop": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "npm run lint && npm run type-check"
          }
        ]
      }
    ]
  }
}

If the command fails (non-zero exit), Claude is informed and can attempt to fix the issues.

Session Startup Dependencies

Ensure dependencies are installed when starting a session:

{
  "hooks": {
    "SessionStart": [
      {
        "matcher": "startup",
        "hooks": [
          {
            "type": "command",
            "command": "npm install --silent"
          }
        ]
      }
    ]
  }
}

The matcher for SessionStart accepts: startup, resume, clear, compact.

Auto-Approve Safe Operations

Skip permission prompts for known-safe tools:

{
  "hooks": {
    "PermissionRequest": [
      {
        "matcher": "Read|Glob|Grep",
        "hooks": [
          {
            "type": "command",
            "command": "echo '{\"hookSpecificOutput\":{\"permissionDecision\":\"allow\"}}'"
          }
        ]
      }
    ]
  }
}

Advanced Patterns

Chaining Multiple Hooks

Multiple hooks on the same event run in parallel:

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Write",
        "hooks": [
          { "type": "command", "command": "npx prettier --write $(jq -r '.tool_input.file_path')" },
          { "type": "command", "command": "npx eslint --fix $(jq -r '.tool_input.file_path')" }
        ]
      }
    ]
  }
}

Both Prettier and ESLint run simultaneously after a file is written.

Conditional Logic with Prompt Hooks

For nuanced decisions, use prompt hooks:

{
  "hooks": {
    "Stop": [
      {
        "hooks": [
          {
            "type": "prompt",
            "prompt": "Review the changes made in this session. Are there any security concerns, breaking changes, or missing test coverage? If yes, explain what additional work is needed."
          }
        ]
      }
    ]
  }
}

Environment Variables

Hooks have access to CLAUDE_PROJECT_DIR for the project root. Use this for portable scripts:

#!/bin/bash
cd "$CLAUDE_PROJECT_DIR"
npm run lint

Debugging Hooks

Verbose Mode

Run Claude with verbose output to see hook execution:

claude --verbose

Test Your Hooks

Before deploying a hook, test it manually:

echo '{"tool_name":"Write","tool_input":{"file_path":"/test.ts"}}' | ./your-hook.sh
echo $?  # Check exit code

Common Pitfalls

1. Forgetting to handle JSON parsing: Always use jq or proper JSON parsing
2. Infinite loops: A hook that triggers another hook that triggers the first
3. Slow hooks blocking workflow: Set appropriate timeouts
4. Hardcoded paths: Use $CLAUDE_PROJECT_DIR for portability

Hook Configuration Best Practices

1. Start with logging hooks: Add visibility before enforcement
2. Test in local settings first: Use .claude/settings.local.json before committing
3. Keep hooks fast: Slow hooks hurt productivity
4. Document your hooks: Future you (and teammates) will thank you
5. Use meaningful exit codes: Exit 2 for intentional blocks, others for errors

What's Next

Hooks automate quality enforcement, but Claude Code's real power comes from connecting to external systems. In Part 3, we'll explore MCP Server Integration—connecting Claude to GitHub, databases, and any API you can imagine.

Up next: Part 3: MCP Server Integration — Extend Claude Code's capabilities by connecting to external services through the Model Context Protocol.

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Sources

• Claude Code Hooks Documentation
• Hooks Getting Started Guide
• Claude Code Hooks Mastery Examples
• Claude Code Best Practices