STARTER_CHARACTER = 🪝

Setup

First, update the reference docs to get the latest from Anthropic:

python ~/.claude/skills/creating-hooks/scripts/update-docs.py

What Hooks Are

Shell commands that execute at lifecycle points in Claude Code. Unlike prompts, hooks are deterministic—they always run when triggered.

Configuration

Hooks live in settings files:

• ~/.claude/settings.json - User settings (all projects)
• .claude/settings.json - Project settings (shared via git)
• .claude/settings.local.json - Local project settings (not committed)

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

Matcher: Pattern to match tool names (case-sensitive)

• Exact match: Write
• Regex: Edit|Write
• All tools: * or omit

Environment variables:

• $CLAUDE_PROJECT_DIR - Absolute path to project root
• $CLAUDE_ENV_FILE - File path for persisting env vars (SessionStart only)

Hook Events

Tool events (matcher applies):

• PreToolUse - Before tool executes
• PostToolUse - After tool completes
• PermissionRequest - Permission dialog shown

Session events:

• SessionStart - Session begins/resumes (matcher: startup/resume/clear/compact)
• SessionEnd - Session ends
• PreCompact - Before compaction (matcher: manual/auto)

Other events:

• UserPromptSubmit - User submits prompt
• Stop - Agent finishes
• SubagentStop - Subagent finishes
• Notification - Alerts sent (matcher: notification type)

Exit Codes

• 0: Success. stdout shown in verbose mode. For UserPromptSubmit/SessionStart, stdout added to context.
• 2: Block. stderr fed to Claude as error message. Blocks the action.
• Other: Non-blocking error. stderr shown to user.

JSON Output

For advanced control, return JSON to stdout with exit code 0:

{
  "continue": false,
  "stopReason": "Message shown when stopping"
}

PreToolUse Control

{
  "hookSpecificOutput": {
    "hookEventName": "PreToolUse",
    "permissionDecision": "allow",
    "permissionDecisionReason": "Auto-approved",
    "updatedInput": { "field": "modified value" }
  }
}

Decisions: "allow" (bypass permission), "deny" (block), "ask" (prompt user)

PostToolUse Feedback

{
  "decision": "block",
  "reason": "Explanation fed to Claude"
}

Stop/SubagentStop Control

{
  "decision": "block",
  "reason": "Must fix X before stopping"
}

Hook Input

Hooks receive JSON via stdin:

{
  "session_id": "abc123",
  "transcript_path": "/path/to/transcript.jsonl",
  "cwd": "/current/dir",
  "permission_mode": "default",
  "hook_event_name": "PreToolUse",
  "tool_name": "Write",
  "tool_input": { "file_path": "/path", "content": "..." }
}

Common Patterns

Auto-format after edit:

{
  "hooks": {
    "PostToolUse": [{
      "matcher": "Edit|Write",
      "hooks": [{
        "type": "command",
        "command": "jq -r '.tool_input.file_path' | xargs -I{} sh -c 'echo {} | grep -q \"\\.ts$\" && npx prettier --write {}'"
      }]
    }]
  }
}

Block dangerous commands:

{
  "hooks": {
    "PreToolUse": [{
      "matcher": "Bash",
      "hooks": [{
        "type": "command",
        "command": "\"$CLAUDE_PROJECT_DIR\"/.claude/hooks/validate-bash.py"
      }]
    }]
  }
}

Inject context on prompt:

{
  "hooks": {
    "UserPromptSubmit": [{
      "hooks": [{
        "type": "command",
        "command": "echo '[REMINDER: Follow TDD]'"
      }]
    }]
  }
}

Desktop notification:

{
  "hooks": {
    "Notification": [{
      "hooks": [{
        "type": "command",
        "command": "osascript -e 'display notification \"Claude needs input\" with title \"Claude Code\"'"
      }]
    }]
  }
}

Hook Scripts

For complex logic, use external scripts. UV single-file format works well:

#!/usr/bin/env -S uv run --script
# /// script
# dependencies = []
# requires-python = ">=3.11"
# ///

import json
import sys

data = json.load(sys.stdin)
tool_input = data.get("tool_input", {})

# Validation logic here

if should_block:
    print("Error message", file=sys.stderr)
    sys.exit(2)

sys.exit(0)

Anti-Patterns

• Using exit code 2 without stderr message (Claude gets no feedback)
• Forgetting to handle JSON parsing errors in scripts
• Blocking without explaining why (Claude will retry the same thing)
• Long-running hooks without timeout (default is 60s)
• Modifying files in PreToolUse (use PostToolUse for modifications)

Reference

• references/anthropic-hooks.md - Complete reference (input schemas, prompt hooks, MCP tools)
• references/anthropic-hooks-guide.md - Quickstart and examples