Creating Claude Hooks

Available Events

Hook Format Requirements

File Location

Project hooks:

.claude/hooks/session-start
.claude/hooks/user-prompt-submit

User-global hooks:

~/.claude/hooks/session-start
~/.claude/hooks/tool-call

Executable Requirements

Every hook MUST:

1. Have a shebang line:

#!/bin/bash
# or
#!/usr/bin/env node
# or
#!/usr/bin/env python3

2. Be executable:

chmod +x .claude/hooks/session-start

3. Handle JSON input from stdin:

#!/bin/bash
INPUT=$(cat)
FILE=$(echo "$INPUT" | jq -r '.input.file_path // empty')

4. Exit with appropriate code:

exit 0  # Success
exit 2  # Block operation
exit 1  # Error (logs but continues)

Input/Output Format

JSON Input Structure

Hooks receive JSON via stdin with event-specific data:

{
  "event": "tool-call",
  "timestamp": "2025-01-15T10:30:00Z",
  "session_id": "abc123",
  "current_dir": "/path/to/project",
  "input": {
    "file_path": "/path/to/file.ts",
    "command": "npm test",
    "old_string": "...",
    "new_string": "..."
  }
}

Stdout Output

• Normal output shows in transcript
• Empty output runs silently
• Use stderr (>&2) for errors

Exit Codes

Schema Validation

Hooks should validate against the JSON schema:

Schema URL: https://github.com/pr-pm/prpm/blob/main/packages/converters/schemas/claude-hook.schema.json

Required frontmatter fields:

• name - Hook identifier (lowercase, hyphens only)
• description - What the hook does
• event - Event type (optional, inferred from filename)
• language - bash, typescript, javascript, python, binary (optional)
• hookType: "hook" - For round-trip conversion

Common Mistakes

Basic Hook Examples

Shell Script Hook

#!/bin/bash
# .claude/hooks/session-start

# Log session start
echo "Session started at $(date)" >> ~/.claude/session.log

# Check environment
if ! command -v node &> /dev/null; then
  echo "Warning: Node.js not installed" >&2
fi

# Output to user
echo "Development environment ready"
exit 0

TypeScript Hook

#!/usr/bin/env node
// .claude/hooks/user-prompt-submit

import { readFileSync } from 'fs';

// Read JSON from stdin
const input = readFileSync(0, 'utf-8');
const data = JSON.parse(input);

// Validate prompt
if (data.prompt.includes('API_KEY')) {
  console.error('Warning: Prompt may contain secrets');
  process.exit(2); // Block
}

console.log('Prompt validated');
process.exit(0);

Best Practices

1. Keep Hooks Fast

Target < 100ms for PreToolUse hooks:

• Cache results where possible
• Run heavy operations in background
• Use specific matchers, not wildcards

2. Handle Errors Gracefully

# Check dependencies exist
if ! command -v jq &> /dev/null; then
  echo "jq not installed, skipping" >&2
  exit 0
fi

# Validate input
FILE=$(echo "$INPUT" | jq -r '.input.file_path // empty')
if [[ -z "$FILE" ]]; then
  echo "No file path provided" >&2
  exit 1
fi

3. Use Shebangs

Always start with shebang:

#!/bin/bash
#!/usr/bin/env node
#!/usr/bin/env python3

4. Secure Sensitive Files

BLOCKED=(".env" ".env.*" "*.pem" "*.key")
for pattern in "${BLOCKED[@]}"; do
  case "$FILE" in
    $pattern)
      echo "Blocked: $FILE is sensitive" >&2
      exit 2
      ;;
  esac
done

5. Quote All Variables

# WRONG - breaks on spaces
prettier --write $FILE

# RIGHT - handles spaces
prettier --write "$FILE"

6. Log for Debugging

LOG_FILE=~/.claude-hooks/debug.log

# Log to file
echo "[$(date)] Processing $FILE" >> "$LOG_FILE"

# Log to stderr (shows in transcript)
echo "Hook running..." >&2

Publishing as PRPM Package

Package Structure

my-hook/
├── prpm.json          # Package manifest
├── HOOK.md            # Hook documentation
└── hook-script.sh     # Hook executable

prpm.json

{
  "name": "@username/hook-name",
  "version": "1.0.0",
  "description": "Brief description shown in search",
  "author": "Your Name",
  "format": "claude",
  "subtype": "hook",
  "tags": ["automation", "security", "formatting"],
  "main": "HOOK.md"
}

HOOK.md Format

---