Creating Hooks

Creating Hooks

Overview

Hooks are event-driven scripts that execute at specific points in Claude Code's lifecycle. They receive JSON input with session data and event-specific information, enabling validation, environment checks, and automated workflows.

When to Use

• Validate tool inputs before execution (PreToolUse)
• Verify outputs after tool completion (PostToolUse)
• Check environment state before processing prompts (UserPromptSubmit)
• Initialize resources at session startup (SessionStart)
• Clean up resources at session end (SessionEnd)

Hook Types

PreToolUse

Runs before tool execution. Use for:

• Environment validation (check dependencies exist)
• Input validation (verify paths, parameters)
• Permission checks (ensure access rights)
• State verification (git status, working directory)

PostToolUse

Runs after tool completion. Use for:

• Output validation (verify file changes)
• Quality checks (run linters, formatters)
• Side effects (update logs, metrics)
• Failure detection (check for errors)

UserPromptSubmit

Runs before processing user input. Use for:

• Input sanitization
• Context injection
• Usage tracking
• Cost estimation

SessionStart

Runs at session initialization. Use for:

• Environment setup
• Dependency checks
• Configuration loading
• Initialization logging

SessionEnd

Runs at session termination. Use for:

• Cleanup tasks
• Result archiving
• Metrics reporting
• Resource deallocation

JSON Input Structure

Common Fields (All Events)

{
  "session_id": "unique-session-identifier",
  "transcript_path": "/path/to/conversation.json",
  "cwd": "/current/working/directory",
  "hook_event_name": "PreToolUse|PostToolUse|UserPromptSubmit|SessionStart|SessionEnd"
}

Event-Specific Fields

PreToolUse:

{
  "tool_name": "Bash",
  "tool_input": {
    "command": "pytest tests/",
    "description": "Run test suite"
  }
}

PostToolUse:

{
  "tool_name": "Edit",
  "tool_input": {
    "file_path": "/path/to/file.py",
    "old_string": "...",
    "new_string": "..."
  },
  "tool_response": {
    "success": true,
    "message": "File edited successfully"
  }
}

UserPromptSubmit:

{
  "prompt": "User's input text here"
}

SessionStart:

{
  "source": "startup|resume"
}

SessionEnd:

{
  "reason": "user_exit|error|timeout"
}

Configuration

Path Resolution with CLAUDE_PROJECT_DIR

Always use $CLAUDE_PROJECT_DIR to reference hook scripts. Claude Code sets this environment variable to your project root, ensuring hooks work regardless of the current working directory.

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Write|Edit",
        "hooks": [
          {
            "type": "command",
            "command": "\"$CLAUDE_PROJECT_DIR\"/.claude/hooks/check-style.sh"
          }
        ]
      }
    ]
  }
}

Why this matters:

• Claude's CWD can change during execution
• Relative paths like ./scripts/hook.sh become fragile
• $CLAUDE_PROJECT_DIR always points to your project root
• Ensures hooks work from any directory

Note: The environment variable is only available when Claude Code spawns the hook command.

Settings File Integration

Add hooks to .claude/settings.json:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "*",
        "hooks": [
          {
            "type": "command",
            "command": "\"$CLAUDE_PROJECT_DIR\"/scripts/pre-tool-hook.sh"
          }
        ]
      }
    ],
    "PostToolUse": [
      {
        "matcher": "Edit|Write",
        "hooks": [
          {
            "type": "command",
            "command": "\"$CLAUDE_PROJECT_DIR\"/scripts/validate-edits.sh"
          }
        ]
      }
    ]
  }
}

Matcher Patterns

• * - Match all tools
• Edit - Match specific tool
• Edit|Write - Match multiple tools
• Bash(git:*) - Match tool with pattern

Process

1. Identify the trigger event - Which lifecycle point needs automation?
2. Design hook script - What validation or action is needed?
3. Parse JSON input - Extract relevant fields from stdin
4. Implement logic - Perform checks or automation
5. Return exit code - 0 for success, non-zero blocks execution
6. Add to settings.json - Configure hook with matcher
7. Test hook - Trigger event and verify behavior

Examples

Example 1: Pre-Tool Git Status Check

Use case: Warn if working directory is dirty before file operations

#!/bin/bash
# scripts/pre-tool-git-check.sh

INPUT=$(cat)
TOOL_NAME=$(echo "$INPUT" | jq -r '.tool_name')

if [[ "$TOOL_NAME" == "Edit" || "$TOOL_NAME" == "Write" ]]; then
  if ! git diff-index --quiet HEAD --; then
    echo "⚠️  Warning: Uncommitted changes in working directory"
    echo "Consider committing before editing files"
  fi
fi

exit 0  # Don't block, just warn

Configuration:

{
  "PreToolUse": [{
    "matcher": "Edit|Write",
    "hooks": [{"type": "command", "command": "\"$CLAUDE_PROJECT_DIR\"/scripts/pre-tool-git-check.sh"}]
  }]
}

Example 2: Post-Tool Code Formatting

Use case: Auto-format Python files after editing

#!/bin/bash
# scripts/post-edit-format.sh

INPUT=$(cat)
TOOL_NAME=$(echo "$INPUT" | jq -r '.tool_name')
FILE_PATH=$(echo "$INPUT" | jq -r '.tool_input.file_path')

if [[ "$TOOL_NAME" == "Edit" && "$FILE_PATH" == *.py ]]; then
  black "$FILE_PATH" --quiet
  echo "✅ Formatted $FILE_PATH with black"
fi

exit 0

Example 3: Session Start Environment Check

Use case: Verify dependencies exist before starting

#!/bin/bash
# scripts/session-start-check.sh

MISSING=()

command -v python >/dev/null || MISSING+=("python")
command -v git >/dev/null || MISSING+=("git")
command -v jq >/dev/null || MISSING+=("jq")

if [ ${#MISSING[@]} -gt 0 ]; then
  echo "❌ Missing dependencies: ${MISSING[*]}"
  exit 1  # Block session
fi

echo "✅ All dependencies available"
exit 0

Best Practices

Path Configuration

• ✅ Do: Always use $CLAUDE_PROJECT_DIR for hook script paths
• ✅ Do: Quote the path: "$CLAUDE_PROJECT_DIR"/scripts/hook.sh
• ❌ Don't: Use relative paths like ./scripts/hook.sh (breaks if CWD changes)
• ❌ Don't: Use absolute paths like /home/user/project/... (not portable)

Design

• ✅ Do: Keep hooks fast (<1 second)
• ✅ Do: Use specific matchers to reduce overhead
• ✅ Do: Return non-zero to block execution
• ❌ Don't: Perform expensive operations in hooks
• ❌ Don't: Block on warnings (use exit 0)

Error Handling

• ✅ Do: Provide clear error messages
• ✅ Do: Log hook failures for debugging
• ✅ Do: Handle missing JSON fields gracefully
• ❌ Don't: Fail silently
• ❌ Don't: Assume JSON structure without validation

JSON Parsing

• ✅ Do: Use jq for robust JSON parsing
• ✅ Do: Provide default values for optional fields
• ✅ Do: Validate required fields exist
• ❌ Don't: Use regex to parse JSON
• ❌ Don't: Assume fields are always present

Performance

• ✅ Do: Exit early when hook doesn't apply
• ✅ Do: Cache expensive checks when possible
• ✅ Do: Use narrow matchers to reduce invocations
• ❌ Don't: Run hooks on every tool unconditionally
• ❌ Don't: Perform network requests without caching

Integration Patterns

With Git

# Check for uncommitted changes
git diff-index --quiet HEAD --

# Get current branch
git branch --show-current

# Check if file is tracked
git ls-files --error-unmatch "$FILE_PATH"

With Linters

# Python
pylint "$FILE_PATH" --score=no --msg-template='{msg_id}: {msg}'

# JavaScript
eslint "$FILE_PATH" --format=compact

# Go
golint "$FILE_PATH"

With Testing

# Run tests related to changed file
pytest "tests/test_${FILENAME}" --quiet

# Fast syntax check only
python -m py_compile "$FILE_PATH"

Common Use Cases

Validation Hooks

• Verify environment variables set
• Check file permissions
• Validate input parameters
• Ensure dependencies installed

Quality Hooks

• Run linters on edited files
• Format code automatically
• Check test coverage
• Validate commit messages

Workflow Hooks

• Update documentation
• Regenerate configuration
• Sync database schemas
• Trigger CI/CD pipelines

Monitoring Hooks

• Log tool usage
• Track session metrics
• Report errors
• Update dashboards

Anti-patterns

• ❌ Don't: Use relative or absolute paths for hook commands

  • ✅ Do: Use $CLAUDE_PROJECT_DIR for portable, reliable paths

• ❌ Don't: Use hooks for long-running tasks

  • ✅ Do: Keep hooks under 1 second

• ❌ Don't: Block on non-critical checks

  • ✅ Do: Use exit 0 for warnings

• ❌ Don't: Parse JSON with string manipulation

  • ✅ Do: Use jq for reliable parsing

• ❌ Don't: Match all tools without filtering

  • ✅ Do: Use specific matchers for relevant tools

• ❌ Don't: Ignore hook failures silently

  • ✅ Do: Provide clear feedback to user

Constraints

• Never inline complex regex directly in hooks.json — double-escaping across JSON and bash layers corrupts the pattern; always delegate to a script file
• Never use \s in grep ERE patterns — use [[:space:]] instead; \s is not supported
• Never rely on \b word boundaries in grep ERE — they are unreliable; use explicit character class delimiters instead
• Never test blocking hooks without also testing false positives alongside bypass vectors — a hook that over-blocks is as broken as one that under-blocks
• Always define long regex as a PATTERN='...' variable in the script rather than embedding it inline in the grep call — it avoids a third layer of quoting and is far easier to read and update

Resources

• Official Docs: https://docs.claude.com/en/docs/claude-code/hooks
• Settings Reference: https://docs.claude.com/en/docs/claude-code/settings
• JSON Parsing: man jq or https://jqlang.github.io/jq/

Custom docs written on creating-hooks topics:

• resources/testing-pretooluse-blocking-hooks.md

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