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name Hook Development
description This skill should be used when the user asks to "create a hook", "add a PreToolUse/PostToolUse/Stop hook", "validate tool use", "implement prompt-based hooks", "use ${CLAUDE_PLUGIN_ROOT}", "set up event-driven automation", "block dangerous commands", or mentions hook events (PreToolUse, PostToolUse, Stop, SubagentStop, SessionStart, SessionEnd, UserPromptSubmit, PreCompact, Notification). Provides comprehensive guidance for creating and implementing Claude Code plugin hooks with focus on advanced prompt-based hooks API.
version 0.1.0

Hook Development for Claude Code Plugins

Overview

Hooks are event-driven automation scripts that execute in response to Claude Code events. Use hooks to validate operations, enforce policies, add context, and integrate external tools into workflows.

Key capabilities:

  • Validate tool calls before execution (PreToolUse)
  • React to tool results (PostToolUse)
  • Enforce completion standards (Stop, SubagentStop)
  • Load project context (SessionStart)
  • Automate workflows across the development lifecycle

Hook Types

Prompt-Based Hooks (Recommended)

Use LLM-driven decision making for context-aware validation:

{
  "type": "prompt",
  "prompt": "Evaluate if this tool use is appropriate: $TOOL_INPUT",
  "timeout": 30
}

Supported events: Stop, SubagentStop, UserPromptSubmit, PreToolUse

Benefits:

  • Context-aware decisions based on natural language reasoning
  • Flexible evaluation logic without bash scripting
  • Better edge case handling
  • Easier to maintain and extend

Command Hooks

Execute bash commands for deterministic checks:

{
  "type": "command",
  "command": "bash ${CLAUDE_PLUGIN_ROOT}/scripts/validate.sh",
  "timeout": 60
}

Use for:

  • Fast deterministic validations
  • File system operations
  • External tool integrations
  • Performance-critical checks

Hook Configuration Formats

Plugin hooks.json Format

For plugin hooks in hooks/hooks.json, use wrapper format:

{
  "description": "Brief explanation of hooks (optional)",
  "hooks": {
    "PreToolUse": [...],
    "Stop": [...],
    "SessionStart": [...]
  }
}

Key points:

  • description field is optional
  • hooks field is required wrapper containing actual hook events
  • This is the plugin-specific format

Example:

{
  "description": "Validation hooks for code quality",
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Write",
        "hooks": [
          {
            "type": "command",
            "command": "${CLAUDE_PLUGIN_ROOT}/hooks/validate.sh"
          }
        ]
      }
    ]
  }
}

Settings Format (Direct)

For user settings in .claude/settings.json, use direct format:

{
  "PreToolUse": [...],
  "Stop": [...],
  "SessionStart": [...]
}

Key points:

  • No wrapper - events directly at top level
  • No description field
  • This is the settings format

Important: The examples below show the hook event structure that goes inside either format. For plugin hooks.json, wrap these in {"hooks": {...}}.

Hook Events

PreToolUse

Execute before any tool runs. Use to approve, deny, or modify tool calls.

Example (prompt-based):

{
  "PreToolUse": [
    {
      "matcher": "Write|Edit",
      "hooks": [
        {
          "type": "prompt",
          "prompt": "Validate file write safety. Check: system paths, credentials, path traversal, sensitive content. Return 'approve' or 'deny'."
        }
      ]
    }
  ]
}

Output for PreToolUse:

{
  "hookSpecificOutput": {
    "permissionDecision": "allow|deny|ask",
    "updatedInput": {"field": "modified_value"}
  },
  "systemMessage": "Explanation for Claude"
}

PostToolUse

Execute after tool completes. Use to react to results, provide feedback, or log.

Example:

{
  "PostToolUse": [
    {
      "matcher": "Edit",
      "hooks": [
        {
          "type": "prompt",
          "prompt": "Analyze edit result for potential issues: syntax errors, security vulnerabilities, breaking changes. Provide feedback."
        }
      ]
    }
  ]
}

Output behavior:

  • Exit 0: stdout shown in transcript
  • Exit 2: stderr fed back to Claude
  • systemMessage included in context

Stop

Execute when main agent considers stopping. Use to validate completeness.

Example:

{
  "Stop": [
    {
      "matcher": "*",
      "hooks": [
        {
          "type": "prompt",
          "prompt": "Verify task completion: tests run, build succeeded, questions answered. Return 'approve' to stop or 'block' with reason to continue."
        }
      ]
    }
  ]
}

Decision output:

{
  "decision": "approve|block",
  "reason": "Explanation",
  "systemMessage": "Additional context"
}

SubagentStop

Execute when subagent considers stopping. Use to ensure subagent completed its task.

Similar to Stop hook, but for subagents.

UserPromptSubmit

Execute when user submits a prompt. Use to add context, validate, or block prompts.

Example:

{
  "UserPromptSubmit": [
    {
      "matcher": "*",
      "hooks": [
        {
          "type": "prompt",
          "prompt": "Check if prompt requires security guidance. If discussing auth, permissions, or API security, return relevant warnings."
        }
      ]
    }
  ]
}

SessionStart

Execute when Claude Code session begins. Use to load context and set environment.

Example:

{
  "SessionStart": [
    {
      "matcher": "*",
      "hooks": [
        {
          "type": "command",
          "command": "bash ${CLAUDE_PLUGIN_ROOT}/scripts/load-context.sh"
        }
      ]
    }
  ]
}

Special capability: Persist environment variables using $CLAUDE_ENV_FILE:

echo "export PROJECT_TYPE=nodejs" >> "$CLAUDE_ENV_FILE"

See examples/load-context.sh for complete example.

SessionEnd

Execute when session ends. Use for cleanup, logging, and state preservation.

PreCompact

Execute before context compaction. Use to add critical information to preserve.

Notification

Execute when Claude sends notifications. Use to react to user notifications.

Hook Output Format

Standard Output (All Hooks)

{
  "continue": true,
  "suppressOutput": false,
  "systemMessage": "Message for Claude"
}
  • continue: If false, halt processing (default true)
  • suppressOutput: Hide output from transcript (default false)
  • systemMessage: Message shown to Claude

Exit Codes

  • 0 - Success (stdout shown in transcript)
  • 2 - Blocking error (stderr fed back to Claude)
  • Other - Non-blocking error

Hook Input Format

All hooks receive JSON via stdin with common fields:

{
  "session_id": "abc123",
  "transcript_path": "/path/to/transcript.txt",
  "cwd": "/current/working/dir",
  "permission_mode": "ask|allow",
  "hook_event_name": "PreToolUse"
}

Event-specific fields:

  • PreToolUse/PostToolUse: tool_name, tool_input, tool_result
  • UserPromptSubmit: user_prompt
  • Stop/SubagentStop: reason

Access fields in prompts using $TOOL_INPUT, $TOOL_RESULT, $USER_PROMPT, etc.

Environment Variables

Available in all command hooks:

  • $CLAUDE_PROJECT_DIR - Project root path
  • $CLAUDE_PLUGIN_ROOT - Plugin directory (use for portable paths)
  • $CLAUDE_ENV_FILE - SessionStart only: persist env vars here
  • $CLAUDE_CODE_REMOTE - Set if running in remote context

Always use ${CLAUDE_PLUGIN_ROOT} in hook commands for portability:

{
  "type": "command",
  "command": "bash ${CLAUDE_PLUGIN_ROOT}/scripts/validate.sh"
}

Plugin Hook Configuration

In plugins, define hooks in hooks/hooks.json:

{
  "PreToolUse": [
    {
      "matcher": "Write|Edit",
      "hooks": [
        {
          "type": "prompt",
          "prompt": "Validate file write safety"
        }
      ]
    }
  ],
  "Stop": [
    {
      "matcher": "*",
      "hooks": [
        {
          "type": "prompt",
          "prompt": "Verify task completion"
        }
      ]
    }
  ],
  "SessionStart": [
    {
      "matcher": "*",
      "hooks": [
        {
          "type": "command",
          "command": "bash ${CLAUDE_PLUGIN_ROOT}/scripts/load-context.sh",
          "timeout": 10
        }
      ]
    }
  ]
}

Plugin hooks merge with user's hooks and run in parallel.

Matchers

Tool Name Matching

Exact match:

"matcher": "Write"

Multiple tools:

"matcher": "Read|Write|Edit"

Wildcard (all tools):

"matcher": "*"

Regex patterns:

"matcher": "mcp__.*__delete.*"  // All MCP delete tools

Note: Matchers are case-sensitive.

Common Patterns

// All MCP tools
"matcher": "mcp__.*"

// Specific plugin's MCP tools
"matcher": "mcp__plugin_asana_.*"

// All file operations
"matcher": "Read|Write|Edit"

// Bash commands only
"matcher": "Bash"

Security Best Practices

Input Validation

Always validate inputs in command hooks:

#!/bin/bash
set -euo pipefail

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

# Validate tool name format
if [[ ! "$tool_name" =~ ^[a-zA-Z0-9_]+$ ]]; then
  echo '{"decision": "deny", "reason": "Invalid tool name"}' >&2
  exit 2
fi

Path Safety

Check for path traversal and sensitive files:

file_path=$(echo "$input" | jq -r '.tool_input.file_path')

# Deny path traversal
if [[ "$file_path" == *".."* ]]; then
  echo '{"decision": "deny", "reason": "Path traversal detected"}' >&2
  exit 2
fi

# Deny sensitive files
if [[ "$file_path" == *".env"* ]]; then
  echo '{"decision": "deny", "reason": "Sensitive file"}' >&2
  exit 2
fi

See examples/validate-write.sh and examples/validate-bash.sh for complete examples.

Quote All Variables

# GOOD: Quoted
echo "$file_path"
cd "$CLAUDE_PROJECT_DIR"

# BAD: Unquoted (injection risk)
echo $file_path
cd $CLAUDE_PROJECT_DIR

Set Appropriate Timeouts

{
  "type": "command",
  "command": "bash script.sh",
  "timeout": 10
}

Defaults: Command hooks (60s), Prompt hooks (30s)

Performance Considerations

Parallel Execution

All matching hooks run in parallel:

{
  "PreToolUse": [
    {
      "matcher": "Write",
      "hooks": [
        {"type": "command", "command": "check1.sh"},  // Parallel
        {"type": "command", "command": "check2.sh"},  // Parallel
        {"type": "prompt", "prompt": "Validate..."}   // Parallel
      ]
    }
  ]
}

Design implications:

  • Hooks don't see each other's output
  • Non-deterministic ordering
  • Design for independence

Optimization

  1. Use command hooks for quick deterministic checks
  2. Use prompt hooks for complex reasoning
  3. Cache validation results in temp files
  4. Minimize I/O in hot paths

Temporarily Active Hooks

Create hooks that activate conditionally by checking for a flag file or configuration:

Pattern: Flag file activation

#!/bin/bash
# Only active when flag file exists
FLAG_FILE="$CLAUDE_PROJECT_DIR/.enable-strict-validation"

if [ ! -f "$FLAG_FILE" ]; then
  # Flag not present, skip validation
  exit 0
fi

# Flag present, run validation
input=$(cat)
# ... validation logic ...

Pattern: Configuration-based activation

#!/bin/bash
# Check configuration for activation
CONFIG_FILE="$CLAUDE_PROJECT_DIR/.claude/plugin-config.json"

if [ -f "$CONFIG_FILE" ]; then
  enabled=$(jq -r '.strictMode // false' "$CONFIG_FILE")
  if [ "$enabled" != "true" ]; then
    exit 0  # Not enabled, skip
  fi
fi

# Enabled, run hook logic
input=$(cat)
# ... hook logic ...

Use cases:

  • Enable strict validation only when needed
  • Temporary debugging hooks
  • Project-specific hook behavior
  • Feature flags for hooks

Best practice: Document activation mechanism in plugin README so users know how to enable/disable temporary hooks.

Hook Lifecycle and Limitations

Hooks Load at Session Start

Important: Hooks are loaded when Claude Code session starts. Changes to hook configuration require restarting Claude Code.

Cannot hot-swap hooks:

  • Editing hooks/hooks.json won't affect current session
  • Adding new hook scripts won't be recognized
  • Changing hook commands/prompts won't update
  • Must restart Claude Code: exit and run claude again

To test hook changes:

  1. Edit hook configuration or scripts
  2. Exit Claude Code session
  3. Restart: claude or cc
  4. New hook configuration loads
  5. Test hooks with claude --debug

Hook Validation at Startup

Hooks are validated when Claude Code starts:

  • Invalid JSON in hooks.json causes loading failure
  • Missing scripts cause warnings
  • Syntax errors reported in debug mode

Use /hooks command to review loaded hooks in current session.

Debugging Hooks

Enable Debug Mode

claude --debug

Look for hook registration, execution logs, input/output JSON, and timing information.

Test Hook Scripts

Test command hooks directly:

echo '{"tool_name": "Write", "tool_input": {"file_path": "/test"}}' | \
  bash ${CLAUDE_PLUGIN_ROOT}/scripts/validate.sh

echo "Exit code: $?"

Validate JSON Output

Ensure hooks output valid JSON:

output=$(./your-hook.sh < test-input.json)
echo "$output" | jq .

Quick Reference

Hook Events Summary

Event When Use For
PreToolUse Before tool Validation, modification
PostToolUse After tool Feedback, logging
UserPromptSubmit User input Context, validation
Stop Agent stopping Completeness check
SubagentStop Subagent done Task validation
SessionStart Session begins Context loading
SessionEnd Session ends Cleanup, logging
PreCompact Before compact Preserve context
Notification User notified Logging, reactions

Best Practices

DO:

  • ✅ Use prompt-based hooks for complex logic
  • ✅ Use ${CLAUDE_PLUGIN_ROOT} for portability
  • ✅ Validate all inputs in command hooks
  • ✅ Quote all bash variables
  • ✅ Set appropriate timeouts
  • ✅ Return structured JSON output
  • ✅ Test hooks thoroughly

DON'T:

  • ❌ Use hardcoded paths
  • ❌ Trust user input without validation
  • ❌ Create long-running hooks
  • ❌ Rely on hook execution order
  • ❌ Modify global state unpredictably
  • ❌ Log sensitive information

Cross-Platform Compatibility (Windows)

The Problem

On Windows, when WSL (Windows Subsystem for Linux) is installed — which is extremely common due to Docker Desktop — the bash command resolves to WSL's C:\Windows\System32\bash.exe instead of Git Bash. Claude Code detects .sh extensions in hook commands and auto-prepends bash, which causes hooks to fail with:

WSL ERROR: CreateProcessCommon: execvpe(/bin/bash) failed: No such file or directory

This affects all command hooks that reference .sh scripts directly.

The Solution: Polyglot .cmd Wrappers

Use a .cmd wrapper that works on both Windows (cmd.exe) and Unix/macOS (bash). The wrapper:

  • On Windows: explicitly invokes Git Bash at its standard installation path
  • On Unix/macOS: passes through to bash natively via the polyglot pattern

How to Use

  1. Create a run-hook.cmd file next to your .sh scripts (see examples/cross-platform-hook.cmd)

  2. Update your hooks.json to call the .cmd wrapper instead of the .sh script directly:

Before (breaks on Windows with WSL):

{
  "type": "command",
  "command": "${CLAUDE_PLUGIN_ROOT}/hooks/my-hook.sh"
}

After (works everywhere):

{
  "type": "command",
  "command": "${CLAUDE_PLUGIN_ROOT}/hooks/run-hook.cmd my-hook.sh"
}
  1. The wrapper passes the script name as an argument and resolves the path relative to itself.

How the Polyglot Works

Lines starting with :; are valid labels in cmd.exe (ignored) and valid no-ops in bash (label syntax). This allows a single file to contain both bash and cmd.exe code paths:

:; # This line is ignored by cmd.exe, executed by bash
:; exec bash "$SCRIPT_DIR/$1"   # bash exec's the target script
:; exit $?                       # bash never reaches here

@echo off
REM cmd.exe starts executing here (skips :; lines)
"C:\Program Files\Git\bin\bash.exe" "%HOOK_SCRIPT%"

Best Practices

  • Always use .cmd wrappers for command hooks in plugins intended for cross-platform use
  • Keep the original .sh scripts unchanged — the wrapper delegates to them
  • The wrapper tries standard Git Bash locations and falls back to finding bash via where git
  • Test on both Windows (cmd /c run-hook.cmd script.sh) and Unix (bash run-hook.cmd script.sh)

Additional Resources

Reference Files

For detailed patterns and advanced techniques, consult:

  • references/patterns.md - Common hook patterns (8+ proven patterns)
  • references/migration.md - Migrating from basic to advanced hooks
  • references/advanced.md - Advanced use cases and techniques

Example Hook Scripts

Working examples in examples/:

  • validate-write.sh - File write validation example
  • validate-bash.sh - Bash command validation example
  • load-context.sh - SessionStart context loading example
  • cross-platform-hook.cmd - Windows/Unix polyglot wrapper for cross-platform hooks

Utility Scripts

Development tools in scripts/:

  • validate-hook-schema.sh - Validate hooks.json structure and syntax
  • test-hook.sh - Test hooks with sample input before deployment
  • hook-linter.sh - Check hook scripts for common issues and best practices

External Resources

Implementation Workflow

To implement hooks in a plugin:

  1. Identify events to hook into (PreToolUse, Stop, SessionStart, etc.)
  2. Decide between prompt-based (flexible) or command (deterministic) hooks
  3. Write hook configuration in hooks/hooks.json
  4. For command hooks, create hook scripts
  5. Use ${CLAUDE_PLUGIN_ROOT} for all file references
  6. Validate configuration with scripts/validate-hook-schema.sh hooks/hooks.json
  7. Test hooks with scripts/test-hook.sh before deployment
  8. Test in Claude Code with claude --debug
  9. Document hooks in plugin README

Focus on prompt-based hooks for most use cases. Reserve command hooks for performance-critical or deterministic checks.